Intel® SPMD Program Compiler User's Guide

The Intel® SPMD Program Compiler (ispc) is a compiler for writing SPMD (single program multiple data) programs to run on the CPU. The SPMD programming approach is widely known to graphics and GPGPU programmers; it is used for GPU shaders and CUDA* and OpenCL* kernels, for example. The main idea behind SPMD is that one writes programs as if they were operating on a single data element (a pixel for a pixel shader, for example), but then the underlying hardware and runtime system executes multiple invocations of the program in parallel with different inputs (the values for different pixels, for example).

The main goals behind ispc are to:

  • Build a variant of the C programming language that delivers good performance to performance-oriented programmers who want to run SPMD programs on CPUs.
  • Provide a thin abstraction layer between the programmer and the hardware--in particular, to follow the lesson from C for serial programs of having an execution and data model where the programmer can cleanly reason about the mapping of their source program to compiled assembly language and the underlying hardware.
  • Harness the computational power of the Single Program, Multiple Data (SIMD) vector units without the extremely low-programmer-productivity activity of directly writing intrinsics.
  • Explore opportunities from close-coupling between C/C++ application code and SPMD ispc code running on the same processor--lightweight function calls between the two languages, sharing data directly via pointers without copying or reformatting, etc.

We are very interested in your feedback and comments about ispc and in hearing your experiences using the system. We are especially interested in hearing if you try using ispc but see results that are not as you were expecting or hoping for. We encourage you to send a note with your experiences or comments to the ispc-users mailing list or to file bug or feature requests with the ispc bug tracker. (Thanks!)

Contents:

Recent Changes to ISPC

See the file ReleaseNotes.txt in the ispc distribution for a list of recent changes to the compiler.

Updating ISPC Programs For Changes In ISPC 1.1

The major changes introduced in the 1.1 release of ispc are first-class support for pointers in the language and new parallel loop constructs. Adding this functionality required a number of syntactic changes to the language. These changes should generally lead to straightforward minor modifications of existing ispc programs.

These are the relevant changes to the language:

  • The syntax for reference types has been changed to match C++'s syntax for references and the reference keyword has been removed. (A diagnostic message is issued if reference is used.)
    • Declarations like reference float foo should be changed to float &foo.
    • Any array parameters in function declaration with a reference qualifier should just have reference removed: void foo(reference float bar[]) can just be void foo(float bar[]).
  • It is now a compile-time error to assign an entire array to another array.
  • A number of standard library routines have been updated to take pointer-typed parameters, rather than references or arrays an index offsets, as appropriate. For example, the atomic_add_global() function previously took a reference to the variable to be updated atomically but now takes a pointer. In a similar fashion, packed_store_active() takes a pointer to a uniform unsigned int as its first parameter rather than taking a uniform unsigned int[] as its first parameter and a uniform int offset as its second parameter.
  • It is no longer legal to pass a varying lvalue to a function that takes a reference parameter; references can only be to uniform lvalue types. In this case, the function should be rewritten to take a varying pointer parameter.
  • There are new iteration constructs for looping over computation domains, foreach and foreach_tiled. In addition to being syntactically cleaner than regular for loops, these can provide performance benefits in many cases when iterating over data and mapping it to program instances. See the Section Parallel Iteration Statements: "foreach" and "foreach_tiled" for more information about these.

Updating ISPC Programs For Changes In ISPC 1.2

The following changes were made to the language syntax and semantics for the ispc 1.2 release:

  • Syntax for the "launch" keyword has been cleaned up; it's now no longer necessary to bracket the launched function call with angle brackets. (In other words, now use launch foo();, rather than launch < foo() >;.)
  • When using pointers, the pointed-to data type is now "uniform" by default. Use the varying keyword to specify varying pointed-to types when needed. (i.e. float *ptr is a varying pointer to uniform float data, whereas previously it was a varying pointer to varying float values.) Use varying float * to specify a varying pointer to varying float data, and so forth.
  • The details of "uniform" and "varying" and how they interact with struct types have been cleaned up. Now, when a struct type is declared, if the struct elements don't have explicit "uniform" or "varying" qualifiers, they are said to have "unbound" variability. When a struct type is instantiated, any unbound variability elements inherit the variability of the parent struct type. See Struct Types for more details.
  • ispc has a new language feature that makes it much easier to use the efficient "(array of) structure of arrays" (AoSoA, or SoA) memory layout of data. A new soa<n> qualifier can be applied to structure types to specify an n-wide SoA version of the corresponding type. Array indexing and pointer operations with arrays SoA types automatically handles the two-stage indexing calculation to access the data. See Structure of Array Types for more details.

Updating ISPC Programs For Changes In ISPC 1.3

This release adds a number of new iteration constructs, which in turn use new reserved words: unmasked, foreach_unique, foreach_active, and in. Any program that happens to have a variable or function with one of these names must be modified to rename that symbol.

Updating ISPC Programs For Changes In ISPC 1.5.0

This release adds support for double precision floating point constants. Double precision floating point constants are floating point number with d suffix and optional exponent part. Here are some examples: 3.14d, 31.4d-1, 1.d, 1.0d, 1d-2. Note that floating point number without suffix is treated as single precision constant.

Updating ISPC Programs For Changes In ISPC 1.6.0

This release adds support for Operators Overloading, so a word operator becomes a keyword and it potentially creates a conflict with existing user function. Also a new library function packed_store_active2() was introduced, which also may create a conflict with existing user functions.

Updating ISPC Programs For Changes In ISPC 1.7.0

This release contains several changes that may affect compatibility with older versions:

  • The algorithm for selecting overloaded functions was extended to cover more types of overloading, and handling of reference types was fixed. At the same time the old scheme, which blindly used the function with "the best score" summed for all arguments, was switched to the C++ approach, which requires "the best score" for each argument. If the best function doesn't exist, a warning is issued in this version. It will be turned into an error in the next version. A simple example: Suppose we have two functions: max(int, int) and max(unsigned int, unsigned int). The new rules lead to an error when calling max(int, unsigned int), as the best choice is ambiguous.
  • Implicit cast of pointer to const type to void* was disallowed. Use explicit cast if needed.
  • A bug which prevented "const" qualifiers from appearing in emitted .h files was fixed. Consequently, "const" qualifiers now properly appearing in emitted .h files may cause compile errors in pre-existing codes.
  • get_ProgramCount() was moved from stdlib to examples/util/util.isph file. You need to include this file to be able to use this function.

Getting Started with ISPC

Installing ISPC

The ispc downloads web page has prebuilt executables for Windows*, Linux* and Mac OS* available for download. Alternatively, you can download the source code from that page and build it yourself; see see the ispc wiki for instructions about building ispc from source.

Once you have an executable for your system, copy it into a directory that's in your PATH. Congratulations--you've now installed ispc.

Compiling and Running a Simple ISPC Program

The directory examples/simple in the ispc distribution includes a simple example of how to use ispc with a short C++ program. See the file simple.ispc in that directory (also reproduced here.)

export void simple(uniform float vin[], uniform float vout[],
                   uniform int count) {
    foreach (index = 0 ... count) {
        float v = vin[index];
        if (v < 3.)
            v = v * v;
        else
            v = sqrt(v);
        vout[index] = v;
    }
}

This program loops over an array of values in vin and computes an output value for each one. For each value in vin, if its value is less than three, the output is the value squared, otherwise it's the square root of the value.

The first thing to notice in this program is the presence of the export keyword in the function definition; this indicates that the function should be made available to be called from application code. The uniform qualifiers on the parameters to simple indicate that the corresponding variables are non-vector quantities--this concept is discussed in detail in the "uniform" and "varying" Qualifiers section.

Each iteration of the foreach loop works on a number of input values in parallel--depending on the compilation target chosen, it may be 4, 8, or even 16 elements of the vin array, processed efficiently with the CPU's SIMD hardware. Here, the variable index takes all values from 0 to count-1. After the load from the array to the variable v, the program can then proceed, doing computation and control flow based on the values loaded. The result from the running program instances is written to the vout array before the next iteration of the foreach loop runs.

On Linux* and Mac OS*, the makefile in that directory compiles this program. For Windows*, open the examples/examples.sln file in Microsoft Visual C++ 2012* to build this (and the other) examples. In either case, build it now! We'll walk through the details of the compilation steps in the following section, Using The ISPC Compiler.) In addition to compiling the ispc program, in this case the ispc compiler also generates a small header file, simple.h. This header file includes the declaration for the C-callable function that the above ispc program is compiled to. The relevant parts of this file are:

#ifdef __cplusplus
extern "C" {
#endif // __cplusplus
    extern void simple(float vin[], float vout[], int32_t count);
#ifdef __cplusplus
}
#endif // __cplusplus

It's not mandatory to #include the generated header file in your C/C++ code (you can alternatively use a manually-written extern declaration of the ispc functions you use), but it's a helpful check to ensure that the function signatures are as expected on both sides.

Here is the main program, simple.cpp, which calls the ispc function above.

#include <stdio.h>
#include "simple.h"

int main() {
    float vin[16], vout[16];
    for (int i = 0; i < 16; ++i)
        vin[i] = i;

    simple(vin, vout, 16);

    for (int i = 0; i < 16; ++i)
        printf("%d: simple(%f) = %f\n", i, vin[i], vout[i]);
}

Note that the call to the ispc function in the middle of main() is a regular function call. (And it has the same overhead as a C/C++ function call, for that matter.)

When the executable simple runs, it generates the expected output:

0: simple(0.000000) = 0.000000
1: simple(1.000000) = 1.000000
2: simple(2.000000) = 4.000000
3: simple(3.000000) = 1.732051
...

For a slightly more complex example of using ispc, see the Mandelbrot set example page on the ispc website for a walk-through of an ispc implementation of that algorithm. After reading through that example, you may want to examine the source code of the various examples in the examples/ directory of the ispc distribution.

Using The ISPC Compiler

To go from a ispc source file to an object file that can be linked with application code, enter the following command

ispc foo.ispc -o foo.o

(On Windows, you may want to specify foo.obj as the output filename.)

Basic Command-line Options

The ispc executable can be run with --help to print a list of accepted command-line arguments. By default, the compiler compiles the provided program (and issues warnings and errors), but doesn't generate any output.

If the -o flag is given, it will generate an output file (a native object file by default).

ispc foo.ispc -o foo.obj

To generate a text assembly file, pass --emit-asm:

ispc foo.ispc -o foo.asm --emit-asm

To generate LLVM bitcode, use the --emit-llvm flag.

Optimizations are on by default; they can be turned off with -O0:

ispc foo.ispc -o foo.obj -O0

On Mac* and Linux*, there is basic support for generating debugging symbols; this is enabled with the -g command-line flag. Using -g causes optimizations to be disabled; to compile with debugging symbols and optimization, -O1 should be provided as well as the -g flag.

The -h flag can also be used to direct ispc to generate a C/C++ header file that includes C/C++ declarations of the C-callable ispc functions and the types passed to it.

The -D option can be used to specify definitions to be passed along to the pre-processor, which runs over the program input before it's compiled. For example, including -DTEST=1 defines the pre-processor symbol TEST to have the value 1 when the program is compiled.

The compiler issues a number of performance warnings for code constructs that compile to relatively inefficient code. These warnings can be silenced with the --wno-perf flag (or by using --woff, which turns off all compiler warnings.) Furthermore, --werror can be provided to direct the compiler to treat any warnings as errors.

Position-independent code (for use in shared libraries) is generated if the --pic command-line argument is provided.

Selecting The Compilation Target

There are three options that affect the compilation target: --arch, which sets the target architecture, --cpu, which sets the target CPU, and --target, which sets the target instruction set.

If none of these options is specified, ispc generates code for the architecture of the system the compiler is running on (i.e. 64-bit x86-64 (--arch=x86-64) on x86 systems and ARM NEON on ARM systems.

To compile to a 32-bit x86 target, for example, supply --arch=x86 on the command line:

ispc foo.ispc -o foo.obj --arch=x86

Currently-supported architectures are x86-64, x86, and arm.

The target CPU determines both the default instruction set used as well as which CPU architecture the code is tuned for. ispc --help provides a list of all of the supported CPUs. By default, the CPU type of the system on which you're running ispc is used to determine the target CPU.

ispc foo.ispc -o foo.obj --cpu=corei7-avx

Finally, --target selects the target instruction set. The target string is of the form [ISA]-i[mask size]x[gang size]. For example, --target=avx2-i32x16 specifies a target with the AVX2 instruction set, a mask size of 32 bits, and a gang size of 16.

The following target ISAs are supported:

Target Description
avx, avx1 AVX (2010-2011 era Intel CPUs)
avx1.1 AVX 1.1 (2012 era "Ivybridge" Intel CPUs)
avx2 AVX 2 target (2013- Intel "Haswell" CPUs)
neon ARM NEON
sse2 SSE2 (early 2000s era x86 CPUs)
sse4 SSE4 (generally 2008-2010 Intel CPUs)

Consult your CPU's manual for specifics on which vector instruction set it supports.

The mask size may be 8, 16, or 32 bits, though not all combinations of ISAs and mask sizes are supported. For best performance, the best general approach is to choose a mask size equal to the size of the most common datatype in your programs. For example, if most of your computation is on 32-bit floating-point values, an i32 target is appropriate. However, if you're mostly doing computation on 8-bit images, i8 is a better choice.

See Basic Concepts: Program Instances and Gangs of Program Instances for more discussion of the "gang size" and its implications for program execution.

Running ispc --help and looking at the output for the --target option gives the most up-to-date documentation about which targets your compiler binary supports.

The naming scheme for compilation targets changed in August 2013; the following table shows the relationship between names in the old scheme and in the new scheme:

Target Former Name
avx1-i32x8 avx, avx1
avx1-i32x16 avx-x2
avx1.1-i32x8 avx1.1
avx1.1-i32x16 avx1.1-x2
avx2-i32x8 avx2
avx2-i32x16 avx2-x2
neon-8 n/a
neon-16 n/a
neon-32 n/a
sse2-i32x4 sse2
sse2-i32x8 sse2-x2
sse4-i32x4 sse4
sse4-i32x8 sse4-x2
sse4-i8x16 n/a
sse4-i16x8 n/a

By default, the target instruction set is chosen based on the most capable one supported by the system on which you're running ispc. You can override this choice with the --target flag; for example, to select Intel® SSE2 with a 32-bit mask and 4 program instances in a gang, use --target=sse2-i32x4. (As with the other options in this section, see the output of ispc --help for a full list of supported targets.)

Generating Generic C++ Output

In addition to generating object files or assembly output for specific targets like NEON, SSE2, SSE4, and AVX, ispc provides an option to generate "generic" C++ output. This

As an example, consider the following simple ispc program:

int foo(int i, int j) {
    return (i < 0) ? 0 : i + j;
}

If this program is compiled with the following command:

ispc foo.ispc --emit-c++ --target=generic-4 -o foo.cpp

Then foo() is compiled to the following C++ code (after various automatically-generated boilerplate code):

__vec4_i32 foo(__vec4_i32 i_llvm_cbe, __vec4_i32 j_llvm_cbe,
               __vec4_i1 __mask_llvm_cbe) {
    return (__select((__signed_less_than(i_llvm_cbe,
                                         __vec4_i32 (0u, 0u, 0u, 0u))),
                     __vec4_i32 (0u, 0u, 0u, 0u),
                    (__add(i_llvm_cbe, j_llvm_cbe))));
}

Note that the original computation has been expressed in terms of a number of vector types (e.g. __vec4_i32 for a 4-wide vector of 32-bit integers and __vec4_i1 for a 4-wide vector of boolean values) and in terms of vector operations on these types like __add() and __select()).

You are then free to provide your own implementations of these types and functions. For example, you might want to target a specific vector ISA, or you might want to instrument these functions for performance measurements.

There is an example implementation of 4-wide variants of the required functions, suitable for use with the generic-4 target in the file examples/intrinsics/sse4.h, and there is an example straightforward C implementation of the 16-wide variants for the generic-16 target in the file examples/intrinsics/generic-16.h. There is not yet comprehensive documentation of these types and the functions that must be provided for them when the C++ target is used, but a review of those two files should provide the basic context.

If you are using C++ source emission, you may also find the --c++-include-file=<filename> command line argument useful; it adds an #include statement with the given filename at the top of the emitted C++ file; this can be used to easily include specific implementations of the vector types and functions.

Compiling For The Intel® Xeon Phi™ Architecture

ispc has beta-level support for compiling for the many-core Intel® Xeon Phi™ architecture (formerly, "Many Integrated Cores" / MIC). This support is based on the "generic" C++ output, described in the previous section.

To compile for Xeon Phi™, first generate intermediate C++ code:

ispc foo.ispc --emit-c++ --target=generic-16 -o foo.cpp \
    --c++-include-file=knc.h

The ispc distribution now includes a header file, examples/intrinsics/knc.h, which maps from the generic C++ output to the corresponding intrinsic operations supported by Intel Xeon Phi™. Thus, to generate an object file, use the Intel C++ Compiler (icpc) compile the C++ code generated by ispc, setting the #include search path so that it can find the examples/intrinsics/knc.h header file in the ispc distribution.

icpc -mmic -Iexamples/intrinsics/ foo.cpp -o foo.o

With the current beta implementation, complex ispc programs are able to run on Xeon Phi™, though there are a number of known limitations:

  • The examples/intrinsics/knc.h header file isn't complete yet; for example, vector operations with int8 and int16 types aren't yet implemented. Programs that operate on varying int32, float, and double data-types (and uniform variables of any data type, and arrays and structures of these types), should operate correctly.
  • If you use the launch functionality to launch tasks across cores, note that the pthreads task system implemented in examples/tasksys.cpp offers several implemenetations for Xeon Phi™. You will need to experiment to understand which one is most appropriate for your workload.
  • The compiler currently emits unaligned memory accesses in many cases where the memory address is actually aligned. This may unnecessarily impact performance.
  • When requesting that ICPC generate code with strict floating point precision compliance (using ICPC option -fp-model strict) or accurate reporting of floating point exceptions (using ICPC option -fp-model except) the compiler will generate code that uses the x87 unit rather than Xeon Phi™'s vector unit. For similar reasons, the options –ansi and –fmath-errno may result in calls to math functions that are implemented in x87 rather than using vector instructions. This will have a significant performance impact. See the ICPC manual for details on these compiler options.

All of these issues are currently actively being addressed and will be fixed in future releases.

If you do use the current version of ispc on Intel Xeon Phi™, please let us know of any bugs or unexpected results. (Also, any interesting results!).

Selecting 32 or 64 Bit Addressing

By default, ispc uses 32-bit arithmetic for performing addressing calculations, even when using a 64-bit compilation target like x86-64. This implementation approach can provide substantial performance benefits by reducing the cost of addressing calculations. (Note that pointers themselves are still maintained as 64-bit quantities for 64-bit targets.)

If you need to be able to address more than 4GB of memory from your ispc programs, the --addressing=64 command-line argument can be provided to cause the compiler to generate 64-bit arithmetic for addressing calculations. Note that it is safe to mix object files where some were compiled with the default --addressing=32 and others were compiled with --addressing=64.

The Preprocessor

ispc automatically runs the C preprocessor on your input program before compiling it. Thus, you can use #ifdef, #define, and so forth in your ispc programs. (This functionality can be disabled with the --nocpp command-line argument.)

A number of preprocessor symbols are automatically defined before the preprocessor runs:

Predefined Preprocessor symbols and their values
Symbol name Value Use
ISPC 1 Detecting that the ispc compiler is processing the file
ISPC_TARGET_{NEON_8,NEON_16,NEON_32,SSE2,SSE4,AVX,AVX11,AVX2,GENERIC} 1 One of these will be set, depending on the compilation target.
ISPC_POINTER_SIZE 32 or 64 Number of bits used to represent a pointer for the target architecture.
ISPC_MAJOR_VERSION 1 Major version of the ispc compiler/language
ISPC_MINOR_VERSION 3 Minor version of the ispc compiler/language
PI 3.1415926535 Mathematics

Debugging

On Linux* and Mac OS*, the -g command-line flag can be supplied to the compiler, which causes it to generate debugging symbols. Running ispc programs in the debugger, setting breakpoints, printing out variables is just the same as debugging C/C++ programs. Similarly, you can directly step up and down the call stack between ispc code and C/C++ code.

One limitation of the current debugging support is that the debugger provides a window into an entire gang's worth of program instances, rather than just a single program instance. (These concepts will be introduced shortly, in Basic Concepts: Program Instances and Gangs of Program Instances). Thus, when a varying variable is printed, the values for each of the program instances are displayed. Along similar lines, the path the debugger follows through program source code passes each statement that any program instance wants to execute (see Control Flow Within A Gang for more details on control flow in ispc.)

While debugging, a variable, __mask, is available to provide the current program execution mask at the current point in the program

Another option for debugging (and the only current option on Windows*) is to use the print statement for printf() style debugging. (See Output Functions for more information.) You can also use the ability to call back to application code at particular points in the program, passing a set of variable values to be logged or otherwise analyzed from there.

The ISPC Parallel Execution Model

Though ispc is a C-based language, it is inherently a language for parallel computation. Understanding the details of ispc's parallel execution model that are introduced in this section is critical for writing efficient and correct programs in ispc.

ispc supports two types of parallelism: task parallelism to parallelize across multiple processor cores and SPMD parallelism to parallelize across the SIMD vector lanes on a single core. Most of this section focuses on SPMD parallelism, but see Tasking Model at the end of this section for discussion of task parallelism in ispc.

This section will use some snippets of ispc code to illustrate various concepts. Given ispc's relationship to C, these should be understandable on their own, but you may want to refer to the The ISPC Language section for details on language syntax.

Basic Concepts: Program Instances and Gangs of Program Instances

Upon entry to a ispc function called from C/C++ code, the execution model switches from the application's serial model to ispc's execution model. Conceptually, a number of ispc program instances start running concurrently. The group of running program instances is a called a gang (harkening to "gang scheduling", since ispc provides certain guarantees about the control flow coherence of program instances running in a gang, detailed in Gang Convergence Guarantees.) An ispc program instance is thus similar to a CUDA* "thread" or an OpenCL* "work-item", and an ispc gang is similar to a CUDA* "warp".

An ispc program expresses the computation performed by a gang of program instances, using an "implicit parallel" model, where the ispc program generally describes the behavior of a single program instance, even though a gang of them is actually executing together. This implicit model is the same that is used for shaders in programmable graphics pipelines, OpenCL* kernels, and CUDA*. For example, consider the following ispc function:

float func(float a, float b) {
     return a + b / 2.;
}

In C, this function describes a simple computation on two individual floating-point values. In ispc, this function describes the computation to be performed by each program instance in a gang. Each program instance has distinct values for the variables a and b, and thus each program instance generally computes a different result when executing this function.

The gang of program instances starts executing in the same hardware thread and context as the application code that called the ispc function; no thread creation or context switching is done under the covers by ispc. Rather, the set of program instances is mapped to the SIMD lanes of the current processor, leading to excellent utilization of hardware SIMD units and high performance.

The number of program instances in a gang is relatively small; in practice, it's no more than 2-4x the native SIMD width of the hardware it is executing on. (Thus, four or eight program instances in a gang on a CPU using the the 4-wide SSE instruction set, and eight or sixteen on a CPU using 8-wide AVX.)

Control Flow Within A Gang

Almost all the standard control-flow constructs are supported by ispc; program instances are free to follow different program execution paths than other ones in their gang. For example, consider a simple if statement in ispc code:

float x = ..., y = ...;
if (x < y) {
   // true statements
}
else {
   // false statements
}

In general, the test x < y may have different result for different program instances in the gang: some of the currently running program instances want to execute the statements for the "true" case and some want to execute the statements for the "false" case.

Complex control flow in ispc programs generally works as expected, computing the same results for each program instance in a gang as would have been computed if the equivalent code ran serially in C to compute each program instance's result individually. However, here we will more precisely define the execution model for control flow in order to be able to precisely define the language's behavior in specific situations.

We will specify the notion of a program counter and how it is updated to step through the program, and an execution mask that indicates which program instances want to execute the instruction at the current program counter. The program counter a single program counter shared by all of the program instances in the gang; it points to a single instruction to be executed next. The execution mask is a per-program-instance boolean value that indicates whether or not side effects from the current instruction should effect each program instance. Thus, for example, if a statement were to be executed with an "all off" mask, there should be no observable side-effects.

Upon entry to an ispc function called by the application, the execution mask is "all on" and the program counter points at the first statement in the function. The following two statements describe the required behavior of the program counter and the execution mask over the course of execution of an ispc function.

1. The program counter will have a sequence of values corresponding to a conservative execution path through the function, wherein if any program instance wants to execute a statement, the program counter will pass through that statement.

2. At each statement the program counter passes through, the execution mask will be set such that its value for a particular program instance is "on" if and only if the program instance wants to execute that statement.

Note that these definition provide the compiler some latitude; for example, the program counter is allowed pass through a series of statements with the execution mask "all off" because doing so has no observable side-effects.

Elsewhere, we will speak informally of the control flow coherence of a program; this notion describes the degree to which the program instances in the gang want to follow the same control flow path through a function (or, conversely, whether most statements are executed with a "mostly on" execution mask or a "mostly off" execution mask.) In general, control flow divergence leads to reductions in SIMD efficiency (and thus performance) as different program instances want to perform different computations.

Control Flow Example: If Statements

As a concrete example of the interplay between program counter and execution mask, one way that an if statement like the one in the previous section can be represented is shown by the following pseudo-code compiler output:

float x = ..., y = ...;
bool test = (x < y);
mask originalMask = get_current_mask();
set_mask(originalMask & test);
if (any_mask_entries_are_enabled()) {
  // true statements
}
set_mask(originalMask & ~test);
if (any_mask_entries_are_enabled()) {
  // false statements
}
set_mask(originalMask);

In other words, the program counter steps through the statements for both the "true" case and the "false" case, with the execution mask set so that no side-effects from the true statements affect the program instances that want to run the false statements, and vice versa. However, a block of statements does not execute if the mask is "all off" upon entry to that block. The execution mask is then restored to the value it had before the if statement.

Control Flow Example: Loops

for, while, and do statements are handled in an analogous fashion. The program counter continues to run additional iterations of the loop until all of the program instances are ready to exit the loop.

Therefore, if we have a loop like the following:

int limit = ...;
for (int i = 0; i < limit; ++i) {
    ...
}

where limit has the value 1 for all of the program instances but one, and has value 1000 for the other one, the program counter will step through the loop body 1000 times. The first time, the execution mask will be all on (assuming it is all on going into the for loop), and the remaining 999 times, the mask will be off except for the program instance with a limit value of 1000. (This would be a loop with poor control flow coherence!)

A continue statement in a loop may be handled either by disabling the execution mask for the program instances that execute the continue and then continuing to step the program counter through the rest of the loop, or by jumping to the loop step statement, if all program instances are disabled after the continue has executed. break statements are handled in a similar fashion.

Gang Convergence Guarantees

The ispc execution model provides an important guarantee about the behavior of the program counter and execution mask: the execution of program instances is maximally converged. Maximal convergence means that if two program instances follow the same control path, they are guaranteed to execute each program statement concurrently. If two program instances follow diverging control paths, it is guaranteed that they will reconverge as soon as possible in the function (if they do later reconverge). [1]

[1]This is another significant difference between the ispc execution model and the one implemented by OpenCL* and CUDA*, which doesn't provide this guarantee.

Maximal convergence means that in the presence of divergent control flow such as the following:

if (test) {
  // true
}
else {
  // false
}

It is guaranteed that all program instances that were running before the if test will also be running after the end of the else block. (This guarantee stems from the notion of having a single program counter for the gang of program instances, rather than the concept of a unique program counter for each program instance.)

Another implication of this property is that it would be illegal for the ispc implementation to execute a function with an 8-wide gang by running it two times, with a 4-wide gang representing half of the original 8-wide gang each time.

It also follows that given the following program:

if (programIndex == 0) {
    while (true)  // infinite loop
        ;
}
print("hello, world\n");

the program will loop infinitely and the print statement will never be executed. (A different execution model that allowed gang divergence might execute the print statement since not all program instances were caught in the infinite loop in the example above.)

The way that "varying" function pointers are handled in ispc is also affected by this guarantee: if a function pointer is varying, then it has a possibly-different value for all running program instances. Given a call to a varying function pointer, ispc must maintains as much execution convergence as possible; the assembly code generated finds the set of unique function pointers over the currently running program instances and calls each one just once, such that the executing program instances when it is called are the set of active program instances that had that function pointer value. The order in which the various function pointers are called in this case is undefined.

Uniform Data

A variable that is declared with the uniform qualifier represents a single value that is shared across the entire gang. (In contrast, the default variability qualifier for variables in ispc, varying, represents a variable that has a distinct storage location for each program instance in the gang.) (Though see the discussion in Struct Types for some subtleties related to uniform and varying when used with structures.)

It is an error to try to assign a varying value to a uniform variable, though uniform values can be assigned to uniform variables. Assignments to uniform variables are not affected by the execution mask (there's no unambiguous way that they could be); rather, they always apply if the program counter pointer passes through a statement that is a uniform assignment.

Uniform Control Flow

One advantage of declaring variables that are shared across the gang as uniform, when appropriate, is the reduction in storage space required. A more important benefit is that it can enable the compiler to generate substantially better code for control flow; when a test condition for a control flow decision is based on a uniform quantity, the compiler can be immediately aware that all of the running program instances will follow the same path at that point, saving the overhead of needing to deal with control flow divergence and mask management. (To distinguish the two forms of control flow, will say that control flow based on varying expressions is "varying" control flow.)

Consider for example an image filtering operation where the program loops over pixels adjacent to the given (x,y) coordinates:

float box3x3(uniform float image[32][32], int x, int y) {
    float sum = 0;
    for (int dy = -1; dy <= 1; ++dy)
        for (int dx = -1; dx <= 1; ++dx)
            sum += image[y+dy][x+dx];
    return sum / 9.;
}

In general each program instance in the gang has different values for x and y in this function. For the box filtering algorithm here, all of the program instances will actually want to execute the same number of iterations of the for loops, with all of them having the same values for dx and dy each time through. If these loops are instead implemented with dx and dy declared as uniform variables, then the ispc compiler can generate more efficient code for the loops. [2]

[2]In this case, a sufficiently smart compiler could determine that dx and dy have the same value for all program instances and thus generate more optimized code from the start, though this optimization isn't yet implemented in ispc.
for (uniform int dy = -1; dy <= 1; ++dy)
    for (uniform int dx = -1; dx <= 1; ++dx)
        sum += image[y+dy][x+dx];

In particular, ispc can avoid the overhead of checking to see if any of the running program instances wants to do another loop iteration. Instead, the compiler can generate code where all instances always do the same iterations.

The analogous benefit comes when using if statements--if the test in an if statement is based on a uniform test, then the result will by definition be the same for all of the running program instances. Thus, the code for only one of the two cases needs to execute. ispc can generate code that jumps to one of the two, avoiding the overhead of needing to run the code for both cases.

Uniform Variables and Varying Control Flow

Recall that in the presence of varying control flow, both the "true" and "false" clauses of an if statement may be executed, with the side effects of the instructions masked so that they only apply to the program instances that are supposed to be executing the corresponding clause. Under this model, we must define the effect of modifying uniform variables in the context of varying control flow.

In general, modifying uniform variables under varying control flow leads to the uniform variable having a value that depends on whether any of the program instances in the gang followed a particular execution path. Consider the following example:

float a = ...;
uniform int b = 0;
if (a == 0) {
    ++b;
    // b is 1
}
else {
    b = 10;
    // b is 10
}
// whether b is 1 or 10 depends on whether any of the values
// of "a" in the executing gang were 0.

Here, if any of the values of a across the gang was non-zero, then b will have a value of 10 after the if statement has executed. However, if all of the values of a in the currently-executing program instances at the start of the if statement had a value of zero, then b would have a value of 1.

Data Races Within a Gang

In order to be able to write well-formed programs where program instances depend on values that are written to memory by other program instances within their gang, it's necessary to have a clear definition of when side-effects from one program instance become visible to other program instances running in the same gang.

In the model implemented by ispc, any side effect from one program instance is visible to other program instances in the gang after the next sequence point in the program. [3]

[3]This is a significant difference between ispc and SPMD languages like OpenCL* and CUDA*, which require barrier synchronization among the running program instances with functions like barrier() or __syncthreads(), respectively, to ensure this condition.

Generally, sequence points include the end of a full expression, before a function is entered in a function call, at function return, and at the end of initializer expressions. The fact that there is no sequence point between the increment of i and the assignment to i in i=i++ is why the effect that expression is undefined in C, for example. See, for example, the Wikipedia page on sequence points for more information about sequence points in C and C++.

In the following example, we have declared an array of values v, with one value for each running program instance. In the below, assume that programCount gives the gang size, and the varying integer value programIndex indexes into the running program instances starting from zero. (Thus, if 8 program instances are running, the first one of them will have a value 0, the next one a value of 1, and so forth up to 7.)

int x = ...;
uniform int tmp[programCount];
tmp[programIndex] = x;
int neighbor = tmp[(programIndex+1)%programCount];

In this code, the running program instances have written their values of x into the tmp array such that the ith element of tmp is equal to the value of x for the ith program instance. Then, the program instances load the value of neighbor from tmp, accessing the value written by their neighboring program instance (wrapping around to the first one at the end.) This code is well-defined and without data races, since the writes to and reads from tmp are separated by a sequence point.

(For this particular application of communicating values from one program instance to another, there are more efficient built-in functions in the ispc standard library; see Cross-Program Instance Operations for more information.)

It is possible to write code that has data races across the gang of program instances. For example, if the following function is called with multiple program instances having the same value of index, then it is undefined which of them will write their value of value to array[index].

void assign(uniform int array[], int index, int value) {
    array[index] = value;
}

As another example, if the values of the array indices i and j have the same values for some of the program instances, and an assignment like the following is performed:

int i = ..., j = ...;
uniform int array[...] = { ... };
array[i] = array[j];

then the program's behavior is undefined, since there is no sequence point between the reads and writes to the same location.

While this rule that says that program instances can safely depend on side-effects from by other program instances in their gang eliminates a class of synchronization requirements imposed by some other SPMD languages, it conversely means that it is possible to write ispc programs that compute different results when run with different gang sizes.

Tasking Model

ispc provides an asynchronous function call (i.e. tasking) mechanism through the launch keyword. (The syntax is documented in the Task Parallelism: "launch" and "sync" Statements section.) A function called with launch executes asynchronously from the function that called it; it may run immediately or it may run concurrently on another processor in the system, for example. (This model is closely modeled on the model introduced by Intel® Cilk(tm).)

If a function launches multiple tasks, there are no guarantees about the order in which the tasks will execute. Furthermore, multiple launched tasks from a single function may execute concurrently.

A function that has launched tasks may use the sync keyword to force synchronization with the launched functions; sync causes a function to wait for all of the tasks it has launched to finish before execution continues after the sync. (Note that sync only waits for the tasks launched by the current function, not tasks launched by other functions).

Alternatively, when a function that has launched tasks returns, an implicit sync waits for all launched tasks to finish before allowing the function to return to its calling function. This feature is important since it enables parallel composition: a function can call second function without needing to be concerned if the second function has launched asynchronous tasks or not--in either case, when the second function returns, the first function can trust that all of its computation has completed.

The ISPC Language

ispc is an extended version of the C programming language, providing a number of new features that make it easy to write high-performance SPMD programs for the CPU. Note that between not only the few small syntactic differences between ispc and C code but more importantly ispc's fundamentally parallel execution model, C code can't just be recompiled to correctly run in parallel with ispc. However, starting with working C code and porting it to ispc can be an efficient way to quickly write ispc programs.

This section describes the syntax and semantics of the ispc language. To understand how to use ispc, you need to understand both the language syntax and ispc's parallel execution model, which was described in the previous section, The ISPC Parallel Execution Model.

Relationship To The C Programming Language

This subsection summarizes the differences between ispc and C; if you are already familiar with C, you may find it most effective to focus on this subsection and just focus on the topics in the remainder of section that introduce new language features. You may also find it helpful to compare the ispc and C++ implementations of various algorithms in the ispc examples/ directory to get a sense of the close relationship between ispc and C.

Specifically, C89 is used as the baseline for comparison in this subsection (this is also the version of C described in the Second Edition of Kernighan and Ritchie's book). (ispc adopts some features from C99 and from C++, which will be highlighted in the below.)

ispc has the same syntax and features for the following as is present in C:

  • Expression syntax and basic types
  • Syntax for variable declarations
  • Control flow structures: if, for, while, do, and switch.
  • Pointers, including function pointers, void *, and C's array/pointer duality (arrays are converted to pointers when passed to functions, etc.)
  • Structs and arrays
  • Support for recursive function calls
  • Support for separate compilation of source files
  • "Short-circuit" evaluation of ||, && and ? : operators
  • The preprocessor

ispc adds a number of features from C++ and C99 to this base:

  • A boolean type, bool, as well as built-in true and false values
  • Reference types (e.g. const float &foo)
  • Comments delimited by //
  • Variables can be declared anywhere in blocks, not just at their start.
  • Iteration variables for for loops can be declared in the for statement itself (e.g. for (int i = 0; ...)
  • The inline qualifier to indicate that a function should be inlined
  • Function overloading by parameter type
  • Hexadecimal floating-point constants
  • Dynamic memory allocation with new and delete.
  • Limited support for overloaded operators (Operators Overloading).

ispc also adds a number of new features that aren't in C89, C99, or C++:

There are a number of features of C89 that are not supported in ispc but are likely to be supported in future releases:

  • There are no types named char, short, or long (or long double). However, there are built-in int8, int16, and int64 types
  • Character constants
  • String constants and arrays of characters as strings
  • goto statements are partially supported (see Unstructured Control Flow: "goto")
  • union types
  • Bitfield members of struct types
  • Variable numbers of arguments to functions
  • Literal floating-point constants (even without a f suffix) are currently treated as being float type, not double. To have a double precision floating point constant use d suffix.
  • The volatile qualifier
  • The register storage class for variables. (Will be ignored).

The following C89 features are not expected to be supported in any future ispc release:

  • "K&R" style function declarations
  • The C standard library
  • Octal integer constants

The following reserved words from C89 are also reserved in ispc:

break, case, const, continue, default, do, double, else, enum, extern, float, for, goto, if, int, NULL, return, signed, sizeof, static, struct, switch, typedef, unsigned, void, and while.

ispc additionally reserves the following words:

bool, delete, export, cdo, cfor, cif, cwhile, false, foreach, foreach_active, foreach_tiled, foreach_unique, in, inline, int8, int16, int32, int64, launch, new, print, soa, sync, task, true, uniform, and varying.

Lexical Structure

Tokens in ispc are delimited by white-space and comments. The white-space characters are the usual set of spaces, tabs, and carriage returns/line feeds. Comments can be delineated with //, which starts a comment that continues to the end of the line, or the start of a comment can be delineated with /* at the start and with */ at the end. Like C/C++, comments can't be nested.

Identifiers in ispc are sequences of characters that start with an underscore or an upper-case or lower-case letter, and then followed by zero or more letters, numbers, or underscores. Identifiers that start with two underscores are reserved for use by the compiler.

Integer numeric constants can be specified in base 10, hexadecimal, or binary. (Octal integer constants aren't supported). Base 10 constants are given by a sequence of one or more digits from 0 to 9. Hexadecimal constants are denoted by a leading 0x and then one or more digits from 0-9, a-f, or A-F. Finally, binary constants are denoted by a leading 0b and then a sequence of 1s and 0s.

Here are three ways of specifying the integer value "15":

int fifteen_decimal = 15;
int fifteen_hex     = 0xf;
int fifteen_binary  = 0b1111;

A number of suffixes can be provided with integer numeric constants. First, "u" denotes that the constant is unsigned, and "ll" denotes a 64-bit integer constant (while "l" denotes a 32-bit integer constant). It is also possible to denote units of 1024, 1024*1024, or 1024*1024*1024 with the SI-inspired suffixes "k", "M", and "G" respectively:

int two_kb = 2k;   // 2048
int two_megs = 2M; // 2 * 1024 * 1024
int one_gig = 1G;  // 1024 * 1024 * 1024

Floating-point constants can be specified in one of three ways. First, they may be a sequence of zero or more digits from 0 to 9, followed by a period, followed by zero or more digits from 0 to 9. (There must be at least one digit before or after the period).

The second option is scientific notation, where a base value is specified as the first form of a floating-point constant but is then followed by an "e" or "E", then a plus sign or a minus sign, and then an exponent.

Finally, floating-point constants may be specified as hexadecimal constants; this form can ensure a perfectly bit-accurate representation of a particular floating-point number. These are specified with an "0x" prefix, followed by a zero or a one, a period, and then the remainder of the mantissa in hexadecimal form, with digits from 0-9, a-f, or A-F. The start of the exponent is denoted by a "p", which is then followed by an optional plus or minus sign and then digits from 0 to 9. For example:

float two = 0x1p+1;  // 2.0
float pi  = 0x1.921fb54442d18p+1;  // 3.1415926535...
float neg = -0x1.ffep+11;  // -4095.

Floating-point constants can optionally have a "f" or "F" suffix (ispc currently treats all floating-point constants as having 32-bit precision, making this suffix not currently have an effect.)

String constants in ispc are denoted by an opening double quote " followed by any character other than a newline, up to a closing double quote. Within the string, a number of special escape sequences can be used to specify special characters. These sequences all start with an initial \ and are listed below:

Escape sequences in strings
\\ backslash: \
\" double quotation mark: "
\' single quotation mark: '
\a bell (alert)
\b backspace character
\f formfeed character
\n newline
\r carriage return
\t horizontal tab
\v vertical tab
\ followed by one or more digits from 0-8 ASCII character in octal notation
\x, followed by one or more digits from 0-9, a-f, A-F ASCII character in hexadecimal notation

ispc doesn't support a string data type; string constants can be passed as the first argument to the print() statement, however. ispc also doesn't support character constants.

The following identifiers are reserved as language keywords: bool, break, case, cdo, cfor, char, cif, cwhile, const, continue, default, do, double, else, enum, export, extern, false, float, for, foreach, foreach_active, foreach_tiled, foreach_unique, goto, if, in, inline, int, int8, int16, int32, int64, launch, NULL, print, return, signed, sizeof, soa, static, struct, switch, sync, task, true, typedef, uniform, union, unsigned, varying, void, volatile, while.

ispc defines the following operators and punctuation:

Operators
Symbols Use
= Assignment
+, -, *, /, % Arithmetic operators
&, |, ^, !, ~, &&, ||, <<, >> Logical and bitwise operators
++, -- Pre/post increment/decrement
<, <=, >, >=, ==, != Relational operators
*=, /=, +=, -=, <<=, >>=, &=, |= Compound assignment operators
?, : Selection operators
; Statement separator
, Expression separator
. Member access

A number of tokens are used for grouping in ispc:

Grouping Tokens
(, ) Parenthesization of expressions, function calls, delimiting specifiers for control flow constructs.
[, ] Array and short-vector indexing
{, } Compound statements

Types

Basic Types and Type Qualifiers

ispc is a statically-typed language. It supports a variety of core basic types:

  • void: "empty" type representing no value.
  • bool: boolean value; may be assigned true, false, or the value of a boolean expression.
  • int8: 8-bit signed integer.
  • unsigned int8: 8-bit unsigned integer.
  • int16: 16-bit signed integer.
  • unsigned int16: 16-bit unsigned integer.
  • int: 32-bit signed integer; may also be specified as int32.
  • unsigned int: 32-bit unsigned integer; may also be specified as unsigned int32.
  • float: 32-bit floating point value
  • int64: 64-bit signed integer.
  • unsigned int64: 64-bit unsigned integer.
  • double: 64-bit double-precision floating point value.

There are also a few built-in types related to pointers and memory:

  • size_t: the maximum size of any object (structure or array)
  • ptrdiff_t: an integer type large enough to represent the difference between two pointers
  • intptr_t: signed integer type that is large enough to represent a pointer value
  • uintptr_t: unsigned integer type large enough to represent a pointer

Implicit type conversion between values of different types is done automatically by the ispc compiler. Thus, a value of float type can be assigned to a variable of int type directly. In binary arithmetic expressions with mixed types, types are promoted to the "more general" of the two types, with the following precedence:

double > uint64 > int64 > float > uint32 > int32 >
    uint16 > int16 > uint8 > int8 > bool

In other words, adding an int64 to a double causes the int64 to be converted to a double, the addition to be performed, and a double value to be returned. If a different conversion behavior is desired, then explicit type-casts can be used, where the destination type is provided in parenthesis around the expression:

double foo = 1. / 3.;
int bar = (float)bar + (float)bar;  // 32-bit float addition

If a bool is converted to an integer numeric type (int, int64, etc.), then the result is the value one if the bool has the value true and has the value zero otherwise.

Variables can be declared with the const qualifier, which prohibits their modification.

const float PI = 3.1415926535;

As in C, the extern qualifier can be used to declare a function or global variable defined in another source file, and the static qualifier can be used to define a variable or function that is only visible in the current scope. The values of static variables declared in functions are preserved across function calls.

"uniform" and "varying" Qualifiers

If a variable has a uniform qualifier, then there is only a single instance of that variable shared by all program instances in a gang. (In other words, it necessarily has the same value across all of the program instances.) In addition to requiring less storage than varying values, uniform variables lead to a number of performance advantages when they are applicable (see Uniform Control Flow, for example.) Varying variables may be qualified with varying, though doing so has no effect, as varying is the default.

uniform variables can be modified as the program executes, but only in ways that preserve the property that they have a single value for the entire gang. Thus, it's legal to add two uniform variables together and assign the result to a uniform variable, but assigning a non-uniform (i.e., varying) value to a uniform variable is a compile-time error.

uniform variables implicitly type-convert to varying types as required:

uniform int x = ...;
int y = ...;
int z = x * y;  // x is converted to varying for the multiply

Arrays themselves aren't uniform or varying, but the elements that they store are:

float foo[10];
uniform float bar[10];

The first declaration corresponds to 10 gang-wide float values in memory, while the second declaration corresponds to 10 float values.

Defining New Names For Types

The typedef keyword can be used to name types:

typedef int64 BigInt;
typedef float Float3[3];

Following C's syntax, the code above defines BigInt to have int64 type and Float3 to have float[3] type.

Also as in C, typedef doesn't create a new type: it just provides an alternative name for an existing type. Thus, in the above example, it is legal to pass a value with float[3] type to a function that has been declared to take a Float3 parameter.

Pointer Types

It is possible to have pointers to data in memory; pointer arithmetic, changing values in memory with pointers, and so forth is supported as in C. As with other basic types, pointers can be both uniform and varying.

** Like other types in ispc, pointers are varying by default, if an explicit uniform qualifier isn't provided. However, the default variability of the pointed-to type is uniform. ** This rule will be illustrated and explained in examples below.

For example, the ptr variable in the code below is a varying pointer to uniform float values. Each program instance has a separate pointer value and the assignment to *ptr generally represents a scatter to memory.

uniform float a[] = ...;
int index = ...;
float * ptr = &a[index];
*ptr = 1;

A uniform pointer can be declared with an appropriately-placed qualifier:

float f = 0;
varying float * uniform pf = &f;  // uniform pointer to a varying float
*pf = 1;

The placement of the uniform qualifier to declare a uniform pointer may be initially surprising, but it matches the form of how, for example, a pointer that is itself const (as opposed to pointing to a const type) is declared in C. (Reading the declaration from right to left gives its meaning: a uniform pointer to a float that is varying.)

A subtlety comes in in cases like the where a uniform pointer points to a varying datatype. In this case, each program instance accesses a distinct location in memory (because the underlying varying datatype is itself laid out with a separate location in memory for each program instance.)

float a;
varying float * uniform pa = &a;
*pa = programIndex;  // same as (a = programIndex)

Also as in C, arrays are silently converted into pointers:

float a[10] = { ... };
varying float * uniform pa = a;     // pointer to first element of a
varying float * uniform pb = a + 5; // pointer to 5th element of a

Any pointer type can be explicitly typecast to another pointer type, as long as the source type isn't a varying pointer when the destination type is a uniform pointer.

float *pa = ...;
int *pb = (int *)pa;  // legal, but beware

Like other types, uniform pointers can be typecast to be varying pointers, however.

Any pointer type can be assigned to a void pointer without a type cast:

float foo(void *);
int *bar = ...;
foo(bar);

There is a special NULL value that corresponds to a NULL pointer. As a special case, the integer value zero can be implicitly converted to a NULL pointer and pointers are implicitly converted to boolean values in conditional expressions.

void foo(float *ptr) {
    if (ptr != 0) { // or, (ptr != NULL), or just (ptr)
       ...

It is legal to explicitly type-cast a pointer type to an integer type and back from an integer type to a pointer type. Note that this conversion isn't performed implicitly, for example for function calls.

Function Pointer Types

Pointers to functions can also be taken and used as in C and C++. The syntax for declaring function pointer types is the same as in those languages; it's generally easiest to use a typedef to help:

int inc(int v) { return v+1; }
int dec(int v) { return v-1; }

typedef int (*FPType)(int);
FPType fptr = inc;  // vs. int (*fptr)(int) = inc;

Given a function pointer, the function it points to can be called:

int x = fptr(1);

It's not necessary to take the address of a function to assign it to a function pointer or to dereference it to call the function.

As with pointers to data in ispc, function pointers can be either uniform or varying. A call through a uniform causes all of the running program instances in the gang to call into the target function; the implications of a call through a varying function pointer are discussed in the section Gang Convergence Guarantees.

Reference Types

ispc also provides reference types (like C++ references) that can be used for passing values to functions by reference, allowing functions can return multiple results or modify existing variables.

void increment(float &f) {
    ++f;
}

As in C++, once a reference is bound to a variable, it can't be rebound to a different variable:

float a = ..., b = ...;
float &r = a;  // makes r refer to a
r = b;  // assigns b to a, doesn't make r refer to b

An important limitation with references in ispc is that references can't be bound to varying lvalues; doing so causes a compile-time error to be issued. This situation is illustrated in the following code, where vptr is a varying pointer type (in other words, there each program instance in the gang has its own unique pointer value)

uniform float * uniform uptr = ...;
float &ra = *uptr;  // ok
uniform float * varying vptr = ...;
float &rb = *vptr;  // ERROR: *ptr is a varying lvalue type

(The rationale for this limitation is that references must be represented as either a uniform pointer or a varying pointer internally. While choosing a varying pointer would provide maximum flexibility and eliminate this restriction, it would reduce performance in the common case where a uniform pointer is all that's needed. As a work-around, a varying pointer can be used in cases where a varying lvalue reference would be desired.)

Enumeration Types

It is possible to define user-defined enumeration types in ispc with the enum keyword, which is followed by an optional enumeration type name and then a brace-delimited list of enumerators with optional values:

enum Color { RED, GREEN, BLUE };
enum Flags {
    UNINITIALIZED = 0,
    INITIALIZED = 2,
    CACHED = 4
};

Each enum declaration defines a new type; an attempt to implicitly convert between enumerations of different types gives a compile-time error, but enumerations of different types can be explicitly cast to one other.

Color c = (Color)CACHED;

Enumerators are implicitly converted to integer types, however, so they can be directly passed to routines that take integer parameters and can be used in expressions including integers, for example. However, the integer result of such an expression must be explicitly cast back to the enumerant type if it to be assigned to a variable with the enumerant type.

Color c = RED;
int nextColor = c+1;
c = (Color)nextColor;

In this particular case, the explicit cast could be avoided using an increment operator.

Color c = RED;
++c;  // c == GREEN now

Short Vector Types

ispc supports a parameterized type to define short vectors. These short vectors can only be used with basic types like float and int; they can't be applied to arrays or structures. Note: ispc does not use these short vectors to facilitate program vectorization; they are purely a syntactic convenience. Using them or writing the corresponding code without them shouldn't lead to any noticeable performance differences between the two approaches.

Syntax similar to C++ templates is used to declare these types:

float<3> foo;   // vector of three floats
double<6> bar;

The length of these vectors can be arbitrarily long, though the expected usage model is relatively short vectors.

You can use typedef to create types that don't carry around the brackets around the vector length:

typedef float<3> float3;

ispc doesn't support templates in general. In particular, not only must the vector length be a compile-time constant, but it's also not possible to write functions that are parameterized by vector length.

uniform int i = foo();
// ERROR: length must be compile-time constant
float<i> vec;
// ERROR: can't write functions parameterized by vector length
float<N> func(float<N> val);

Arithmetic on these short vector types works as one would expect; the operation is applied component-wise to the values in the vector. Here is a short example:

float<3> func(float<3> a, float<3> b) {
    a += b;    // add individual elements of a and b
    a *= 2.;   // multiply all elements of a by 2
    bool<3> test = a < b;  // component-wise comparison
    return test ? a : b;   // return each minimum component
}

As shown by the above code, scalar types automatically convert to corresponding vector types when used in vector expressions. In this example, the constant 2. above is converted to a three-vector of 2s for the multiply in the second line of the function implementation.

Type conversion between other short vector types also works as one would expect, though the two vector types must have the same length:

float<3> foo = ...;
int<3> bar = foo;    // ok, cast elements to ints
int<4> bat = foo;    // ERROR: different vector lengths
float<4> bing = foo; // ERROR: different vector lengths

For convenience, short vectors can be initialized with a list of individual element values:

float x = ..., y = ..., z = ...;
float<3> pos = { x, y, z };

There are two mechanisms to access the individual elements of these short vector data types. The first is with the array indexing operator:

float<4> foo;
for (uniform int i = 0; i < 4; ++i)
    foo[i] = i;

ispc also provides a specialized mechanism for naming and accessing the first few elements of short vectors based on an overloading of the structure member access operator. The syntax is similar to that used in HLSL, for example.

float<3> position;
position.x = ...;
position.y = ...;
position.z = ...;

More specifically, the first element of any short vector type can be accessed with .x or .r, the second with .y or .g, the third with .z or .b, and the fourth with .w or .a. Just like using the array indexing operator with an index that is greater than the vector size, accessing an element that is beyond the vector's size is undefined behavior and may cause your program to crash.

It is also possible to construct new short vectors from other short vector values using this syntax, extended for "swizzling". For example,

float<3> position = ...;
float<3> new_pos = position.zyx;  // reverse order of components
float<2> pos_2d = position.xy;

Though a single element can be assigned to, as in the examples above, it is not currently possible to use swizzles on the left-hand side of assignment expressions:

int8<2> foo = ...;
int8<2> bar = ...;
foo.yz = bar;   // Error: can't assign to left-hand side of expression

Array Types

Arrays of any type can be declared just as in C and C++:

float a[10];
uniform int * varying b[20];

Multidimensional arrays can be specified as arrays of arrays; the following declares an array of 5 arrays of 15 floats.

float a[5][15];

The size of arrays must be a compile-time constant, though array size can be determined from array initializer lists; see the following section, Declarations and Initializers, for details. One exception to this is that functions can be declared to take "unsized arrays" as parameters:

void foo(float array[], int length);

Finally, the name of an array will be automatically implicitly converted to a uniform pointer to the array type if needed:

int a[10];
int * uniform ap = a;

Struct Types

Aggregate data structures can be built using struct.

struct Foo {
    float time;
    int flags[10];
};

As in C++, after a struct is declared, an instance can be created using the struct's name:

Foo f;

Alternatively, struct can be used before the structure name:

struct Foo f;

Members in a structure declaration may each have uniform or varying qualifiers, or may have no rate qualifier, in which case their variability is initially "unbound".

struct Bar {
    uniform int a;
    varying int b;
    int c;
};

In the declaration above, the variability of c is unbound. The variability of struct members that are unbound is resolved when a struct is defined; if the struct is uniform, then unbound members are uniform, and if the struct is varying, then unbound members are varying.

Bar vb;
uniform Bar ub;

Here, b is a varying Bar (since varying is the default variability). If Bar is defined as above, then vb.a is still a uniform int, since its varaibility was bound in the original declaration of the Bar type. Similarly, vb.b is varying. The variability of vb.c is varying, since vb is varying.

(Similarly, ub.a is uniform, ub.b is varying, and ub.c is uniform.)

In most cases, it's worthwhile to declare struct members with unbound variability so that all have the same variability for both uniform and varying structs. In particular, if a struct has a member with bound uniform type, it's not possible to index into an array of the struct type with a varying index. Consider the following example:

struct Foo { uniform int a; };
uniform Foo f[...] = ...;
int index = ...;
Foo fv = f[index];  // ERROR

Here, the Foo type has a member with bound uniform variability. Because index has a different value for each program instance in the above code, the value of f[index] needs to be able to store a different value of Foo::a for each program instance. However, a varying Foo still has only a single a member, since a was declared with uniform variability in the declaration of Foo. Therefore, the indexing operation in the last line results in an error.

Operators Overloading

ISPC has limited support for overloaded operators for struct types. Only binary operators are supported currently, namely they are: *, /, %, +, -, >> and <<. Operators overloading support is similar to the one in C++ language. To overload an operator for struct S, you need to declare and implement a function using keyword operator, which accepts two parameters of type struct S or struct S& and returns either of these types. For example:

struct S { float re, im;};
struct S operator*(struct S a, struct S b) {
    struct S result;
    result.re = a.re * b.re - a.im * b.im;
    result.im = a.re * b.im + a.im * b.re;
    return result;
}

void foo(struct S a, struct S b) {
    struct S mul = a*b;
    print("a.re:   %\na.im:   %\n", a.re, a.im);
    print("b.re:   %\nb.im:   %\n", b.re, b.im);
    print("mul.re: %\nmul.im: %\n", mul.re, mul.im);
}

Structure of Array Types

If data can be laid out in memory so that the executing program instances access it via loads and stores of contiguous sections of memory, overall performance can be improved noticably. One way to improve this memory access coherence is to lay out structures in "structure of arrays" (SOA) format in memory; the benefits from SOA layout are discussed in more detail in the Use "Structure of Arrays" Layout When Possible section in the ispc Performance Guide.

ispc provides two key language-level capabilities for laying out and accessing data in SOA format:

  • An soa keyword that transforms a regular struct into an SOA version of the struct.
  • Array indexing syntax for SOA arrays that transparently handles SOA indexing.

As an example, consider a simple struct declaration:

struct Point { float x, y, z; };

With the soa rate qualifier, an array of SOA variants of this structure can be declared:

soa<8> Point pts[...];

The in-memory layout of the Point instances has had the SOA transformation applied, such that there are 8 x values in memory followed by 8 y values, and so forth. Here is the effective declaration of soa<8> Point:

struct { uniform float x[8], y[8], z[8]; };

Given an array of SOA data, array indexing (and pointer arithmetic) is done so that the appropriate values from the SOA array are accessed. For example, given:

soa<8> Point pts[...];
uniform float x = pts[10].x;

The generated code effectively accesses the second 8-wide SOA structure and then loads the third x value from it. In general, one can write the same code to access arrays of SOA elements as one would write to access them in AOS layout.

Note that it directly follows from SOA layout that the layout of a single element of the array isn't contiguous in memory--pts[1].x and pts[1].y are separated by 7 float values in the above example.

There are a few limitations to the current implementation of SOA types in ispc; these may be relaxed in future releases:

  • It's illegal to typecast to soa data to void pointers.
  • Reference types are illegal in SOA structures
  • All members of SOA structures must have no rate qualifiers--specifically, it's illegal to have an explicitly-qualified uniform or varying member of a structure that has soa applied to it.

Declarations and Initializers

Variables are declared and assigned just as in C:

float foo = 0, bar[5];
float bat = func(foo);

More complex declarations are also possible:

void (*fptr_array[16])(int, int);

Here, fptr_array is an array of 16 pointers to functions that have void return type and take two int parameters.

If a variable is declared without an initializer expression, then its value is undefined until a value is assigned to it. Reading an undefined variable is undefined behavior.

Any variable that is declared at file scope (i.e. outside a function) is a global variable. If a global variable is qualified with the static keyword, then its only visible within the compilation unit in which it was defined. As in C/C++, a variable with a static qualifier inside a functions maintains its value across function invocations.

As in C++, variables don't need to be declared at the start of a basic block:

int foo = ...;
if (foo < 2) { ... }
int bar = ...;

Variables can also be declared in for statement initializers:

for (int i = 0; ...)

Arrays can be initialized with individual element values in braces:

int bar[2][4] = { { 1, 2, 3, 4 }, { 5, 6, 7, 8 } };

An array with an initializer expression can be declared with some or all of its dimensions unspecified. In this case, the "shape" of the initializer expression is used to determine the array dimensions:

// This corresponds to bar[2][4], due to the initializer expression
int bar[][] = { { 1, 2, 3, 4 }, { 5, 6, 7, 8 } };

Structures can also be initialized by providing element values in braces:

struct Color { float r, g, b; };
....
Color d = { 0.5, .75, 1.0 }; // r = 0.5, ...

Arrays of structures and arrays inside structures can be initialized with the expected syntax:

struct Foo { int x; float bar[3]; };
Foo fa[2] = { { 1, { 2, 3, 4 } }, { 10, { 20, 30, 40 } } };
// now, fa[1].bar[2] == 30, and so forth

Expressions

All of the operators from C that you'd expect for writing expressions are present. Rather than enumerating all of them, here is a short summary of the range of them available in action.

unsigned int i = 0x1234feed;
unsigned int j = (i << 3) ^ ~(i - 3);
i += j / 6;
float f = 1.234e+23;
float g = j * f / (2.f * i);
double h = (g < 2) ? f : g/5;

Structure member access and array indexing also work as in C.

struct Foo { float f[5]; int i; };
Foo foo = { { 1,2,3,4,5 }, 2 };
return foo.f[4] - foo.i;

The address-of operator, pointer dereference operator, and pointer member operator also work as expected.

struct Foo { float a, b, c; };
Foo f;
Foo * uniform fp = &f;
(*fp).a = 0;
fp->b = 1;

As in C and C++, evaluation of the || and && logical operators as well as the selection operator ? : is "short-circuited"; the right hand side won't be evaluated if the value from the left-hand side determines the logical operator's value. For example, in the following code, array[index] won't be evaluated for values of index that are greater than or equal to NUM_ITEMS.

if (index < NUM_ITEMS && array[index] > 0) {
    // ...
}

Short-circuiting may impose some overhead in the generated code; for cases where short-circuiting is undesirable due to performance impact, see the section Logical and Selection Operations, which introduces helper functions in the standard library that provide these operations without short-circuiting.

Dynamic Memory Allocation

ispc programs can dynamically allocate (and free) memory, using syntax based on C++'s new and delete operators:

int count = ...;
int *ptr = new int[count];
// use ptr...
delete[] ptr;

In the above code, each program instance allocates its own count sized array of uniform int values, uses that memory, and then deallocates that memory. Uses of new and delete in ispc programs are implemented as calls to C library's aligned memory allocation routines, which are platform dependent (posix_memalign() and free() on Linux and Mac and _aligned_malloc() and _aligned_free() on Windows). So it's advised to pair ISPC's new and delete with each other, but not with C/C++ memory management functions.

Note that the rules for uniform and varying for new are analogous to the corresponding rules for pointers (as described in Pointer Types). Specifically, if a specific rate qualifier isn't provided with the new expression, then the default is that a "varying" new is performed, where each program instance performs a unique allocation. The allocated type, in turn, is by default uniform.

After a pointer has been deleted, it is illegal to access the memory it points to. However, that deletion happens on a per-program-instance basis. In other words, consider the following code:

int *ptr = new int[count];
// use ptr
if (count > 1000)
    delete[] ptr;
// ...

Here, the program instances where count is greater than 1000 have deleted the dynamically allocated memory pointed to by ptr, but the other program instances have not. As such, it's illegal for the former set of program instances to access *ptr, but it's perfectly fine for the latter set to continue to use the memory ptr points to. Note that it is illegal to delete a pointer value returned by new more than one time.

Sometimes, it's useful to be able to do a single allocation for the entire gang of program instances. A new statement can be qualified with uniform to indicate a single memory allocation:

float * uniform ptr = uniform new float[10];

While a regular call to new returns a varying pointer (i.e. a distinct pointer to separately-allocated memory for each program instance), a uniform new performs a single allocation and returns a uniform pointer. Recall that with a uniform new, the default variability of the allocated type is varying, so the above code is allocating an array of ten varying float values.

When using uniform new, it's important to be aware of a subtlety; if the returned pointer is stored in a varying pointer variable (as may be appropriate and useful for the particular program being written), then the varying pointer may inadvertently be passed to a subsequent delete statement, which is an error: effectively

varying float * ptr = uniform new float[10];
// use ptr...
delete ptr;  // ERROR: varying pointer is deleted

In this case, ptr will be deleted multiple times, once for each executing program instance, which is an error (unless it happens that only a single program instance is active in the above code.)

When using new statements, it's important to make an appropriate choice of uniform or varying, for both the new operator itself as well as the type of data being allocated, based on the program's needs. Consider the following four memory allocations:

uniform float * uniform p1 = uniform new uniform float[10];
float * uniform p2 = uniform new float[10];
float * p3 = new float[10];
varying float * p4 = new varying float[10];

Assuming that a float is 4 bytes in memory and if the gang size is 8 program instances, then the first allocation represents a single allocation of 10 uniform float values (40 bytes), the second is a single allocation of 10 varying float values (8*4*10 = 320 bytes), the third is 8 allocations of 10 uniform float values (8 allocations of 40 bytes each), and the last performs 8 allocations of 320 bytes each.

Note in particular that varying allocations of varying data types are rarely desirable in practice. In that case, each program instance is performing a separate allocation of varying float memory. In this case, it's likely that the program instances will only access a single element of each varying float, which is wasteful. (This in turn is partially why the allocated type is uniform by default with both pointers and new statements.)

Although ispc doesn't support constructors or destructors like C++, it is possible to provide initializer values with new statements:

struct Point { float x, y, z; };
Point *pptr = new Point(10, 20, 30);

Here for example, the "x" element of the returned Point is initialized to have the value 10 and so forth. In general, the rules for how initializer values provided in new statements are used to initialize complex data types follow the same rules as initializers for variables described in Declarations and Initializers.

Control Flow

ispc supports most of C's control flow constructs, including if, switch, for, while, do. It has limited support for goto, detailed below. It also supports variants of C's control flow constructs that provide hints about the expected runtime coherence of the control flow at that statement. It also provides parallel looping constructs, foreach and foreach_tiled, all of which will be detailed in this section.

Conditional Statements: "if"

The if statement behaves precisely as in C; the code in the "true" block only executes if the condition evaluates to true, and if an optional else clause is provided, the code in the "else" block only executes if the condition is false.

float x = ..., y = ...;
if (x < 0.)
    y = -y;
else
    x *= 2.;

Conditional Statements: "switch"

The switch conditional statement is also available, again with the same behavior as in C; the expression used in the switch must be of integer type (but it can be uniform or varying). As in C, if there is no break statement at the end of the code for a given case, execution "falls through" to the following case. These features are demonstrated in the code below.

int x = ...;
switch (x) {
case 0:
case 1:
    foo(x);
    /* fall through */
case 5:
    x = 0;
    break;
default:
    x *= x;
}

Iteration Statements

In addition to the standard iteration statements for, while, and do, inherited from C/C++, ispc provides a number of additional specialized ways to iterate over data.

Basic Iteration Statements: "for", "while", and "do"

ispc supports for, while, and do loops, with the same specification as in C. As in C++, variables can be declared in the for statement itself:

for (uniform int i = 0; i < 10; ++i) {
  // loop body
}
// i is now no longer in scope

You can use break and continue statements in for, while, and do loops; break breaks out of the current enclosing loop, while continue has the effect of skipping the remainder of the loop body and jumping to the loop step.

Note that all of these looping constructs have the effect of executing independently for each of the program instances in a gang; for example, if one of them executes a continue statement, other program instances executing code in the loop body that didn't execute the continue will be unaffected by it.

Iteration over active program instances: "foreach_active"

The foreach_active construct specifies a loop that serializes over the active program instances: the loop body executes once for each active program instance, and with only that program instance executing.

As an example of the use of this construct, consider an application where each program instance independently computes an offset into a shared array that is being updated:

uniform float array[...] = { ... };
int index = ...;
++array[index];

If more than one active program instance computes the same value for index, the above code has undefined behavior (see the section Data Races Within a Gang for details.) The increment of array[index] could instead be written inside a foreach_active statement:

foreach_active (i) {
    ++array[index];
}

The variable name provided in parenthesis after the foreach_active keyword (here, index), causes a const uniform int64 local variable of that name to be declared, where the variable takes the programIndex value of the program instance executing at each loop iteration.

In the code above, because only one program instance is executing at a time when the loop body executes, the update to array is well-defined. Note that for this particular example, the "local atomic" operations in the standard library could be used instead to safely update array. However, local atomics functions aren't always available or appropriate for more complex cases.)

continue statements may be used inside foreach_active loops, though break and return are prohibited. The order in which the active program instances are processed in the loop is not defined.

See the Using "foreach_active" Effectively Section in the ispc Performance Guide for more details about foreach_active.

Iteration over unique elements: "foreach_unique"

It can be useful to iterate over the elements of a varying variable, processing the subsets of them that have the same value together. For example, consider a varying variable x that has the values {1, 2, 2, 1, 1, 0, 0, 0}, where the program is running on a target with a gang size of 8 program instances. Here, x has three unique values across the program instances: 0, 1, and 2.

The foreach_unique looping construct allows us to iterate over these unique values. In the code below, the foreach_unique loop body executes once for each of the three unique values, with execution mask set to match the program instances where the varying value matches the current unique value being processed.

int x = ...; // assume {1, 2, 2, 1, 1, 0, 0, 0}
foreach_unique (val in x) {
    extern void func(uniform int v);
    func(val);
}

In the above, func() will be called three times, once with value 0, once with value 1, and once with value 2. When it is called for value 0, only the last three program instances will be executing, and so forth. The order in which the loop executes for the unique values isn't defined.

The varying expression that provides the values to be iterated over is only evaluated once, and it must be of an atomic type (float, int, etc.), an enum type, or a pointer type. The iteration variable val is a variable of const uniform type of the iteration type; it can't be modified within the loop. Finally, break and return statements are illegal within the loop body, but continue statements are allowed.

Parallel Iteration Statements: "foreach" and "foreach_tiled"

The foreach and foreach_tiled constructs specify loops over a possibly multi-dimensional domain of integer ranges. Their role goes beyond "syntactic sugar"; they provide one of the two key ways of expressing parallel computation in ispc.

In general, a foreach or foreach_tiled statement takes one or more dimension specifiers separated by commas, where each dimension is specified by identifier = start ... end, where start is a signed integer value less than or equal to end, specifying iteration over all integer values from start up to and including end-1. An arbitrary number of iteration dimensions may be specified, with each one spanning a different range of values. Within the foreach loop, the given identifiers are available as const varying int32 variables. The execution mask starts out "all on" at the start of each foreach loop iteration, but may be changed by control flow constructs within the loop.

It is illegal to have a break statement or a return statement within a foreach loop; a compile-time error will be issued in this case. (It is legal to have a break in a regular for loop that's nested inside a foreach loop.) continue statements are legal in foreach loops; they have the same effect as in regular for loops: a program instances that executes a continue statement effectively skips over the rest of the loop body for the current iteration.

It is also currently illegal to have nested foreach statements; this limitation will be removed in a future release of ispc.

As a specific example, consider the following foreach statement:

foreach (j = 0 ... height, i = 0 ... width) {
    // loop body--process data element (i,j)
}

It specifies a loop over a 2D domain, where the j variable goes from 0 to height-1 and i goes from 0 to width-1. Within the loop, the variables i and j are available and initialized accordingly.

foreach loops actually cause the given iteration domain to be automatically mapped to the program instances in the gang, so that all of the data can be processed, in gang-sized chunks. As a specific example, consider a simple foreach loop like the following, on a target where the gang size is 8:

foreach (i = 0 ... 16) {
    // perform computation on element i
}

One possible valid execution path of this loop would be for the program counter the step through the statements of this loop just 16/8==2 times; the first time through, with the varying int32 variable i having the values (0,1,2,3,4,5,6,7) over the program instances, and the second time through, having the values (8,9,10,11,12,13,14,15), thus mapping the available program instances to all of the data by the end of the loop's execution.

In general, however, you shouldn't make any assumptions about the order in which elements of the iteration domain will be processed by a foreach loop. For example, the following code exhibits undefined behavior:

uniform float a[10][100];
foreach (i = 0 ... 10, j = 0 ... 100) {
    if (i == 0)
        a[i][j] = j;
    else
        // Error: can't assume that a[i-1][j] has been set yet
        a[i][j] = a[i-1][j];

The foreach statement generally subdivides the iteration domain by selecting sets of contiguous elements in the inner-most dimension of the iteration domain. This decomposition approach generally leads to coherent memory reads and writes, but may lead to worse control flow coherence than other decompositions.

Therefore, foreach_tiled decomposes the iteration domain in a way that tries to map locations in the domain to program instances in a way that is compact across all of the dimensions. For example, on a target with an 8-wide gang size, the following foreach_tiled statement might process the iteration domain in chunks of 2 elements in j and 4 elements in i each time. (The trade-offs between these two constructs are discussed in more detail in the ispc Performance Guide.)

foreach_tiled (j = 0 ... height, i = 0 ... width) {
    // loop body--process data element (i,j)
}

Parallel Iteration with "programIndex" and "programCount"

In addition to foreach and foreach_tiled, ispc provides a lower-level mechanism for mapping SPMD program instances to data to operate on via the built-in programIndex and programCount variables.

programIndex gives the index of the SIMD-lane being used for running each program instance. (In other words, it's a varying integer value that has value zero for the first program instance, and so forth.) The programCount builtin gives the total number of instances in the gang. Together, these can be used to uniquely map executing program instances to input data. [4]

[4]programIndex is analogous to get_global_id() in OpenCL* and threadIdx in CUDA*.

As a specific example, consider an ispc function that needs to perform some computation on an array of data.

for (uniform int i = 0; i < count; i += programCount) {
    float d = data[i + programIndex];
    float r = ....
    result[i + programIndex] = r;
}

Here, we've written a loop that explicitly loops over the data in chunks of programCount elements. In each loop iteration, the running program instances effectively collude amongst themselves using programIndex to determine which elements to work on in a way that ensures that all of the data elements will be processed. In this particular case, a foreach loop would be preferable, as foreach naturally handles the case where programCount doesn't evenly divide the number of elements to be processed, while the loop above assumes that case implicitly.

Unstructured Control Flow: "goto"

goto statements are allowed in ispc programs under limited circumstances; specifically, only when the compiler can determine that if any program instance executes a goto statement, then all of the program instances will be running at that statement, such that all will follow the goto.

Put another way: it's illegal for there to be "varying" control flow statements in scopes that enclose a goto statement. An error is issued if a goto is used in this situation.

The syntax for adding labels to ispc programs and jumping to them with goto is the same as in C. The following code shows a goto based equivalent of a for loop where the induction variable i goes from zero to ten.

  uniform int i = 0;
check:
  if (i > 10)
      goto done;
  // loop body
  ++i;
  goto check;
done:
  // ...

"Coherent" Control Flow Statements: "cif" and Friends

ispc provides variants of all of the standard control flow constructs that allow you to supply a hint that control flow is expected to be coherent at a particular point in the program's execution. These mechanisms provide the compiler a hint that it's worth emitting extra code to check to see if the control flow is in fact coherent at run-time, in which case a simpler code path can often be executed.

The first of these statements is cif, indicating an if statement that is expected to be coherent. The usage of cif in code is just the same as if:

cif (x < y) {
    ...
} else {
    ...
}

cif provides a hint to the compiler that you expect that most of the executing SPMD programs will all have the same result for the if condition.

Along similar lines, cfor, cdo, and cwhile check to see if all program instances are running at the start of each loop iteration; if so, they can run a specialized code path that has been optimized for the "all on" execution mask case.

Functions and Function Calls

Like C, functions must be declared in ispc before they are called, though a forward declaration can be used before the actual function definition. Also like C, arrays are passed to functions by reference. Recursive function calls are legal:

int gcd(int a, int b) {
    if (a == 0)
        return b;
    else
        return gcd(b%a, a);
}

Functions can be declared with a number of qualifiers that affect their visibility and capabilities. As in C/C++, functions have global visibility by default. If a function is declared with a static qualifier, then it is only visible in the file in which it was declared.

static void lerp(float t, float a, float b) {
    return (1.-t)*a + t*b;
}

Any function that can be launched with the launch construct in ispc must have a task qualifier; see Task Parallelism: "launch" and "sync" Statements for more discussion of launching tasks in ispc.

A function can also be given the unmasked qualifier; this qualifier indicates that all program instances should be made active at the start of the function execution (or, equivalently, that the current execution mask shouldn't be passed to the function from the function call site.) If it is known that a function will always be called when all program instances are executing, adding this qualifier can slightly improve performance. See the Section Re-establishing The Execution Mask for more discussion of unmasked program code.

Functions that are intended to be called from C/C++ application code must have the export qualifier. This causes them to have regular C linkage and to have their declarations included in header files, if the ispc compiler is directed to generated a C/C++ header file for the file it compiled.

export uniform float inc(uniform float v) {
    return v+1;
}

Finally, any function defined with an inline qualifier will always be inlined by ispc; inline is not a hint, but forces inlining. The compiler will opportunistically inline short functions depending on their complexity, but any function that should always be inlined should have the inline qualifier.

Function Overloading

Functions can be overloaded by parameter type. Given multiple definitions of a function, ispc uses the following model to choose the best function: each conversion of two types has its cost. ispc tries to find conversion with the smallest cost. When ispc can't find any conversion it means that this function is not suitable. Then ispc sums costs for all arguments and chooses the function with the smallest final cost. If the chosen function has some arguments which costs are bigger than their costs in other function this treats as ambiguous. Costs of type conversions placed from small to big:

  1. Parameter types match exactly.
  2. Function parameter type is reference and parameters match when any reference-type parameter are considered equivalent to their underlying type.
  3. Function parameter type is const-reference and parameters match when any reference-type parameter are considered equivalent to their underlying type ignoring const attributes.
  4. Parameters match exactly, except constant attributes. [NO CONSTANT ATTRIBUTES LATER]
  5. Parameters match exactly, except reference attributes. [NO REFERENCES ATTRIBUTES LATER]
  6. Parameters match with only type conversions that don't risk losing any information (for example, converting an int16 value to an int32 parameter value.)
  7. Parameters match with only promotions from uniform to varying types.
  8. Parameters match using arbitrary type conversion, without changing variability from uniform to varying (e.g., int to float, float to int.)
  9. Parameters match with widening and promotions from uniform to varying types. (combination of "6" and "7")
  10. Parameters match using arbitrary type conversion, including also changing variability from uniform to varying.
  • If function parameter type is reference and neither "2" nor "3" aren't suitable, function is not suitable
  • If "10" isn't suitable, function is not suitable

Re-establishing The Execution Mask

As discussed in Functions and Function Calls, a function that is declared with an unmasked qualifier starts execution with all program instances running, regardless of the execution mask at the site of the function call. A block of statements can also be enclosed with unmasked to have the same effect within a function:

int a = ..., b = ...;
if (a < b) {
    // only program instances where a < b are executing here
    unmasked {
        // now all program instances are executing
    }
    // and again only the a < b instances
}

unmasked can be useful in cases where the programmer wants to "change the axis of parallelism" or use nested parallelism, as shown in the following code:

uniform WorkItem items[...] = ...;
foreach (itemNum = 0 ... numItems) {
    // do computation on items[itemNum] to determine if it needs
    // further processing...
    if (/* itemNum needs processing */) {
        foreach_active (i) {
            unmasked {
                uniform int uItemNum = extract(itemNum, i);
                // apply entire gang of program instances to uItemNum
            }
        }
    }
}

The general idea is that we are first using SPMD parallelism to determine which of the items requires further processing, checking a gang's worth of them concurrently inside the foreach loop. Assuming that only a subset of them needs further processing, would be wasteful to do this work within the foreach loop in the same program instance that made the initial determination of whether more work as needed; in this case, all of the program instances corresponding to items that didn't need further processing would be inactive, with corresponding unused computational capability in the system.

In the above code, this issue is avoided by working on each of the items requiring more processing in turn with foreach_active and then using unmasked to re-establish execution of all of the program instances. The entire gang can in turn be applied to the computation to be done for each items[itemNum].

The unmasked statement should be used with care; it can lead to a number of surprising cases of undefined program behavior. For example, consider the following code:

void func(float);
float a = ...;
float b;
if (a < 0) {
    b = 0;
    unmasked {
        if (b == 0)
            func(a);
    }
}

The variable a is initialized to some value and b is declared but not initialized, and thus has an undefined value. Within the if test, we have assigned zero to b, though only for the program instances currently executing--i.e. those where a < 0. After re-establishing the executing mask with unmasked, we then compare b to zero--this comparison is well-defined (and "true") for the program instances where a < 0, but it is undefined for any program instances where that isn't the case, since the value of b is undefined for those program instances. Similar surprising cases can arise when writing to varying variables within unmasked code.

As a general rule, code within an unmasked block, or a function with the unmasked qualifier should use great care when accessing varying variables that were declared in an outer scope.

Task Parallel Execution

In addition to the facilities for using SPMD for parallelism across the SIMD lanes of one processing core, ispc also provides facilities for parallel execution across multiple cores though an asynchronous function call mechanism via the launch keyword. A function called with launch executes as an asynchronous task, often on another core in the system.

Task Parallelism: "launch" and "sync" Statements

One option for combining task-parallelism with ispc is to just use regular task parallelism in the C/C++ application code (be it through Intel® Cilk(tm), Intel® Thread Building Blocks or another task system), and for tasks to use ispc for SPMD parallelism across the vector lanes as appropriate. Alternatively, ispc also has support for launching tasks from ispc code. The approach is similar to Intel® Cilk's task launch feature. (Check the examples/mandelbrot_tasks example to see how it is used.)

Any function that is launched as a task must be declared with the task qualifier:

task void func(uniform float a[], uniform int index) {
    ...
    a[index] = ....
}

Tasks must return void; a compile time error is issued if a non-void task is defined.

Given a task declaration, a task can be launched with launch:

uniform float a[...] = ...;
launch func(a, 1);

Program execution continues asynchronously after a launch statement in a function; thus, a function shouldn't access values written by a task it has launched within the function without synchronization. A function can use a sync statement to wait for all launched tasks to finish:

launch func(a, 1);
sync;
// now safe to use computed values in a[]...

Alternatively, any function that launches tasks has an automatically-added implicit sync statement before it returns, so that functions that call a function that launches tasks don't have to worry about outstanding asynchronous computation from that function.

The task generated by a launch statement is a single gang's worth of work. The same program instances are respectively active and inactive at the start of the task as were active and inactive when their launch statement executed. To make all program instances in the launched gang be active, the unmasked construct can be used (see Re-establishing The Execution Mask.)

There are two ways to write code that launches a group multiple tasks. First, one task can be launched at a time, with parameters passed to the task to help it determine what part of the overall computation it's responsible for:

for (uniform int i = 0; i < 100; ++i)
    launch func(a, i);

This code launches 100 tasks, each of which presumably does some computation that is keyed off of given the value i. In general, one should launch many more tasks than there are processors in the system to ensure good load-balancing, but not so many that the overhead of scheduling and running tasks dominates the computation.

Alternatively, a number of tasks may be launched from a single launch statement. We might instead write the above example with a single launch like this:

launch[100] func2(a);

Where an integer value (not necessarily a compile-time constant) is provided to the launch keyword in square brackets; this number of tasks will be enqueued to be run asynchronously. Within each of the tasks, two special built-in variables are available--taskIndex, and taskCount. The first, taskIndex, ranges from zero to one minus the number of tasks provided to launch, and taskCount equals the number of launched tasks. Thus, in this example we might use taskIndex in the implementation of func2 to determine which array element to process.

task void func2(uniform float a[]) {
    ...
    a[taskIndex] = ...
}

Inside functions with the task qualifier, two additional built-in variables are provided in addition to taskIndex and taskCount: threadIndex and threadCount. threadCount gives the total number of hardware threads that have been launched by the task system. threadIndex provides an index between zero and threadCount-1 that gives a unique index that corresponds to the hardware thread that is executing the current task. The threadIndex can be used for accessing data that is private to the current thread and thus doesn't require synchronization to access under parallel execution.

The tasking system also supports multi-dimensional partitioning (currently up to three dimensions). To launch a 3D grid of tasks, for example with N0, N1 and N2 tasks in x-, y- and z-dimension respectively

float data[N2][N1][N0]
task void foo_task()
{
   data[taskIndex2][taskIndex1][threadIndex0] = taskIndex;
}

we use the following launch expressions:

launch [N2][N1][N0] foo_task()

or

launch [N0,N1,N2] foo_task()

Value of taskIndex is equal to taskIndex0 + taskCount0*(taskIndex1 + taskCount1*taskIndex2) and it ranges from 0 to taskCount-1, where taskCount = taskCount0*taskCount1*taskCount2. If N1 or/and N2 are not specified in the launch expression, a value of 1 is assumed. Finally, for an one-dimensional grid of tasks, taskIndex is equivalent to taskIndex0 and taskCount is equivalent to taskCount0.

Task Parallelism: Runtime Requirements

If you use the task launch feature in ispc, you must provide C/C++ implementations of three specific functions that manage launching and synchronizing parallel tasks; these functions must be linked into your executable. Although these functions may be implemented in any language, they must have "C" linkage (i.e. their prototypes must be declared inside an extern "C" block if they are defined in C++.)

By using user-supplied versions of these functions, ispc programs can easily interoperate with software systems that have existing task systems for managing parallelism. If you're using ispc with a system that isn't otherwise multi-threaded and don't want to write custom implementations of them, you can use the implementations of these functions provided in the examples/tasksys.cpp file in the ispc distributions.

If you are implementing your own task system, the remainder of this section discusses the requirements for these calls. You will also likely want to review the example task systems in examples/tasksys.cpp for reference. If you are not implementing your own task system, you can skip reading the remainder of this section.

Here are the declarations of the three functions that must be provided to manage tasks in ispc:

void *ISPCAlloc(void **handlePtr, int64_t size, int32_t alignment);
void ISPCLaunch(void **handlePtr, void *f, void *data, int count0, int count1, int count2);
void ISPCSync(void *handle);

All three of these functions take an opaque handle (or a pointer to an opaque handle) as their first parameter. This handle allows the task system runtime to distinguish between calls to these functions from different functions in ispc code. In this way, the task system implementation can efficiently wait for completion on just the tasks launched from a single function.

The first time one of ISPCLaunch() or ISPCAlloc() is called in an ispc function, the void * pointed to by the handlePtr parameter will be NULL. The implementations of these function should then initialize *handlePtr to a unique handle value of some sort. (For example, it might allocate a small structure to record which tasks were launched by the current function.) In subsequent calls to these functions in the emitted ispc code, the same value for handlePtr will be passed in, such that loading from *handlePtr will retrieve the value stored in the first call.

At function exit (or at an explicit sync statement), a call to ISPCSync() will be generated if *handlePtr is non-NULL. Therefore, the handle value is passed directly to ISPCSync(), rather than a pointer to it, as in the other functions.

The ISPCAlloc() function is used to allocate small blocks of memory to store parameters passed to tasks. It should return a pointer to memory with the given size and alignment. Note that there is no explicit ISPCFree() call; instead, all memory allocated within an ispc function should be freed when ISPCSync() is called.

ISPCLaunch() is called to launch one or more asynchronous tasks. Each launch statement in ispc code causes a call to ISPCLaunch() to be emitted in the generated code. The three parameters after the handle pointer to the function are relatively straightforward; the void *f parameter holds a pointer to a function to call to run the work for this task, data holds a pointer to data to pass to this function, and count0, count1 and count2 are the number of instances of this function to enqueue for asynchronous execution. (In other words, count0, count1 and count2 correspond to the value n0, n1 and n2 in a multiple-task launch statement like launch[n2][n1][n0] or launch [n0,n1,n2] respectively.)

The signature of the provided function pointer f is

void (*TaskFuncPtr)(void *data, int threadIndex, int threadCount,
                    int taskIndex, int taskCount,
                    int taskIndex0, int taskIndex1, int taskIndex2,
                    int taskCount0, int taskCount1, int taskCount2);

When this function pointer is called by one of the hardware threads managed by the task system, the data pointer passed to ISPCLaunch() should be passed to it for its first parameter; threadCount gives the total number of hardware threads that have been spawned to run tasks and threadIndex should be an integer index between zero and threadCount uniquely identifying the hardware thread that is running the task. (These values can be used to index into thread-local storage.)

The value of taskCount should be the total number of tasks launched in the launch statement (it must be equal to taskCount0*taskCount1*taskCount2) that caused the call to ISPCLaunch() and each of the calls to this function should be given a unique value of taskIndex, taskIndex0, taskIndex1 and taskIndex2 between zero and taskCount, taskCount0, taskCount1 and taskCount2 respectively, with taskIndex = taskIndex0 + taskCount0*(taskIndex1 + taskCount1*taskIndex2), to distinguish which of the instances of the set of launched tasks is running.

The ISPC Standard Library

ispc has a standard library that is automatically available when compiling ispc programs. (To disable the standard library, pass the --nostdlib command-line flag to the compiler.)

Basic Operations On Data

Logical and Selection Operations

Recall from Expressions that ispc short-circuits the evaluation of logical and selection operators: given an expression like (index < count && array[index] == 0), then array[index] == 0 is only evaluated if index < count is true. This property is useful for writing expressions like the preceding one, where the second expression may not be safe to evaluate in some cases.

This short-circuiting can impose overhead in the generated code; additional operations are required to test the first value and to conditionally jump over the code that evaluates the second value. The ispc compiler does try to mitigate this cost by detecting cases where it is both safe and inexpensive to evaluate both expressions, and skips short-circuiting in the generated code in this case (without there being any programmer-visible change in program behavior.)

For cases where the compiler can't detect this case but the programmer wants to avoid short-circuiting behavior, the standard library provides a few helper functions. First, and() and or() provide non-short-circuiting logical AND and OR operations.

bool and(bool a, bool b)
bool or(bool a, bool b)
uniform bool and(uniform bool a, uniform bool b)
uniform bool or(uniform bool a, uniform bool b)

And there are three variants of select() that select between two values based on a boolean condition. These are the variants of select() for the int8 type:

int8 select(bool v, int8 a, int8 b)
int8 select(uniform bool v, int8 a, int8 b)
uniform int8 select(uniform bool v, uniform int8 a, uniform int8 b)

There are also variants for int16, int32, int64, float, and double types.

Bit Operations

The various variants of popcnt() return the population count--the number of bits set in the given value.

uniform int popcnt(uniform int v)
int popcnt(int v)
uniform int popcnt(bool v)

A few functions determine how many leading bits in the given value are zero and how many of the trailing bits are zero; there are also unsigned variants of these functions and variants that take int64 and unsigned int64 types.

int32 count_leading_zeros(int32 v)
uniform int32 count_leading_zeros(uniform int32 v)
int32 count_trailing_zeros(int32 v)
uniform int32 count_trailing_zeros(uniform int32 v)

Sometimes it's useful to convert a bool value to an integer using sign extension so that the integer's bits are all on if the bool has the value true (rather than just having the value one). The sign_extend() functions provide this functionality:

int sign_extend(bool value)
uniform int sign_extend(uniform bool value)

The intbits() and floatbits() functions can be used to implement low-level floating-point bit twiddling. For example, intbits() returns an unsigned int that is a bit-for-bit copy of the given float value. (Note: it is not the same as (int)a, but corresponds to something like *((int *)&a) in C.

float floatbits(unsigned int a);
uniform float floatbits(uniform unsigned int a);
unsigned int intbits(float a);
uniform unsigned int intbits(uniform float a);

The intbits() and floatbits() functions have no cost at runtime; they just let the compiler know how to interpret the bits of the given value. They make it possible to efficiently write functions that take advantage of the low-level bit representation of floating-point values.

For example, the abs() function in the standard library is implemented as follows:

float abs(float a) {
    unsigned int i = intbits(a);
    i &= 0x7fffffff;
    return floatbits(i);
}

This code directly clears the high order bit to ensure that the given floating-point value is positive. This compiles down to a single andps instruction when used with an Intel® SSE target, for example.

Math Functions

The math functions in the standard library provide a relatively standard range of mathematical functionality.

A number of different implementations of the transcendental math functions are available; the math library to use can be selected with the --math-lib= command line argument. The following values can be provided for this argument.

  • default: ispc's default built-in math functions. These have reasonably high precision. (e.g. sin has a maximum absolute error of approximately 1.45e-6 over the range -10pi to 10pi.)
  • fast: more efficient but lower accuracy versions of the default ispc implementations.
  • svml: use Intel "Short Vector Math Library". Use icpc to link your final executable so that the appropriate libraries are linked.
  • system: use the system's math library. On many systems, these functions are more accurate than both of ispc's implementations. Using these functions may be quite inefficient; the system math functions only compute one result at a time (i.e. they aren't vectorized), so ispc has to call them once per active program instance. (This is not the case for the other three options.)

Basic Math Functions

In addition to an absolute value call, abs(), signbits() extracts the sign bit of the given value, returning 0x80000000 if the sign bit is on (i.e. the value is negative) and zero if it is off.

float abs(float a)
uniform float abs(uniform float a)
unsigned int signbits(float x)

Standard rounding functions are provided. (On machines that support Intel® SSE or Intel® AVX, these functions all map to variants of the roundss and roundps instructions, respectively.)

float round(float x)
uniform float round(uniform float x)
float floor(float x)
uniform float floor(uniform float x)
float ceil(float x)
uniform float ceil(uniform float x)

rcp() computes an approximation to 1/v. The amount of error is different on different architectures.

float rcp(float v)
uniform float rcp(uniform float v)

A standard set of minimum and maximum functions is available. These functions also map to corresponding intrinsic functions.

float min(float a, float b)
uniform float min(uniform float a, uniform float b)
float max(float a, float b)
uniform float max(uniform float a, uniform float b)
unsigned int min(unsigned int a, unsigned int b)
uniform unsigned int min(uniform unsigned int a,
                         uniform unsigned int b)
unsigned int max(unsigned int a, unsigned int b)
uniform unsigned int max(uniform unsigned int a,
                         uniform unsigned int b)

The clamp() functions clamp the provided value to the given range. (Their implementations are based on min() and max() and are thus quite efficient.)

float clamp(float v, float low, float high)
uniform float clamp(uniform float v, uniform float low,
                    uniform float high)
unsigned int clamp(unsigned int v, unsigned int low,
                   unsigned int high)
uniform unsigned int clamp(uniform unsigned int v,
                           uniform unsigned int low,
                           uniform unsigned int high)

The isnan() functions test whether the given value is a floating-point "not a number" value:

bool isnan(float v)
uniform bool isnan(uniform float v)
bool isnan(double v)
uniform bool isnan(uniform double v)

A number of functions are also available for performing operations on 8- and 16-bit quantities; these map to specialized instructions that perform these operations on targets that support them. avg_up() computes the average of the two values, rounding up if their average is halfway between two integers (i.e., it computes (a+b+1)/2).

int8 avg_up(int8 a, int8 b)
unsigned int8 avg_up(unsigned int8 a, unsigned int8 b)
int16 avg_up(int16 a, int16 b)
unsigned int16 avg_up(unsigned int16 a, unsigned int16 b)

avg_down() computes the average of the two values, rounding down (i.e., it computes (a+b)/2).

int8 avg_down(int8 a, int8 b)
unsigned int8 avg_down(unsigned int8 a, unsigned int8 b)
int16 avg_down(int16 a, int16 b)
unsigned int16 avg_down(unsigned int16 a, unsigned int16 b)

Transcendental Functions

The square root of a given value can be computed with sqrt(), which maps to hardware square root intrinsics when available. An approximate reciprocal square root, 1/sqrt(v) is computed by rsqrt(). Like rcp(), the error from this call is different on different architectures.

float sqrt(float v)
uniform float sqrt(uniform float v)
float rsqrt(float v)
uniform float rsqrt(uniform float v)

ispc provides a standard variety of calls for trigonometric functions:

float sin(float x)
uniform float sin(uniform float x)
float cos(float x)
uniform float cos(uniform float x)
float tan(float x)
uniform float tan(uniform float x)

The corresponding inverse functions are also available:

float asin(float x)
uniform float asin(uniformfloat x)
float acos(float x)
uniform float acos(uniform float x)
float atan(float x)
uniform float atan(uniform float x)
float atan2(float y, float x)
uniform float atan2(uniform float y, uniform float x)

If both sine and cosine are needed, then the sincos() call computes both more efficiently than two calls to the respective individual functions:

void sincos(float x, varying float * uniform s, varying float * uniform c)
void sincos(uniform float x, uniform float * uniform s,
            uniform float * uniform c)

The usual exponential and logarithmic functions are provided.

float exp(float x)
uniform float exp(uniform float x)
float log(float x)
uniform float log(uniform float x)
float pow(float a, float b)
uniform float pow(uniform float a, uniform float b)

A few functions that end up doing low-level manipulation of the floating-point representation in memory are available. As in the standard math library, ldexp() multiplies the value x by 2^n, and frexp() directly returns the normalized mantissa and returns the normalized exponent as a power of two in the pw2 parameter.

float ldexp(float x, int n)
uniform float ldexp(uniform float x, uniform int n)
float frexp(float x, varying int * uniform pw2)
uniform float frexp(uniform float x,
                    uniform int * uniform pw2)

Saturating Arithmetic

A saturation (no overflow possible) addition, substraction, multiplication and division of all integer types are provided by the ispc standard library.

int8 saturating_add(uniform int8 a, uniform int8 b)
int8 saturating_add(varying int8 a, varying int8 b)
unsigned int8 saturating_add(uniform unsigned int8 a, uniform unsigned int8 b)
unsigned int8 saturating_add(varying unsigned int8 a, varying unsigned int8 b)

int8 saturating_sub(uniform int8 a, uniform int8 b)
int8 saturating_sub(varying int8 a, varying int8 b)
unsigned int8 saturating_sub(uniform unsigned int8 a, uniform unsigned int8 b)
unsigned int8 saturating_sub(varying unsigned int8 a, varying unsigned int8 b)

int8 saturating_mul(uniform int8 a, uniform int8 b)
int8 saturating_mul(varying int8 a, varying int8 b)
unsigned int8 saturating_mul(uniform unsigned int8 a, uniform unsigned int8 b)
unsigned int8 saturating_mul(varying unsigned int8 a, varying unsigned int8 b)

int8 saturating_div(uniform int8 a, uniform int8 b)
int8 saturating_div(varying int8 a, varying int8 b)
unsigned int8 saturating_div(uniform unsigned int8 a, uniform unsigned int8 b)
unsigned int8 saturating_div(varying unsigned int8 a,varying unsigned int8 b)

In addition to the int8 variants of saturating arithmetic functions listed above, there are versions that supports int16, int32 and int64 values as well.

Pseudo-Random Numbers

A simple random number generator is provided by the ispc standard library. State for the RNG is maintained in an instance of the RNGState structure, which is seeded with seed_rng().

struct RNGState;
void seed_rng(varying RNGState * uniform state, varying int seed)
void seed_rng(uniform RNGState * uniform state, uniform int seed)

Note that if the same varying seed value is used for all of the program instances (e.g. RNGState state; seed_rng(&state, 1);), then all of the program instances in the gang will see the same sequence of pseudo-random numbers. If this behavior isn't desred, you may want to add the programIndex value to the provided seed or otherwise ensure that the seed has a unique value for each program instance.

After the RNG is seeded, the random() function can be used to get a pseudo-random unsigned int32 value and the frandom() function can be used to get a pseudo-random float value.

unsigned int32 random(varying RNGState * uniform state)
float frandom(varying RNGState * uniform state)
uniform unsigned int32 random(RNGState * uniform state)
uniform float frandom(uniform RNGState * uniform state)

Random Numbers

Some recent CPUs (including those based on the Intel(r) Ivy Bridge micro-architecture), provide support for generating true random numbers. A few standard library functions make this functionality available:

bool rdrand(uniform int32 * uniform ptr)
bool rdrand(varying int32 * uniform ptr)
bool rdrand(uniform int32 * varying ptr)

If the processor doesn't have sufficient entropy to generate a random number, then this function fails and returns false. Otherwise, if the processor is successful, the random value is stored in the given pointer and true is returned. Therefore, this function should generally be used as follows, called repeatedly until it is successful:

int r;
while (rdrand(&r) == false)
    ; // empty loop body

In addition to the int32 variants of rdrand() listed above, there are versions that return int16, float, and int64 values as well.

Note that when compiling to targets other than avx1.1 and avx2, the rdrand() functions always return false.

Output Functions

ispc has a simple print statement for printing values during program execution. In the following short ispc program, there are three uses of the print statement:

export void foo(uniform float f[4], uniform int i) {
    float x = f[programIndex];
    print("i = %, x = %\n", i, x);
    if (x < 2) {
        ++x;
        print("added to x = %\n", x);
    }
    print("last print of x = %\n", x);
}

There are a few things to note. First, the function is called print, not printf (unlike C). Second, the formatting string passed to this function only uses a single percent sign to denote where the corresponding value should be printed. You don't need to match the types of formatting operators with the types being passed. However, you can't currently use the rich data formatting options that printf provides (e.g. constructs like %.10f.).

If this function is called with the array of floats (0,1,2,3) passed in for the f parameter and the value 10 for the i parameter, it generates the following output on a four-wide compilation target:

i = 10, x = [0.000000,1.000000,2.000000,3.000000]
added to x = [1.000000,2.000000,((2.000000)),((3.000000))]
last print of x = [1.000000,2.000000,2.000000,3.000000]

When a varying variable is printed, the values for program instances that aren't currently executing are printed inside double parenthesis, indicating inactive program instances. The elements for inactive program instances may have garbage values, though in some circumstances it can be useful to see their values.

Assertions

The ispc standard library includes a mechanism for adding assert() statements to ispc program code. Like assert() in C, the assert() function takes a single boolean expression as an argument. If the expression evaluates to false at runtime, then a diagnostic error message printed and the abort() function is called.

When called with a varying quantity, an assertion triggers if the expression evaluates to false for any any of the executing program instances at the point where it is called. Thus, given code like:

int x = programIndex - 2;  // (-2, -1, 0, ... )
if (x > 0)
    assert(x > 0);

The assert() statement will not trigger, since the condition isn't true for any of the executing program instances at that point. (If this assert() statement was outside of this if, then it would of course trigger.)

To disable all of the assertions in a file that is being compiled (e.g., for an optimized release build), use the --opt=disable-assertions command-line argument.

Cross-Program Instance Operations

ispc programs are often used to express independently-executing programs performing computation on separate data elements. (i.e. pure data-parallelism). However, it's often the case where it's useful for the program instances to be able to cooperate in computing results. The cross-lane operations described in this section provide primitives for communication between the running program instances in the gang.

The lanemask() function returns an integer that encodes which of the current SPMD program instances are currently executing. The i'th bit is set if the i'th program instance lane is currently active.

uniform int lanemask()

To broadcast a value from one program instance to all of the others, a broadcast() function is available. It broadcasts the value of the value parameter for the program instance given by index to all of the running program instances.

int8 broadcast(int8 value, uniform int index)
int16 broadcast(int16 value, uniform int index)
int32 broadcast(int32 value, uniform int index)
int64 broadcast(int64 value, uniform int index)
float broadcast(float value, uniform int index)
double broadcast(double value, uniform int index)

The rotate() function allows each program instance to find the value of the given value that their neighbor offset steps away has. For example, on an 8-wide target, if value has the value (1, 2, 3, 4, 5, 6, 7, 8) across the gang of running program instances, then rotate(value, -1) causes the first program instance to get the value 8, the second program instance to get the value 1, the third 2, and so forth. The provided offset value can be positive or negative, and may be greater than the size of the gang (it is masked to ensure valid offsets).

int8 rotate(int8 value, uniform int offset)
int16 rotate(int16 value, uniform int offset)
int32 rotate(int32 value, uniform int offset)
int64 rotate(int64 value, uniform int offset)
float rotate(float value, uniform int offset)
double rotate(double value, uniform int offset)

The shift() function allows each program instance to find the value of the given value that their neighbor offset steps away has. This is similar to rotate() with the exception that values are not circularly shifted. Instead, zeroes are shifted in where appropriate.

int8 shift(int8 value, uniform int offset)
int16 shift(int16 value, uniform int offset)
int32 shift(int32 value, uniform int offset)
int64 shift(int64 value, uniform int offset)
float shift(float value, uniform int offset)
double shift(double value, uniform int offset)

Finally, the shuffle() functions allow two variants of fully general shuffling of values among the program instances. For the first version, each program instance's value of permutation gives the program instance from which to get the value of value. The provided values for permutation must all be between 0 and the gang size.

int8 shuffle(int8 value, int permutation)
int16 shuffle(int16 value, int permutation)
int32 shuffle(int32 value, int permutation)
int64 shuffle(int64 value, int permutation)
float shuffle(float value, int permutation)
double shuffle(double value, int permutation)

The second variant of shuffle() permutes over the extended vector that is the concatenation of the two provided values. In other words, a value of 0 in an element of permutation corresponds to the first element of value0, the value of two times the gang size, minus one corresponds to the last element of value1, etc.)

int8 shuffle(int8 value0, int8 value1, int permutation)
int16 shuffle(int16 value0, int16 value1, int permutation)
int32 shuffle(int32 value0, int32 value1, int permutation)
int64 shuffle(int64 value0, int64 value1, int permutation)
float shuffle(float value0, float value1, int permutation)
double shuffle(double value0, double value1, int permutation)

Finally, there are primitive operations that extract and set values in the SIMD lanes. You can implement all of the broadcast, rotate, shift, and shuffle operations described above in this section from these routines, though in general, not as efficiently. These routines are useful for implementing other reductions and cross-lane communication that isn't included in the above, though. Given a varying value, extract() returns the i'th element of it as a single uniform value. .

uniform int8 extract(int8 x, uniform int i)
uniform int16 extract(int16 x, uniform int i)
uniform int32 extract(int32 x, uniform int i)
uniform int64 extract(int64 x, uniform int i)
uniform float extract(float x, uniform int i)

Similarly, insert returns a new value where the i th element of x has been replaced with the value v

int8 insert(int8 x, uniform int i, uniform int8 v)
int16 insert(int16 x, uniform int i, uniform int16 v)
int32 insert(int32 x, uniform int i, uniform int32 v)
int64 insert(int64 x, uniform int i, uniform int64 v)
float insert(float x, uniform int i, uniform float v)

Reductions

A number of routines are available to evaluate conditions across the running program instances. For example, any() returns true if the given value v is true for any of the SPMD program instances currently running, all() returns true if it true for all of them, and none() returns true if v is always false.

uniform bool any(bool v)
uniform bool all(bool v)
uniform bool none(bool v)

You can also compute a variety of reductions across the program instances. For example, the values of the given value in each of the active program instances are added together by the reduce_add() function.

uniform int16 reduce_add(int8 x)
uniform unsigned int16 reduce_add(unsigned int8 x)
uniform int32 reduce_add(int16 x)
uniform unsigned int32 reduce_add(unsigned 16int x)
uniform int64 reduce_add(int32 x)
uniform unsigned int64 reduce_add(unsigned int32 x)
uniform int64 reduce_add(int64 x)
uniform unsigned int64 reduce_add(unsigned int64 x)

uniform float reduce_add(float x)
uniform double reduce_add(double x)

You can also use functions to compute the minimum value of the given value across all of the currently-executing program instances.

uniform int32 reduce_min(int32 a)
uniform unsigned int32 reduce_min(unsigned int32 a)
uniform int64 reduce_min(int64 a)
uniform unsigned int64 reduce_min(unsigned int64 a)

uniform float reduce_min(float a)
uniform double reduce_min(double a)

Equivalent functions are available to comptue the maximum of the given varying variable over the active program instances.

uniform int32 reduce_max(int32 a)
uniform unsigned int32 reduce_max(unsigned int32 a)
uniform int64 reduce_max(int64 a)
uniform unsigned int64 reduce_max(unsigned int64 a)

uniform float reduce_max(float a)
uniform double reduce_max(double a)

Finally, you can check to see if a particular value has the same value in all of the currently-running program instances:

uniform bool reduce_equal(int32 v)
uniform bool reduce_equal(unsigned int32 v)
uniform bool reduce_equal(int64 v)
uniform bool reduce_equal(unsigned int64 v)

uniform bool reduce_equal(float v)
uniform bool reduce_equal(double)

There are also variants of these functions that return the value as a uniform in the case where the values are all the same. (There is discussion of an application of this variant to improve memory access performance in the Performance Guide.

uniform bool reduce_equal(int32 v, uniform int32 * uniform sameval)
uniform bool reduce_equal(unsigned int32 v,
                          uniform unsigned int32 * uniform sameval)
uniform bool reduce_equal(int64 v, uniform int64 * uniform sameval)
uniform bool reduce_equal(unsigned int64 v,
                          uniform unsigned int64 * uniform sameval)

uniform bool reduce_equal(float v, uniform float * uniform sameval)
uniform bool reduce_equal(double, uniform double * uniform sameval)

If called when none of the program instances are running, reduce_equal() will return false.

There are also a number of functions to compute "scan"s of values across the program instances. For example, the exclusive_scan_and() function computes, for each program instance, the sum of the given value over all of the preceding program instances. (The scans currently available in ispc are all so-called "exclusive" scans, meaning that the value computed for a given element does not include the value provided for that element.) In C code, an exclusive add scan over an array might be implemented as:

void scan_add(int *in_array, int *result_array, int count) {
    result_array[0] = 0;
    for (int i = 1; i < count; ++i)
        result_array[i] = result_array[i-1] + in_array[i-1];
}

ispc provides the following scan functions--addition, bitwise-and, and bitwise-or are available:

int32 exclusive_scan_add(int32 v)
unsigned int32 exclusive_scan_add(unsigned int32 v)
float exclusive_scan_add(float v)
int64 exclusive_scan_add(int64 v)
unsigned int64 exclusive_scan_add(unsigned int64 v)
double exclusive_scan_add(double v)
int32 exclusive_scan_and(int32 v)
unsigned int32 exclusive_scan_and(unsigned int32 v)
int64 exclusive_scan_and(int64 v)
unsigned int64 exclusive_scan_and(unsigned int64 v)
int32 exclusive_scan_or(int32 v)
unsigned int32 exclusive_scan_or(unsigned int32 v)
int64 exclusive_scan_or(int64 v)
unsigned int64 exclusive_scan_or(unsigned int64 v)

The use of exclusive scan to generate variable amounts of output from program instances into a compact output buffer is discussed in the FAQ.

Data Movement

Setting and Copying Values In Memory

There are a few functions for copying blocks of memory and initializing values in memory. Along the lines of the equivalently-named routines in the C Standard libary, memcpy copies a given number of bytes starting from a source location in memory to a destination locaiton, where the two regions of memory are guaranteed by the caller to be non-overlapping. Alternatively, memmove can be used to copy data if the buffers may overlap.

void memcpy(void * uniform dst, void * uniform src, uniform int32 count)
void memmove(void * uniform dst, void * uniform src, uniform int32 count)
void memcpy(void * varying dst, void * varying src, int32 count)
void memmove(void * varying dst, void * varying src, int32 count)

Note that there are variants of these functions that take both uniform and varying pointers. Also note that sizeof(float) and sizeof(uniform float) return different values, so programmers should take care when calculating count.

To initialize values in memory, the memset routine can be used. (It also behaves like the function of the same name in the C Standard Library.) It sets the given number of bytes of memory starting at the given location to the value provided.

void memset(void * uniform ptr, uniform int8 val, uniform int32 count)
void memset(void * varying ptr, int8 val, int32 count)

There are also variants of all of these functions that take 64-bit values for the number of bytes of memory to operate on:

void memcpy64(void * uniform dst, void * uniform src, uniform int64 count)
void memcpy64(void * varying dst, void * varying src, int64 count)
void memmove64(void * uniform dst, void * uniform src, uniform int64 count)
void memmove64(void * varying dst, void * varying src, int64 count)
void memset64(void * uniform ptr, uniform int8 val, uniform int64 count)
void memset64(void * varying ptr, int8 val, int64 count)

Packed Load and Store Operations

The standard library also offers routines for writing out and reading in values from linear memory locations for the active program instances. The packed_load_active() functions load consecutive values starting at the given location, loading one consecutive value for each currently-executing program instance and storing it into that program instance's val variable. They return the total number of values loaded.

uniform int packed_load_active(uniform int * uniform base,
                               varying int * uniform val)
uniform int packed_load_active(uniform unsigned int * uniform base,
                               varying unsigned int * uniform val)

Similarly, the packed_store_active() functions store the val values for each program instances that executed the packed_store_active() call, storing the results consecutively starting at the given location. They return the total number of values stored.

uniform int packed_store_active(uniform int * uniform base,
                                int val)
uniform int packed_store_active(uniform unsigned int * uniform base,
                                unsigned int val)

There are also packed_store_active2() functions with exactly the same signatures and the same semantic except that they may write one extra element to the output array (but still returning the same value as packed_store_active()). These functions suggest different branch free implementation on most of supported targets, which usually (but not always) performs better than packed_store_active(). It's advised to test function performance on user's scenarios on particular target hardware before using it.

As an example of how these functions can be used, the following code shows the use of packed_store_active().

uniform int negative_indices(uniform float a[], uniform int length,
                             uniform int indices[]) {
    uniform int numNeg = 0;
    foreach (i = 0 ... length) {
        if (a[i] < 0.)
            numNeg += packed_store_active(&indices[numNeg], i);
    }
    return numNeg;
}

The function takes an array of floating point values a, with length given by the length parameter. This function also takes an output array, indices, which is assumed to be at least as long as length. It then loops over all of the elements of a and, for each element that is less than zero, stores that element's offset into the indices array. It returns the total number of negative values. For example, given an input array a[8] = { 10, -20, 30, -40, -50, -60, 70, 80 }, it returns a count of four negative values, and initializes the first four elements of indices[] to the values { 1, 3, 4, 5 } corresponding to the array indices where a[i] was less than zero.

Data Conversions

Converting Between Array-of-Structures and Structure-of-Arrays Layout

Applications often lay data out in memory in "array of structures" form. Though convenient in C/C++ code, this layout can make ispc programs less efficient than they would be if the data was laid out in "structure of arrays" form. (See the section Use "Structure of Arrays" Layout When Possible in the performance guide for extended discussion of this topic.)

The standard library does provide a few functions that efficiently convert between these two formats, for cases where it's not possible to change the application to use "structure of arrays layout". Consider an array of 3D (x,y,z) position data laid out in a C array like:

// C++ code
float pos[] = { x0, y0, z0, x1, y1, z1, x2, ... };

In an ispc program, we might want to load a set of (x,y,z) values and do a computation based on them. The natural expression of this:

extern uniform float pos[];
uniform int base = ...;
float x = pos[base + 3 * programIndex];     // x = { x0 x1 x2 ... }
float y = pos[base + 1 + 3 * programIndex]; // y = { y0 y1 y2 ... }
float z = pos[base + 2 + 3 * programIndex]; // z = { z0 z1 z2 ... }

leads to irregular memory accesses and reduced performance. Alternatively, the aos_to_soa3() standard library function could be used:

extern uniform float pos[];
uniform int base = ...;
float x, y, z;
aos_to_soa3(&pos[base], x, y, z);

This routine loads three times the gang size values from the given array starting at the given offset, returning three varying results. There are both int32 and float variants of this function:

void aos_to_soa3(uniform float a[], varying float * uniform v0,
                 varying float * uniform v1, varying float * uniform v2)
void aos_to_soa3(uniform int32 a[], varying int32 * uniform v0,
                 varying int32 * uniform v1, varying int32 * uniform v2)

After computation is done, corresponding functions convert back from the SoA values in ispc varying variables and write the values back to the given array, starting at the given offset.

extern uniform float pos[];
uniform int base = ...;
float x, y, z;
aos_to_soa3(&pos[base], x, y, z);
// do computation with x, y, z
soa_to_aos3(x, y, z, &pos[base]);
void soa_to_aos3(float v0, float v1, float v2, uniform float a[])
void soa_to_aos3(int32 v0, int32 v1, int32 v2, uniform int32 a[])

There are also variants of these functions that convert 4-wide values between AoS and SoA layouts. In other words, aos_to_soa4() converts AoS data in memory laid out like r0 g0 b0 a0 r1 g1 b1 a1 ... to four varying variables with values r0 r1..., g0 g1..., b0 b1..., and a0 a1..., reading a total of four times the gang size values from the given array, starting at the given offset.

void aos_to_soa4(uniform float a[], varying float * uniform v0,
                 varying float * uniform v1, varying float * uniform v2,
                 varying float * uniform v3)
void aos_to_soa4(uniform int32 a[], varying int32 * uniform v0,
                 varying int32 * uniform v1, varying int32 * uniform v2,
                 varying int32 * uniform v3)
void soa_to_aos4(float v0, float v1, float v2, float v3, uniform float a[])
void soa_to_aos4(int32 v0, int32 v1, int32 v2, int32 v3, uniform int32 a[])

Conversions To and From Half-Precision Floats

There are functions to convert to and from the IEEE 16-bit floating-point format. Note that there is no half data-type, and it isn't possible to do floating-point math directly with half types in ispc; these functions facilitate converting to and from half-format data in memory.

To use them, half-format data should be loaded into an int16 and the half_to_float() function used to convert it to a 32-bit floating point value. To store a value to memory in half format, the float_to_half() function returns the 16 bits that are the closest match to the given float, in half format.

float half_to_float(unsigned int16 h)
uniform float half_to_float(uniform unsigned int16 h)
int16 float_to_half(float f)
uniform int16 float_to_half(uniform float f)

There are also faster versions of these functions that don't worry about handling floating point infinity, "not a number" and denormalized numbers correctly. These are faster than the above functions, but are less precise.

float half_to_float_fast(unsigned int16 h)
uniform float half_to_float_fast(uniform unsigned int16 h)
int16 float_to_half_fast(float f)
uniform int16 float_to_half_fast(uniform float f)

Converting to sRGB8

The sRGB color space is used in many applications in graphics and imaging; see the Wikipedia page on sRGB for more information. The ispc standard library provides two functions for converting floating-point color values to 8-bit values in the sRGB space.

int float_to_srgb8(float v)
uniform int float_to_srgb8(uniform float v)

Systems Programming Support

Atomic Operations and Memory Fences

The standard set of atomic memory operations are provided by the standard library, including variants to handle both uniform and varying types as well as "local" and "global" atomics.

Local atomics provide atomic behavior across the program instances in a gang, but not across multiple gangs or memory operations in different hardware threads. To see why they are needed, consider a histogram calculation where each program instance in the gang computes which bucket a value lies in and then increments a corresponding counter. If the code is written like this:

uniform int count[N_BUCKETS] = ...;
float value = ...;
int bucket = clamp(value / N_BUCKETS, 0, N_BUCKETS);
++count[bucket];  // ERROR: undefined behavior if collisions

then the program's behavior is undefined: whenever multiple program instances have values that map to the same value of bucket, then the effect of the increment is undefined. (See the discussion in the Data Races Within a Gang section; in the case here, there isn't a sequence point between one program instance updating count[bucket] and the other program instance reading its value.)

The atomic_add_local() function can be used in this case; as a local atomic it is atomic across the gang of program instances, such that the expected result is computed.

...
int bucket = clamp(value / N_BUCKETS, 0, N_BUCKETS);
atomic_add_local(&count[bucket], 1);

It uses this variant of the 32-bit integer atomic add routine:

int32 atomic_add_local(uniform int32 * uniform ptr, int32 delta)

The semantics of this routine are typical for an atomic add function: the pointer here points to a single location in memory (the same one for all program instances), and for each executing program instance, the value stored in the location that ptr points to has that program instance's value "delta" added to it atomically, and the old value at that location is returned from the function.

One thing to note is that the type of the value being added to is a uniform integer, while the increment amount and the return value are varying. In other words, the semantics of this call are that each running program instance individually issues the atomic operation with its own delta value and gets the previous value back in return. The atomics for the running program instances may be issued in arbitrary order; it's not guaranteed that they will be issued in programIndex order, for example.

Global atomics are more powerful than local atomics; they are atomic across both the program instances in the gang as well as atomic across different gangs and different hardware threads. For example, for the global variant of the atomic used above,

int32 atomic_add_global(uniform int32 * uniform ptr, int32 delta)

if multiple processors simultaneously issue atomic adds to the same memory location, the adds will be serialized by the hardware so that the correct result is computed in the end.

Here are the declarations of the int32 variants of these functions. There are also int64 equivalents as well as variants that take unsigned int32 and int64 values.

int32 atomic_add_{local,global}(uniform int32 * uniform ptr, int32 value)
int32 atomic_subtract_{local,global}(uniform int32 * uniform ptr, int32 value)
int32 atomic_min_{local,global}(uniform int32 * uniform ptr, int32 value)
int32 atomic_max_{local,global}(uniform int32 * uniform ptr, int32 value)
int32 atomic_and_{local,global}(uniform int32 * uniform ptr, int32 value)
int32 atomic_or_{local,global}(uniform int32 * uniform ptr, int32 value)
int32 atomic_xor_{local,global}(uniform int32 * uniform ptr, int32 value)
int32 atomic_swap_{local,global}(uniform int32 * uniform ptr, int32 value)

Support for float and double types is also available. For local atomics, all but the logical operations are available. (There are corresponding double variants of these, not listed here.)

float atomic_add_local(uniform float * uniform ptr, float value)
float atomic_subtract_local(uniform float * uniform ptr, float value)
float atomic_min_local(uniform float * uniform ptr, float value)
float atomic_max_local(uniform float * uniform ptr, float value)
float atomic_swap_local(uniform float * uniform ptr, float value)

For global atomics, only atomic swap is available for these types:

float atomic_swap_global(uniform float * uniform ptr, float value)
double atomic_swap_global(uniform double * uniform ptr, double value)

Finally, "swap" (but none of these other atomics) is available for pointer types:

void *atomic_swap_{local,global}(void * * uniform ptr, void * value)

There are also variants of the atomic that take uniform values for the operand and return a uniform result. These correspond to a single atomic operation being performed for the entire gang of program instances, rather than one per program instance.

uniform int32 atomic_add_{local,global}(uniform int32 * uniform ptr,
                                        uniform int32 value)
uniform int32 atomic_subtract_{local,global}(uniform int32 * uniform ptr,
                                             uniform int32 value)
uniform int32 atomic_min_{local,global}(uniform int32 * uniform ptr,
                                        uniform int32 value)
uniform int32 atomic_max_{local,global}(uniform int32 * uniform ptr,
                                        uniform int32 value)
uniform int32 atomic_and_{local,global}(uniform int32 * uniform ptr,
                                        uniform int32 value)
uniform int32 atomic_or_{local,global}(uniform int32 * uniform ptr,
                                        uniform int32 value)
uniform int32 atomic_xor_{local,global}(uniform int32 * uniform ptr,
                                        uniform int32 value)
uniform int32 atomic_swap_{local,global}(uniform int32 * uniform ptr,
                                         uniform int32 newval)

And similarly for pointers:

uniform void *atomic_swap_{local,global}(void * * uniform ptr,
                                         void *newval)

Be careful that you use the atomic function that you mean to; consider the following code:

extern uniform int32 counter;
int32 myCounter = atomic_add_global(&counter, 1);

One might write code like this with the intent that each running program instance increments the counter by one and gets the old value of the counter (for example, to store results into unique locations in an array). However, the above code calls the second variant of atomic_add_global(), which takes a uniform int value to add to the counter and only performs one atomic operation. The counter will be increased by just one, and all program instances will receive the same value back (thanks to the uniform int32 return value being silently converted to a varying int32.) Writing the code this way, for example, will cause the desired atomic add function to be called.

extern uniform int32 counter;
int32 myCounter = atomic_add_global(&counter, (varying int32)1);

There is a third variant of each of these atomic functions that takes a varying pointer; this allows each program instance to issue an atomic operation to a possibly-different location in memory. (Of course, the proper result is still returned if some or all of them happen to point to the same location in memory!)

int32 atomic_add_{local,global}(uniform int32 * varying ptr, int32 value)
int32 atomic_subtract_{local,global}(uniform int32 * varying ptr, int32 value)
int32 atomic_min_{local,global}(uniform int32 * varying ptr, int32 value)
int32 atomic_max_{local,global}(uniform int32 * varying ptr, int32 value)
int32 atomic_and_{local,global}(uniform int32 * varying ptr, int32 value)
int32 atomic_or_{local,global}(uniform int32 * varying ptr, int32 value)
int32 atomic_xor_{local,global}(uniform int32 * varying ptr, int32 value)
int32 atomic_swap_{local,global}(uniform int32 * varying ptr, int32 value)

And:

void *atomic_swap_{local,global}(void * * ptr, void *value)

There are also atomic "compare and exchange" functions. Compare and exchange atomically compares the value in "val" to "compare"--if they match, it assigns "newval" to "val". In either case, the old value of "val" is returned. (As with the other atomic operations, there are also unsigned and 64-bit variants of this function. Furthermore, there are float, double, and void * variants as well.)

int32 atomic_compare_exchange_{local,global}(uniform int32 * uniform ptr,
                                             int32 compare, int32 newval)
uniform int32 atomic_compare_exchange_{local,global}(uniform int32 * uniform ptr,
                                uniform int32 compare, uniform int32 newval)

ispc also has a standard library routine that inserts a memory barrier into the code; it ensures that all memory reads and writes prior to be barrier complete before any reads or writes after the barrier are issued. See the Linux kernel documentation on memory barriers for an excellent writeup on the need for and the use of memory barriers in multi-threaded code.

void memory_barrier();

Note that this barrier is not needed for coordinating reads and writes among the program instances in a gang; it's only needed for coordinating between multiple hardware threads running on different cores. See the section Data Races Within a Gang for the guarantees provided about memory read/write ordering across a gang.

Prefetches

The standard library has a variety of functions to prefetch data into the processor's cache. While modern CPUs have automatic prefetchers that do a reasonable job of prefetching data to the cache before its needed, high performance applications may find it helpful to prefetch data before it's needed.

For example, this code shows how to prefetch data to the processor's L1 cache while iterating over the items in an array.

uniform int32 array[...];
for (uniform int i = 0; i < count; ++i) {
    // do computation with array[i]
    prefetch_l1(&array[i+32]);
}

The standard library has routines to prefetch to the L1, L2, and L3 caches. It also has a variant, prefetch_nt(), that indicates that the value being prefetched isn't expected to be used more than once (so should be high priority to be evicted from the cache). Furthermore, it has versions of these functions that take both uniform and varying pointer types.

void prefetch_{l1,l2,l3,nt}(void * uniform ptr)
void prefetch_{l1,l2,l3,nt}(void * varying ptr)

System Information

The value of a high-precision hardware clock counter is returned by the clock() routine; its value increments by one each processor cycle. Thus, taking the difference between the values returned by clock() at different points in program execution gives the number of cycles between those points in the program.

uniform int64 clock()

Note that clock() flushes the processor pipeline. It has an overhead of a hundred or so cycles, so for very fine-grained measurements, it may be worthwhile to measure the cost of calling clock() and subtracting that value from reported results.

A routine is also available to find the number of CPU cores available in the system:

uniform int num_cores()

This value can be useful for adapting the granularity of parallel task decomposition depending on the number of processors in the system.

Interoperability with the Application

One of ispc's key goals is to make it easy to interoperate between the C/C++ application code and parallel code written in ispc. This section describes the details of how this works and describes a number of the pitfalls.

Interoperability Overview

As described in Compiling and Running a Simple ISPC Program it's relatively straightforward to call ispc code from C/C++. First, any ispc functions to be called should be defined with the export keyword:

export void foo(uniform float a[]) {
    ...
}

This function corresponds to the following C-callable function:

void foo(float a[]);

(Recall from the "uniform" and "varying" Qualifiers section that uniform types correspond to a single instances of the corresponding type in C/C++.)

In addition to variables passed from the application to ispc in the function call, you can also share global variables between the application and ispc. To do so, just declare the global variable as usual (in either ispc or application code), and add an extern declaration on the other side.

For example, given this ispc code:

// ispc code
uniform float foo;
extern uniform float bar[10];

And this C++ code:

// C++ code
extern "C" {
  extern float foo;
  float bar[10];
}

Both the foo and bar global variables can be accessed on each side. Note that the extern "C" declaration is necessary from C++, since ispc uses C linkage for functions and globals.

ispc code can also call back to C/C++. On the ispc side, any application functions to be called must be declared with the extern "C" qualifier.

extern "C" void foo(uniform float f, uniform float g);

Unlike in C++, extern "C" doesn't take braces to delineate multiple functions to be declared; thus, multiple C functions to be called from ispc must be declared as follows:

extern "C" void foo(uniform float f, uniform float g);
extern "C" uniform int bar(uniform int a);

It is illegal to overload functions declared with extern "C" linkage; ispc issues an error in this case.

Only a single function call is made back to C++ for the entire gang of running program instances. Furthermore, function calls back to C/C++ are not made if none of the program instances want to make the call. For example, given code like:

uniform float foo = ...;
float x = ...;
if (x != 0)
    foo = appFunc(foo);

appFunc() will only be called if one or more of the running program instances evaluates true for x != 0. If the application code would like to determine which of the running program instances want to make the call, a mask representing the active SIMD lanes can be passed to the function.

extern "C" float appFunc(uniform float x,
                         uniform int activeLanes);

If the function is then called as:

...
x = appFunc(x, lanemask());

The activeLanes parameter will have the value one in the 0th bit if the first program instance is running at this point in the code, one in the first bit for the second instance, and so forth. (The lanemask() function is documented in Cross-Program Instance Operations.) Application code can thus be written as:

float appFunc(float x, int activeLanes) {
    for (int i = 0; i < programCount; ++i)
        if ((activeLanes & (1 << i)) != 0) {
            // do computation for i'th SIMD lane
        }
}

In some cases, it can be desirable to generate a single call for each executing program instance, rather than one call for a gang. For example, the code below shows how one might call an existing math library routine that takes a scalar parameter.

extern "C" uniform double erf(uniform double);
double v = ...;
double result;
foreach_active (instance) {
    uniform double r = erf(extract(v, instance));
    result = insert(result, instance, r);
}

This code calls erf() once for each active program instance, passing it the program instance's value of v and storing the result in the instance's result value.

Data Layout

In general, ispc tries to ensure that struct types and other complex datatypes are laid out in the same way in memory as they are in C/C++. Matching structure layout is important for easy interoperability between C/C++ code and ispc code.

The main complexity in sharing data between ispc and C/C++ often comes from reconciling data structures between ispc code and application code; it can be useful to declare the shared structures in ispc code and then examine the generated header file (which will have the C/C++ equivalents of them.) For example, given a structure in ispc:

// ispc code
struct Node {
   int count;
   float pos[3];
};

If a uniform Node structure is used in the parameters to an export ed function, then the header file generated by the ispc compiler will have a declaration like:

// C/C++ code
struct Node {
   int count;
   float pos[3];
};

Because varying types have size that depends on the size of the gang of program instances, ispc has restrictrictions on using varying types in parameters to functions with the export qualifier. ispc `` prohibits parameters to exported functions to have varying type unless the parameter is of pointer type.  (That is, ``varying float isn't allowed, but varying float * uniform (uniform pointer to varying float) is permitted.) Care must be taken by the programmer to ensure that the data being accessed through any pointers to varying data has the correct organization.

Similarly, struct types shared with the application can also have embedded pointers.

// C code
struct Foo {
    float *foo, *bar;
};

On the ispc side, the corresponding struct declaration is:

// ispc
struct Foo {
    float * uniform foo, * uniform bar;
};

If a pointer to a varying struct type appears in an exported function, the generated header file will have a definition like (for 8-wide SIMD):

// C/C++ code
struct Node {
  int count[8];
  float pos[3][8];
};

In the case of multiple target compilation, ispc will generate multiple header files and a "general" header file with definitions for multiple sizes. Any pointers to varyings in exported functions will be rewritten as void *. At runtime, the ispc dispatch mechanism will cast these pointers to the appropriate types. Programmers can provide C/C++ code with a mechanism to determine the gang width used at runtime by ispc by creating an exported function that simply returns the value of programCount. An example of such a function is provided in the file examples/util/util.isph included in the ispc distribution.

There is one subtlety related to data layout to be aware of: ispc stores uniform short-vector types in memory with their first element at the machine's natural vector alignment (i.e. 16 bytes for a target that is using Intel® SSE, and so forth.) This implies that these types will have different layout on different compilation targets. As such, applications should in general avoid accessing uniform short vector types from C/C++ application code if possible.

Data Alignment and Aliasing

There are two important constraints that must be adhered to when passing pointers from the application to ispc programs.

The first is that it is required that it be valid to read memory at the first element of any array that is passed to ispc. In practice, this should just happen naturally, but it does mean that it is illegal to pass a NULL pointer as a parameter to a ispc function called from the application.

The second constraint is that pointers and references in ispc programs must not alias. The ispc compiler assumes that different pointers can't end up pointing to the same memory location, either due to having the same initial value, or through array indexing in the program as it executed.

This aliasing constraint also applies to reference parameters to functions. Given a function like:

void func(int &a, int &b) {
    a = 0;
    if (b == 0) { ... }
}

Then the same variable must not be passed to func(). This is another case of aliasing, and if the caller calls the function as func(x, x), it's not guaranteed that the if test will evaluate to true, due to the compiler's requirement of no aliasing.

(In the future, ispc will have a mechanism to indicate that pointers may alias.)

Restructuring Existing Programs to Use ISPC

ispc is designed to enable you to incorporate SPMD parallelism into existing code with minimal modification; features like the ability to share memory and data structures between C/C++ and ispc code and the ability to directly call back and forth between ispc and C/C++ are motivated by this. These features also make it easy to incrementally transform a program to use ispc; the most computationally-intensive localized parts of the computation can be transformed into ispc code while the remainder of the system is left as is.

For a given section of code to be transitioned to run in ispc, the next question is how to parallelize the computation. Generally, there will be obvious loops inside which a large amount of computation is done ("for each ray", "for each pixel", etc.) Mapping these to the SPMD computational style is often effective.

Carefully choose how to do the exact mapping of computation to SPMD program instances. This choice can impact the mix of gather/scatter memory access versus coherent memory access, for example. (See more on this topic in the ispc Performance Tuning Guide.) This decision can also impact the coherence of control flow across the running SPMD program instances, which can also have a significant effect on performance; in general, creating groups of work that will tend to do similar computation across the SPMD program instances improves performance.

Optimization Notice

Intel compilers, associated libraries and associated development tools may include or utilize options that optimize for instruction sets that are available in both Intel and non-Intel microprocessors (for example SIMD instruction sets), but do not optimize equally for non-Intel microprocessors. In addition, certain compiler options for Intel compilers, including some that are not specific to Intel micro-architecture, are reserved for Intel microprocessors. For a detailed description of Intel compiler options, including the instruction sets and specific microprocessors they implicate, please refer to the "Intel Compiler User and Reference Guides" under "Compiler Options." Many library routines that are part of Intel compiler products are more highly optimized for Intel microprocessors than for other microprocessors. While the compilers and libraries in Intel compiler products offer optimizations for both Intel and Intel-compatible microprocessors, depending on the options you select, your code and other factors, you likely will get extra performance on Intel microprocessors.

Intel compilers, associated libraries and associated development tools may or may not optimize to the same degree for non-Intel microprocessors for optimizations that are not unique to Intel microprocessors. These optimizations include Intel® Streaming SIMD Extensions 2 (Intel® SSE2), Intel® Streaming SIMD Extensions 3 (Intel® SSE3), and Supplemental Streaming SIMD Extensions 3 (Intel SSSE3) instruction sets and other optimizations. Intel does not guarantee the availability, functionality, or effectiveness of any optimization on microprocessors not manufactured by Intel. Microprocessor-dependent optimizations in this product are intended for use with Intel microprocessors.

While Intel believes our compilers and libraries are excellent choices to assist in obtaining the best performance on Intel and non-Intel microprocessors, Intel recommends that you evaluate other compilers and libraries to determine which best meet your requirements. We hope to win your business by striving to offer the best performance of any compiler or library; please let us know if you find we do not.