Frequently Asked Questions About Intel® ISPC

This document includes a number of frequently (and not frequently) asked questions about ispc, the Intel® Implicit SPMD Program Compiler (Intel® ISPC). The source to this document is in the file docs/faq.rst in the ispc source distribution.

Understanding ispc's Output

How can I see the assembly language generated by ispc?

The --emit-asm flag causes assembly output to be generated. If the -o command-line flag is also supplied, the assembly is stored in the given file, or printed to standard output if - is specified for the filename. For example, given the simple ispc program:

export uniform int foo(uniform int a, uniform int b) {
    return a+b;

If the SSE4 target is used, then the following assembly is printed:

        addl    %esi, %edi
        movl    %edi, %eax

How can I have the assembly output be printed using Intel assembly syntax?

The ispc compiler is currently only able to emit assembly with AT+T syntax, where the destination operand is the last operand after an instruction. If you'd prefer Intel assembly output, one option is to use Agner Fog's objconv tool: have ispc emit a native object file and then use objconv to disassemble it, specifying the assembler syntax that you prefer. objconv is available for download here.

Why are there multiple versions of exported ispc functions in the assembly output?

Two generations of all functions qualified with export are generated: one of them is for being be called by other ispc functions, and the other is to be called by the application. The application callable function has the original function's name, while the ispc-callable function has a mangled name that encodes the types of the function's parameters.

The crucial difference between these two functions is that the application-callable function doesn't take a parameter encoding the current execution mask, while ispc-callable functions have a hidden mask parameter. An implication of this difference is that the export function starts with the execution mask "all on". This allows a number of improvements in the generated code, particularly on architectures that don't have support for masked load and store instructions.

As an example, consider this short function, which loads a vector's worth values from two arrays in memory, adds them, and writes the result to an output array.

export void foo(uniform float a[], uniform float b[],
                uniform float result[]) {
    float aa = a[programIndex], bb = b[programIndex];
    result[programIndex] = aa+bb;

Here is the assembly code for the application-callable instance of the function.

        movups        (%rsi), %xmm1
        movups        (%rdi), %xmm0
        addps         %xmm1, %xmm0
        movups        %xmm0, (%rdx)

And here is the assembly code for the ispc-callable instance of the function.

        movmskps      %xmm0, %eax
        cmpl          $15, %eax
        je            LBB0_3
        testl         %eax, %eax
        jne           LBB0_4
        movups        (%rsi), %xmm1
        movups        (%rdi), %xmm0
        addps         %xmm1, %xmm0
        movups        %xmm0, (%rdx)
####  Code elided; handle mixed mask case..

There are a few things to notice in this code. First, the current program mask is coming in via the %xmm0 register and the initial few instructions in the function essentially check to see if the mask is all on or all off. If the mask is all on, the code at the label LBB0_3 executes; it's the same as the code that was generated for _foo above. If the mask is all off, then there's nothing to be done, and the function can return immediately.

In the case of a mixed mask, a substantial amount of code is generated to load from and then store to only the array elements that correspond to program instances where the mask is on. (This code is elided below). This general pattern of having two-code paths for the "all on" and "mixed" mask cases is used in the code generated for almost all but the most simple functions (where the overhead of the test isn't worthwhile.)

How can I more easily see gathers and scatters in generated assembly?

Because CPU vector ISAs don't have native gather and scatter instructions, these memory operations are turned into sequences of a series of instructions in the code that ispc generates. In some cases, it can be useful to see where gathers and scatters actually happen in code; there is an otherwise undocumented command-line flag that provides this information.

Consider this simple program:

void set(uniform int a[], int value, int index) {
    a[index] = value;

When compiled normally to the SSE4 target, this program generates this extensive code sequence, which makes it more difficult to see what the program is actually doing.

        pmulld        LCPI0_0(%rip), %xmm1
        movmskps      %xmm2, %eax
        testb         $1, %al
        je            LBB0_2
        movd          %xmm1, %ecx
        movd          %xmm0, (%rcx,%rdi)
        testb         $2, %al
        je            LBB0_4
        pextrd        $1, %xmm1, %ecx
        pextrd        $1, %xmm0, (%rcx,%rdi)
        testb         $4, %al
        je            LBB0_6
        pextrd        $2, %xmm1, %ecx
        pextrd        $2, %xmm0, (%rcx,%rdi)
        testb        $8, %al
        je            LBB0_8
        pextrd        $3, %xmm1, %eax
        pextrd        $3, %xmm0, (%rax,%rdi)

If this program is compiled with the --opt=disable-handle-pseudo-memory-ops command-line flag, then the scatter is left as an unresolved function call. The resulting program won't link without unresolved symbols, but the assembly output is much easier to understand:

        movaps        %xmm0, %xmm3
        pmulld        LCPI0_0(%rip), %xmm1
        movdqa        %xmm1, %xmm0
        movaps        %xmm3, %xmm1
        jmp        ___pseudo_scatter_base_offsets32_32 ## TAILCALL

Language Details

What is the difference between "int *foo" and "int foo[]"?

In C and C++, declaring a function to take a parameter int *foo and int foo[] results in the same type for the parameter. Both are pointers to integers. In ispc, these are different types. The first one is a varying pointer to a uniform integer value in memory, while the second results in a uniform pointer to the start of an array of varying integer values in memory.

To understand why the first is a varying pointer to a uniform integer, first recall that types without explicit rate qualifiers (uniform, varying, or soa<>) are varying by default. Second, recall from the discussion of pointer types in the ispc User's Guide that pointed-to types without rate qualifiers are uniform by default. (This second rule is discussed further below, in Why are pointed-to types "uniform" by default?.) The type of int *foo follows from these.

Conversely, in a function body, int foo[10] represents a declaration of a 10-element array of varying int values. In that we'd certainly like to be able to pass such an array to a function that takes a int [] parameter, the natural type for an int [] parameter is a uniform pointer to varying integer values.

In terms of compatibility with C/C++, it's unfortunate that this distinction exists, though any other set of rules seems to introduce more awkwardness than this one. (Though we're interested to hear ideas to improve these rules!).

Why are pointed-to types "uniform" by default?

In ispc, types without rate qualifiers are "varying" by default, but types pointed to by pointers without rate qualifiers are "uniform" by default. Why this difference?

int foo;  // no rate qualifier, "varying int".
uniform int *foo;  // pointer type has no rate qualifier, pointed-to does.
                   // "varying pointer to uniform int".
int *foo;  // neither pointer type nor pointed-to type ("int") have
           // rate qualifiers. Pointer type is varying by default,
           // pointed-to is uniform. "varying pointer to uniform int".
varying int *foo;   // varying pointer to varying int

The first rule, having types without rate qualifiers be varying by default, is a default that keeps the number of "uniform" or "varying" qualifiers in ispc programs low. Most ispc programs use mostly "varying" variables, so this rule allows most variables to be declared without also requiring rate qualifiers.

On a related note, this rule allows many C/C++ functions to be used to define equivalent functions in the SPMD execution model that ispc provides with little or no modification:

// scalar add in C/C++, SPMD/vector add in ispc
int add(int a, int b) { return a + b; }

This motivation also explains why uniform int *foo represents a varying pointer; having pointers be varying by default if they don't have rate qualifiers similarly helps with porting code from C/C++ to ispc.

The tricker issue is why pointed-to types are "uniform" by default. In our experience, data in memory that is accessed via pointers is most often uniform; this generally includes all data that has been allocated and initialized by the C/C++ application code. In practice, "varying" types are more generally (but not exclusively) used for local data in ispc functions. Thus, making the pointed-to type uniform by default leads to more concise code for the most common cases.

What am I getting an error about assigning a varying lvalue to a reference type?

Given code like the following:

uniform float a[...];
int index = ...;
float &r = a[index];

ispc issues the error "Initializer for reference-type variable "r" must have a uniform lvalue type.". The underlying issue stems from how references are represented in the code generated by ispc. Recall that ispc supports both uniform and varying pointer types--a uniform pointer points to the same location in memory for all program instances in the gang, while a varying pointer allows each program instance to have its own pointer value.

References are represented a pointer in the code generated by ispc, though this is generally opaque to the user; in ispc, they are specifically uniform pointers. This design decision was made so that given code like this:

extern void func(float &val);
float foo = ...;

Then the reference would be handled efficiently as a single pointer, rather than unnecessarily being turned into a gang-size of pointers.

However, an implication of this decision is that it's not possible for references to refer to completely different things for each of the program instances. (And hence the error that is issued). In cases where a unique per-program-instance pointer is needed, a varying pointer should be used instead of a reference.


How can I supply an initial execution mask in the call from the application?

Recall that when execution transitions from the application code to an ispc function, all of the program instances are initially executing. In some cases, it may desired that only some of them are running, based on a data-dependent condition computed in the application program. This situation can easily be handled via an additional parameter from the application.

As a simple example, consider a case where the application code has an array of float values and we'd like the ispc code to update just specific values in that array, where which of those values to be updated has been determined by the application. In C++ code, we might have:

int count = ...;
float *array = new float[count];
bool *shouldUpdate = new bool[count];
// initialize array and shouldUpdate
ispc_func(array, shouldUpdate, count);

Then, the ispc code could process this update as:

export void ispc_func(uniform float array[], uniform bool update[],
                      uniform int count) {
    foreach (i = 0 ... count) {
        cif (update[i] == true)
            // update array[i+programIndex]...

(In this case a "coherent" if statement is likely to be worthwhile if the update array will tend to have sections that are either all-true or all-false.)

How can I generate a single binary executable with support for multiple instruction sets?

ispc can also generate output that supports multiple target instruction sets, also generating code that chooses the most appropriate one at runtime if multiple targets are specified with the --target command-line argument.

For example, if you run the command:

ispc foo.ispc -o foo.o --target=sse2,sse4-x2,avx-x2

Then four object files will be generated: foo_sse2.o, foo_sse4.o, foo_avx.o, and foo.o [1]. Link all of these into your executable, and when you call a function in foo.ispc from your application code, ispc will determine which instruction sets are supported by the CPU the code is running on and will call the most appropriate version of the function available.

[1]Similarly, if you choose to generate assembly language output or LLVM bitcode output, multiple versions of those files will be created.

In general, the version of the function that runs will be the one in the most general instruction set that is supported by the system. If you only compile SSE2 and SSE4 variants and run on a system that supports AVX, for example, then the SSE4 variant will be executed. If the system is not able to run any of the available variants of the function (for example, trying to run a function that only has SSE4 and AVX variants on a system that only supports SSE2), then the standard library abort() function will be called.

One subtlety is that all non-static global variables (if any) must have the same size and layout with all of the targets used. For example, if you have the global variables:

uniform int foo[2*programCount];
int bar;

and compile to both SSE2 and AVX targets, both of these variables will have different sizes (the first due to program count having the value 4 for SSE2 and 8 for AVX, and the second due to varying types having different numbers of elements with the two targets--essentially the same issue as the first.) ispc issues an error in this case.

How can I determine at run-time which vector instruction set's instructions were selected to execute?

ispc doesn't provide any API that allows querying which vector ISA's instructions are running when multi-target compilation was used. However, this can be solved in "user space" by writing a small helper function. Specifically, if you implement a function like this

export uniform int isa() {
#if defined(ISPC_TARGET_SSE2)
    return 0;
#elif defined(ISPC_TARGET_SSE4)
    return 1;
#elif defined(ISPC_TARGET_AVX)
    return 2;
    return -1;

And then call it from your application code at runtime, it will return 0, 1, or 2, depending on which target's instructions are running.

The way this works is a little surprising, but it's a useful trick. Of course the preprocessor #if checks are all compile-time only operations. What's actually happening is that the function is compiled multiple times, once for each target, with the appropriate ISPC_TARGET preprocessor symbol set. Then, a small dispatch function is generated for the application to actually call. This dispatch function in turn calls the appropriate version of the function based on the CPU of the system it's executing on, which in turn returns the appropriate value.

In a similar fashion, it's possible to find out at run-time the value of programCount for the target that's actually being used.

export uniform int width() { return programCount; }

Is it possible to inline ispc functions in C/C++ code?

If you're willing to use the clang C/C++ compiler that's part of the LLVM tool suite, then it is possible to inline ispc code with C/C++ (and conversely, to inline C/C++ calls in ispc). Doing so can provide performance advantages when calling out to short functions written in the "other" language. Note that you don't need to use clang to compile all of your C/C++ code, but only for the files where you want to be able to inline. In order to do this, you must have a full installation of LLVM version 3.0 or later, including the clang compiler.

The basic approach is to have the various compilers emit LLVM intermediate representation (IR) code and to then use tools from LLVM to link together the IR from the compilers and then re-optimize it, which gives the LLVM optimizer the opportunity to do additional inlining and cross-function optimizations. If you have source files foo.ispc and foo.cpp, first emit LLVM IR:

ispc --emit-llvm -o foo_ispc.bc foo.ispc
clang -O2 -c -emit-llvm -o foo_cpp.bc foo.cpp

Next, link the two IR files into a single file and run the LLVM optimizer on the result:

llvm-link foo_ispc.bc foo_cpp.bc -o - | opt -O3 -o foo_opt.bc

And finally, generate a native object file:

llc -filetype=obj foo_opt.bc -o foo.o

This file can in turn be linked in with the rest of your object files when linking your applicaiton.

(Note that if you're using the AVX instruction set, you must provide the -mattr=+avx flag to llc.)

Why is it illegal to pass "varying" values from C/C++ to ispc functions?

If any of the types in the parameter list to an exported function is "varying" (including recursively, and members of structure types, etc.), then ispc will issue an error and refuse to compile the function:

% echo "export int add(int x) { return ++x; }" | ispc
<stdin>:1:12: Error: Illegal to return a "varying" type from exported function "foo"
<stdin>:1:20: Error: Varying parameter "x" is illegal in an exported function.

While there's no fundamental reason why this isn't possible, recall the definition of "varying" variables: they have one value for each program instance in the gang. As such, the number of values and amount of storage required to represent a varying variable depends on the gang size (i.e. programCount), which can have different values depending on the compilation target.

ispc therefore prohibits passing "varying" values between the application and the ispc program in order to prevent the application-side code from depending on a particular gang size, in order to encourage portability to different gang sizes. (A generally desirable programming practice.)

For cases where the size of data is actually fixed from the application side, the value can be passed via a pointer to a short uniform array, as follows:

export void add4(uniform int ptr[4]) {
    foreach (i = 0 ... 4)

On the 4-wide SSE instruction set, this compiles to a single vector add instruction (and associated move instructions), while it still also efficiently computes the correct result on 8-wide AVX targets.

Programming Techniques

What primitives are there for communicating between SPMD program instances?

The broadcast(), rotate(), and shuffle() standard library routines provide a variety of mechanisms for the running program instances to communicate values to each other during execution. Note that there's no need to synchronize the program instances before communicating between them, due to the synchronized execution model of gangs of program instances in ispc.

How can a gang of program instances generate variable amounts of output efficiently?

It's not unusual to have a gang of program instances where each program instance generates a variable amount of output (perhaps some generate no output, some generate one output value, some generate many output values and so forth), and where one would like to have the output densely packed in an output array. The exclusive_scan_add() function from the standard library is quite useful in this situation.

Consider the following function:

uniform int func(uniform float outArray[], ...) {
   int numOut = ...;  // figure out how many to be output
   float outLocal[MAX_OUT]; // staging area

   // each program instance in the gang puts its results in
   //  outLocal[0], ..., outLocal[numOut-1]

   int startOffset = exclusive_scan_add(numOut);
   for (int i = 0; i < numOut; ++i)
       outArray[startOffset + i] = outLocal[i];
   return reduce_add(numOut);

Here, each program instance has computed a number, numOut, of values to output, and has stored them in the outLocal array. Assume that four program instances are running and that the first one wants to output one value, the second two values, and the third and fourth three values each. In this case, exclusive_scan_add() will return the values (0, 1, 3, 6) to the four program instances, respectively.

The first program instance will then write its one result to outArray[0], the second will write its two values to outArray[1] and outArray[2], and so forth. The reduce_add() call at the end returns the total number of values that all of the program instances have written to the array.

FIXME: add discussion of foreach_active as an option here once that's in

Is it possible to use ispc for explicit vector programming?

The typical model for programming in ispc is an implicit parallel model, where one writes a program that is apparently doing scalar computation on values and the program is then vectorized to run in parallel across the SIMD lanes of a processor. However, ispc also has some support for explicit vector unit programming, where the vectorization is explicit. Some computations may be more effectively described in the explicit model rather than the implicit model.

This support is provided via uniform instances of short vectors Specifically, if this short program

export uniform float<8> madd(uniform float<8> a, uniform float<8> b,
                             uniform float<8> c) {
    return a + b * c;

is compiled with the AVX target, ispc generates the following assembly:

    vmulps  %ymm2, %ymm1, %ymm1
    vaddps  %ymm0, %ymm1, %ymm0

(And similarly, if compiled with a 4-wide SSE target, two mulps and two addps instructions are generated, and so forth.)

Note that ispc doesn't currently support control-flow based on uniform short vector types; it is thus not possible to write code like:

export uniform int<8> count(uniform float<8> a, uniform float<8> b) {
    uniform int<8> sum = 0;
    while (a++ < b)

How can I debug my ispc programs using Valgrind?

The valgrind memory checker is an extremely useful memory checker for Linux and OSX; it detects a range of memory errors, including accessing memory after it has been freed, accessing memory beyond the end of an array, accessing uninitialized stack variables, and so forth. In general, applications that use ispc code run with valgrind without modification and valgrind will detect the same range of memory errors in ispc code that it does in C/C++ code.

One issue to be aware of is that until recently, valgrind only supported the SSE2 vector instructions; if you are using a version of valgrind older than the 3.7.0 release (5 November 2011), you should compile your ispc programs with --target=sse2 before running them through valgrind. (Note that if no target is specified, then ispc chooses a target based on the capabilities of the system you're running ispc on.) If you run an ispc program that uses instructions that valgrind doesn't support, you'll see an error message like:

vex amd64->IR: unhandled instruction bytes: 0xC5 0xFA 0x10 0x0 0xC5 0xFA 0x11 0x84
==46059== valgrind: Unrecognised instruction at address 0x100002707.

The just-released valgrind 3.7.0 adds support for the SSE4.2 instruction set; if you're using that version (and your system supports SSE4.2), then you can use --target=sse4 when compiling to run with valgrind.

Note that valgrind does not yet support programs that use the AVX instruction set.

foreach statements generate more complex assembly than I'd expect; what's going on?

Given a simple foreach loop like the following:

void foo(uniform float a[], uniform int count) {
    foreach (i = 0 ... count)
        a[i] *= 2;

the ispc compiler generates approximately 40 instructions--why isn't the generated code simpler?

There are two main components to the code: one handles programCount-sized chunks of elements of the array, and the other handles any excess elements at the end of the array that don't completely fill a gang. The code for the main loop is essentially what one would expect: a vector of values are loaded from the array, the multiply is done, and the result is stored.

LBB0_2:                                 ## %foreach_full_body
    movslq  %edx, %rdx
    vmovups (%rdi,%rdx), %ymm1
    vmulps  %ymm0, %ymm1, %ymm1
    vmovups %ymm1, (%rdi,%rdx)
    addl    $32, %edx
    addl    $8, %eax
    cmpl    %ecx, %eax
    jl      LBB0_2

Then, there is a sequence of instructions that handles any additional elements at the end of the array. (These instructions don't execute if there aren't any left-over values to process, but they do lengthen the amount of generated code.)

## BB#4:                                ## %partial_inner_only
      vmovd   %eax, %xmm0
      vinsertf128     $1, %xmm0, %ymm0, %ymm0
      vpermilps       $0, %ymm0, %ymm0 ## ymm0 = ymm0[0,0,0,0,4,4,4,4]
      vextractf128    $1, %ymm0, %xmm3
      vmovd   %esi, %xmm2
      vmovaps LCPI0_1(%rip), %ymm1
      vextractf128    $1, %ymm1, %xmm4
      vpaddd  %xmm4, %xmm3, %xmm3
      # ....
      vmulps  LCPI0_0(%rip), %ymm1, %ymm1
      vmaskmovps      %ymm1, %ymm0, (%rdi,%rax)

If you know that the number of elements to be processed will always be an exact multiple of the 8, 16, etc., then adding a simple assignment to count like the one below gives the compiler enough information to be able to eliminate the code for the additional array elements.

void foo(uniform float a[], uniform int count) {
    // This assignment doesn't change the value of count
    // if it's a multiple of 16, but it gives the compiler
    // insight into this fact, allowing for simpler code to
    // be generated for the foreach loop.
    count = (count & ~(16-1));
    foreach (i = 0 ... count)
        a[i] *= 2;

With this new version of foo(), only the code for the first loop above is generated.

How do I launch an individual task for each active program instance?

Recall from the discussion of "launch" in the ispc User's Guide that a launch statement launches a single task corresponding to a single gang of executing program instances, where the indices of the active program instances are the same as were active when the launch statement executed.

In some situations, it's desirable to be able to launch an individual task for each executing program instance. For example, we might be performing an iterative computation where a subset of the program instances determine that an item they are responsible for requires additional processing.

bool itemNeedsMoreProcessing(int);
int itemNum = ...;
if (itemNeedsMoreProcessing(itemNum)) {
    // do additional work

For performance reasons, it may be desirable to apply an entire gang's worth of comptuation to each item that needs additional processing; there may be available parallelism in this computation such that we'd like to process each of the items with SPMD computation.

In this case, the foreach_active and unmasked constructs can be applied together to accomplish this goal.

// do additional work
task void doWork(uniform int index);
foreach_active (index) {
    unmasked {
        launch doWork(extract(itemNum, index));

Recall that the body of the foreach_active loop runs once for each active program instance, with each active program instance's programIndex value available in index in the above. In the loop, we can re-establish an "all on" execution mask, enabling execution in all of the program instances in the gang, such that execution in doWork() starts with all instances running. (Alternatively, the unmasked block could be in the definition of doWork().)