Targeting Processors Manually

Manual Processor Dispatch Programming Guidelines

Use the __declspec(cpu_dispatch(cpuid, cpuid,...)) syntax in your code to provide a list of targeted processors along with an empty function body/function stub. Use the __declspec(cpu_specific(cpuid)) in your code to declare each function version targeted at particular type[s] of processors.

The following table lists the values for cpuid:

Note

If no other matching Intel processor type is detected, the generic version of the function is executed. If you want the program to execute on non-Intel processors, a generic function version must be provided. You can control the degree of optimization of the generic function version and the processor features that it assumes.

The cpuid attributes are not case sensitive. The body of a function declared with __declspec(cpu_dispatch) must be empty, and is referred to as a stub (an empty-bodied function).

The following example illustrates how the cpu_dispatch and cpu_specific keywords can be used to create function versions for the 2nd generation Intel Core processor family, for the Intel Core processor family, for the Intel Core 2 Duo processor family, and for other Intel and compatible, non-Intel processors. Each processor-specific function body might contain processor-specific intrinsic functions, or it might be placed in a separate source file and compiled with a processor-specific compiler option.

#include <stdio.h>

// need to create specific function versions for the following processors:

__declspec(cpu_dispatch(core_2nd_gen_avx, core_i7_sse4_2, core_2_duo_ssse3, generic ))

void dispatch_func() {};      //  stub that will call the appropriate specific function version   

__declspec(cpu_specific(core_2nd_gen_avx))

void dispatch_func() {

printf("\nCode for 2nd generation Intel Core processors with support for AVX goes here\n");

}

__declspec(cpu_specific(core_i7_sse4_2))

void dispatch_func() {

printf("\nCode for Intel Core processors with support for SSE4.2 goes here\n");

}

__declspec(cpu_specific(core_2_duo_ssse3))

void dispatch_func() {

printf("\nCode for Intel Core 2 Duo processors with support for SSSE3 goes here\n");

}

__declspec(cpu_specific(generic))

void dispatch_func() {

printf("\nCode for non-Intel processors and generic Intel processors goes here\n");

}

int main() { 

dispatch_func();

printf("Return from dispatch_func\n");

return 0;

}

Considerations

Before using manual dispatch, consider whether the benefits outweigh the additional effort and possible performance issues. You may encounter any one or all of the following issues when using manual processor dispatch in your code:

• some types of inlining may be disabled

• code and executable sizes increase considerably

• additional performance overhead may be introduced because of additional function calls

Test your application on all targeted platforms before release.

Using Pragmas to Target Processors Manually

You can also use the #pragma intel optimize target_arch, to flag those routines in your code that you want to execute on specified types of Intel® processors. This pragma controls the -x or /Qx option at a routine level, overriding the option values specified at the command-line. The pragma uses the same values as the -x or /Qxoption to target processors. The following example illustrates how to use the pragma to target a routine bar() to execute only on Intel® AVX supported processors regardless of what the command-line has specified.

#pragma intel optimize target_arch=AVX

void bar()

{

...

}