_allow_cpu_features

Intel® C++ Compiler Classic Developer Guide and Reference

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ID 767249

Date 12/16/2022

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_allow_cpu_features

Tells the compiler that the code region may be targeted for processors with the specified features. The compiler may then generate optimized code for the specified features.

Syntax

extern void _allow_cpu_features(unsigned __int64);

Arguments

unsigned __int64

an unsigned __int64 bitset representing one or more cpuid features:

_FEATURE_GENERIC_IA32

_FEATURE_FPU

_FEATURE_CMOV

_FEATURE_MMX

_FEATURE_FXSAVE

_FEATURE_SSE

_FEATURE_SSE2

_FEATURE_SSE3

_FEATURE_SSSE3

_FEATURE_SSE4_1

_FEATURE_SSE4_2

_FEATURE_MOVBE

_FEATURE_POPCNT

_FEATURE_PCLMULQDQ

_FEATURE_AES

_FEATURE_F16C

_FEATURE_AVX

_FEATURE_RDRND

_FEATURE_FMA

_FEATURE_BMI

_FEATURE_LZCNT

_FEATURE_HLE

_FEATURE_RTM

_FEATURE_AVX2

_FEATURE_ADX

_FEATURE_RDSEED

_FEATURE_AVX512DQ

_FEATURE_AVX512F

_FEATURE_AVX512ER

_FEATURE_AVX512PF

_FEATURE_AVX512CD

_FEATURE_AVX512BW

_FEATURE_AVX512VL

_FEATURE_SHA

_FEATURE_MPX

_FEATURE_AVX512IFMA52

_FEATURE_AVX512VBMI

_FEATURE_AVX512_4FMAPS

_FEATURE_AVX512_4VNNIW

Description

Use this intrinsic function to use the specified processor feature at a code block level. The function only affects the scope of the code following the function call. Ensure that the code block will run only on processors with the specified features. If the code runs on a processor without the specified feature, the program may fail with an illegal instruction exception.

The function accepts a single argument that is a bitmask. In cases where one ISA depends on another, the higher ISA typically implies the lower. For example, the following arguments produce the same assembly code:

_FEATURE_SSE2|_FEATURE_AVX|_FEATURE_AVX512F
_FEATURE_AVX512F

The argument can only add features to those specified by the [Q]x or -m (Linux* and macOS) or /arch (Windows*) options, it cannot remove features.

This function does not itself cause the compiler to generate multiple code paths. To do that, you need to use _may_i_use_cpu_feature().

NOTE:

See the Release Notes for the latest information about this function.

To use specified processor features at a function level, use the cpu_dispatch or the cpu_specific attribute or the optimization_parameter pragma.

To use specified processor features at the file level, use the [Q]x compiler option.

The following example demonstrates how to use this intrinsic function to allow the compiler to generate the necessary code to use the Advanced Vector Extensions (AVX) and Streaming SIMD Extensions 2 (SSE2) features in the processor.

#include <string.h>
#include <immintrin.h> 

#define MAXIMGS            20
#define MAXNAME            512

typedef struct {
  int x;                  /* image X axis size              */
  int y;                  /* image Y axis size              */
  int bpp;                /* image bits */
  char name[MAXNAME];     /* image full filename */
  unsigned char * data;   /* pointer to raw byte image data */
} rawimage;

extern rawimage * imagelist[MAXIMGS];
extern int numimages;

rawimage* CreateImage(char * filename) 
{ 
       rawimage* newimage = NULL;
       int i, len, intable;

       intable=0;
       if (numimages!=0) {

             _allow_cpu_features(_FEATURE_SSE2 | _FEATURE_AVX);
             for (i=0; i<numimages; i++) {
                    if (!strcmp(filename, imagelist[i]->name)) {
                           newimage=imagelist[i];
                           intable=1;
                    }
             }
       }

       if (!intable) {
             newimage=(rawimage *)malloc(sizeof(rawimage));
             if (newimage != NULL) {
                    strcpy(newimage->name, filename);

                    imagelist[numimages]=newimage;  /* add new one to the table       */ 
                    numimages++;                    /* increment the number of images */
             }
       }

       return newimage;
}

Returns

Returns nothing.

Parent topic: Intrinsics for All Intel® Architectures

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