CODEWITHSUNDEEP
c++gccavxfma
Violet Giraffe
Violet Giraffe

Reputation: 33627

Using AVX with GCC: __builtin_ia32_addpd256 not declared

If I #include <immintrin.h> I get this error:

error: '__builtin_ia32_addpd256' was not declared in this scope

I have defined __AVX__ and __FMA__ macros to make AVX avilable, but apparently this isn't enough. There is no error if I use compiler flag -mavx instead of the macros, but that solution is not acceptable. So, what else should I define to use AVX?

Upvotes: 0

Views: 3635

Answers (2)

sergfc
sergfc

Reputation: 61

The proper solution might be to have a specific file that contains the processor specific intrinsic. And you set -mavx -mfma options only to this file. The program itself determine which version to call at runtime.

I use GCC helpers to get the best optimized version at runtime.

func_avx_fma.c

void domagic_avx_fma(...) {}

func_general.c

void domagic_general(...) {}

helper.c

void domagic_avx_fma(...);
void domagic_general(...);
typedef void (*domagic_func_t)(...);
domagic_func_t resolve_domagic()
{
     __builtin_cpu_init();
     if (__builtin_cpu_supports("avx") && __builtin_cpu_supports("fma")) {
         return domagic_avx_fma;
     }
     return domagic_general;
}
void domagic(...) __attribute__ ((ifunc ("resolve_domagic")));

program.c

void domagic(...);
int main() { 
    domagic(...);
}

To compile

$ gcc -c func_avx_fma.c -o func_avx_fma.o -O3 -mfma -mavx
$ gcc -c func_general.c -o func_general.o -O3
$ gcc -c helper.c -o helper.o
$ ...

This approach works great on x86 (x86_64) but not all targets support these helpers

Upvotes: 1

Paul R
Paul R

Reputation: 213210

You shouldn't be defining __AVX__ and __FMA__ yourself - these get defined automatically when you enable the correct compiler options, e.g.

gcc -Wall -mavx ...

You can check this yourself if you're interested:

No AVX:

$ gcc -dM -E - < /dev/null | egrep "AVX|FMA"
$

AVX:

$ gcc -mavx -dM -E - < /dev/null | egrep "AVX|FMA"
#define __AVX__ 1
$

AVX + FMA:

$ gcc -mavx -mfma -dM -E - < /dev/null | egrep "AVX|FMA"
#define __AVX__ 1
#define __FMA__ 1
$

Upvotes: 5

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