What's the proper way to use different versions of SSE intrinsics in GCC?

Question

I will ask my question by giving an example. Now I have a function called do_something().

It has three versions: do_something(), do_s

Answer 1

If you are using GCC 4.9 or above on an i686 or x86_64 machine, then you are supposed to be able to use intrinsics regardless of your -march=XXX and -mXXX options. You could write your do_something() accordingly:

void do_something()
{
    byte temp[18];

    if (HasSSE2())
    {
        const __m128i i = _mm_loadu_si128((const __m128i*)(ptr));
        ...
    }
    else if (HasSSSE3())
    {
        const __m128i MASK = _mm_set_epi8(12,13,14,15, 8,9,10,11, 4,5,6,7, 0,1,2,3);
        _mm_storeu_si128(reinterpret_cast<__m128i*>(temp),
           _mm_shuffle_epi8(_mm_loadu_si128((const __m128i*)(ptr)), MASK));
    }
    else
    {
        // Do the byte swap/endian reversal manually
        ...
    }
}

You have to supply HasSSE2(), HasSSSE3() and friends. Also see Intrinsics for CPUID like informations?.

Also see GCC Issue 57202 - Please make the intrinsics headers like immintrin.h be usable without compiler flags. But I don't believe the feature works. I regularly encounter compile failures because GCC does not make intrinsics available.