My C++ code uses SSE and now I want to improve it to support AVX when it is available. So I detect when AVX is available and call a function that uses AVX commands. I use Win7 SP1 + VS2010 SP1 and a CPU with AVX.
To use AVX, it is necessary to include this:
#include "immintrin.h"
and then you can use intrinsics AVX functions like _mm256_mul_ps, _mm256_add_ps etc.
The problem is that by default, VS2010 produces code that works very slowly and shows the warning:
warning C4752: found Intel(R) Advanced Vector Extensions; consider using /arch:AVX
It seems VS2010 actually does not use AVX instructions, but instead, emulates them. I added /arch:AVX to the compiler options and got good results. But this option tells the compiler to use AVX commands everywhere when possible. So my code may crash on CPU that does not support AVX!
So the question is how to make VS2010 compiler to produce AVX code but only when I specify AVX intrinsics directly. For SSE it works, I just use SSE intrinsics functions and it produce SSE code without any compiler options like /arch:SSE. But for AVX it does not work for some reason.