How to detect SSE/SSE2/AVX/AVX2/AVX-512/AVX-128-FMA/KCVI availability at compile-time?

Question

I'm trying to optimize some matrix computations and I was wondering if it was possible to detect at compile-time if SSE/SSE2/AVX/AVX2/AVX-512/AVX-128-FMA/KCVI^[1] is enabled by the compiler ? Ideally for GCC and Clang, but I can manage with only one of them.

I'm not sure it is possible and perhaps I will use my own macro, but I'd prefer detecting it rather and asking the user to select it.

^[1] "KCVI" stands for Knights Corner Vector Instruction optimizations. Libraries like FFTW detect/utilize these newer instruction optimizations.

What exactly do you want to test for? Do you want to test that the compiler will produce AVX instructions? It is important to keep in mind that just because the compiler is ready to produce them does not mean that the CPU your program will eventually run will also support it (even if both compilation and execution happen on the same machine). — ArjunShankar
@ArjunShankar I want to know if for instance avx was enabled during compilation with -mavx. — Baptiste Wicht
Also, note that CPU support and OS support are two different things. The CPU may support SSE, but the OS may not support SSE (which requires the OS to save XMM registers during a context switch). See, for example, Checking for SSE on OSDev wiki. — jww

Paul R Paul R · Accepted Answer · 2015-03-09T10:25:26

Most compilers will automatically define:

__SSE__
__SSE2__
__SSE3__
__AVX__
__AVX2__

etc, according to whatever command line switches you are passing. You can easily check this with gcc (or gcc-compatible compilers such as clang), like this:

$ gcc -msse3 -dM -E - < /dev/null | egrep "SSE|AVX" | sort
#define __SSE__ 1
#define __SSE2__ 1
#define __SSE2_MATH__ 1
#define __SSE3__ 1
#define __SSE_MATH__ 1

or:

$ gcc -mavx2 -dM -E - < /dev/null | egrep "SSE|AVX" | sort
#define __AVX__ 1
#define __AVX2__ 1
#define __SSE__ 1
#define __SSE2__ 1
#define __SSE2_MATH__ 1
#define __SSE3__ 1
#define __SSE4_1__ 1
#define __SSE4_2__ 1
#define __SSE_MATH__ 1
#define __SSSE3__ 1

or to just check the pre-defined macros for a default build on your particular platform:

$ gcc -dM -E - < /dev/null | egrep "SSE|AVX" | sort
#define __SSE2_MATH__ 1
#define __SSE2__ 1
#define __SSE3__ 1
#define __SSE_MATH__ 1
#define __SSE__ 1
#define __SSSE3__ 1

More recent Intel processors support AVX-512, which is not a monolithic instruction set. One can see the support available from GCC (version 6.2) for two examples below.

Here is Knights Landing:

$ gcc -march=knl -dM -E - < /dev/null | egrep "SSE|AVX" | sort
#define __AVX__ 1
#define __AVX2__ 1
#define __AVX512CD__ 1
#define __AVX512ER__ 1
#define __AVX512F__ 1
#define __AVX512PF__ 1
#define __SSE__ 1
#define __SSE2__ 1
#define __SSE2_MATH__ 1
#define __SSE3__ 1
#define __SSE4_1__ 1
#define __SSE4_2__ 1
#define __SSE_MATH__ 1
#define __SSSE3__ 1

Here is Skylake AVX-512:

$ gcc -march=skylake-avx512 -dM -E - < /dev/null | egrep "SSE|AVX" | sort
#define __AVX__ 1
#define __AVX2__ 1
#define __AVX512BW__ 1
#define __AVX512CD__ 1
#define __AVX512DQ__ 1
#define __AVX512F__ 1
#define __AVX512VL__ 1
#define __SSE__ 1
#define __SSE2__ 1
#define __SSE2_MATH__ 1
#define __SSE3__ 1
#define __SSE4_1__ 1
#define __SSE4_2__ 1
#define __SSE_MATH__ 1
#define __SSSE3__ 1

Intel has disclosed additional AVX-512 subsets (see ISA extensions). GCC (version 7) supports compiler flags and preprocessor symbols associated with the 4FMAPS, 4VNNIW, IFMA, VBMI and VPOPCNTDQ subsets of AVX-512:

for i in 4fmaps 4vnniw ifma vbmi vpopcntdq ; do echo "==== $i ====" ; gcc -mavx512$i -dM -E - < /dev/null | egrep "AVX512" | sort ; done
==== 4fmaps ====
#define __AVX5124FMAPS__ 1
#define __AVX512F__ 1
==== 4vnniw ====
#define __AVX5124VNNIW__ 1
#define __AVX512F__ 1
==== ifma ====
#define __AVX512F__ 1
#define __AVX512IFMA__ 1
==== vbmi ====
#define __AVX512BW__ 1
#define __AVX512F__ 1
#define __AVX512VBMI__ 1
==== vpopcntdq ====
#define __AVX512F__ 1
#define __AVX512VPOPCNTDQ__ 1

How to detect SSE/SSE2/AVX/AVX2/AVX-512/AVX-128-FMA/KCVI availability at compile-time?

2 Answers