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assemblygccx86clangavx

Does -mavx2 implies -mavx and -msse4.2

From what I understand every CPU that supports AVX2 supports AVX, and every CPU that supports AVX supports SSE4.2. Does that mean that just specifying -mavx2 is enough or there is a difference between:

-msse4.2 -mavx -mavx2

and

-mavx2

All I found in GCC docs is this(AVX SSE relationship):

GCC depresses SSEx instructions when -mavx is used. Instead, it generates new AVX instructions or AVX equivalence for all SSEx instructions when needed.

I presume this just means that GCC will use better AVX instructions instead of SSE instructions, so I guess this implies when you have -mavx that msse4.2 is useless, but I could not find anything wrt -mavx2 -mavx relationship.

I also looked up the clang docs, could not find anything.

Note: I am compiling C++ code, but I presume answer is same for C and C++ compilers.

Solution

  • You can look at the active flags for gcc by adding -Q --help=target to the command line.

    Pipe into files, compare. The two sets of options produced identical settings for me.