Disable AVX2 functions on non-Haswell processors

Question

I have written some AVX2 code to run on a Haswell i7 processor. The same codebase is also used on non-Haswell processors, where the same code should be replaced with their SSE e

Answer 1

If you just want to do this at compile-time then you can do this:

#ifdef __AVX2__
    // AVX2 code
#elif __SSE__
    // SSE code
#else
    // scalar code
#endif

Note that when you compile with gcc -mavx2 ... then __AVX2__ gets defined automatically. Similarly for __SSE__. (Note also that you can check what's pre-defined by your compiler for any given command line switching using the incantation gcc -dM -E -mavx2 - < /dev/null.)

If you want to do run-time dispatching though then that's a little more complicated.