title: Is there a way to enable avx2 intruction set without auto-vectorization by LLVM
body: <p>Recently I met a problem that my avx2 optimized program may crash on old machines like 2010 mac, which does not support avx2 intruction set. At the same time, I can ensure that all my avx2 code is surrounded by dynamically detection of instruction, which will not be run on an avx2-free machine.
  So i digged into this problem and found that the crash is caused by auto-vectorization conducted by llvm itself. I tried -fno-vectorize and -fno-slp-vectorize but found that once -mavx2 is set, the program will be auto vectorized.
  Is there a way to disable auto-vectorization in llvm with -mavx2 set? Because without -mavx2, my handwritten avx2 code may not be compiled successfully.</p>