https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69471
--- Comment #6 from Thiago Macieira <thiago at kde dot org> --- Clang is not affected: $ clang -dM -E -xc /dev/null -march=sandybridge -march=native | grep AVX #define __AVX2__ 1 #define __AVX__ 1 $ clang -dM -E -xc /dev/null -march=native -march=sandybridge | grep AVX #define __AVX__ 1 Instead of enabling the CPU features your CPU has, Clang tries to guess which CPU you have and will apply it. This has side-effects for non-arch-specific items like AES. ICC is similarly affected, despite claiming it isn't: $ icc -dM -E -xc /dev/null -march=sandybridge -march=native | grep AVX icc: command line warning #10121: overriding '-march=sandybridge' with '-march=native' icc: command line warning #10121: overriding '-march=sandybridge' with '-march=native' #define __AVX_I__ 1 #define __AVX__ 1 #define __AVX2__ 1 $ icc -dM -E -xc /dev/null -march=native -march=sandybridge | grep AVX icc: command line warning #10121: overriding '-march=native' with '-march=sandybridge' #define __AVX_I__ 1 #define __AVX__ 1 #define __AVX2__ 1 It says it's overriding, but doesn't override.