[Bug target/69471] "-march=native" unintentionally breaks further -march/-mtune flags

[thiago at kde dot org]

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.