GCC 8 doesn't generate AVX512 instructions for skylake

[marcin.krotkiewski]

Hi, all!

I compile the STREAM benchmark using gcc 8.1:

gcc -march=skylake-avx512 -O3 -fopenmp stream.c -o stream

The generated binary doesn't use ZMM registers (AVX512), only YMM 
registers (AVX2). The same code compiled with GCC 7.2 looks as expected, 
i.e., 8-wide vector instructions are used.

Apparently Intel compiler doesn't generate ZMM instructions unless told 
so (-qopt-zmm-usage=high). John McCalpin says the reason is the extra 
heating (and the need to step down the frequency), e.g.

https://software.intel.com/en-us/forums/intel-isa-extensions/topic/747994


Does anyone know, if the same limitation is implemented in GCC 8? If so, 
is there a flag to force GCC to generate AVX512 instructions?

Thanks!