[Bug c++/80859] Performance Problems with OpenMP 4.5 support

["jakub at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80859

--- Comment #14 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Thorsten Kurth from comment #13)
> the compiler options are just -fopenmp. I am sure it does not have to do
> anything with vectorization as I compare the code runtime with and without
> the target directives and thus vectorization should be the same between
> them. The remaining OpenMP sections are the same. In our work we have not
> seen 10x because of insufficient vectorization, it is usually because of
> cache locality but that is the same for OMP 4.5 and OMP 3 because the loops
> are not touched.
> I do not specify an ISA choice, but I will try specifying KNL now and will
> tell you what the compiler is going to do.

The compiler doesn't optimize by default (i.e. default is -O0), so if you are
measuring -O0 -fopenmp performance or code size, that is something that is
completely uninteresting.  For -O0 the most important is compilation speed, not
quality of generated code.  For runtime performance of generated code only -O2,
-O3 or -Ofast are optimization levels that make sense.