[Bug libfortran/51119] MATMUL slow for large matrices

[Joost.VandeVondele at mat dot ethz.ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51119

--- Comment #36 from Joost VandeVondele <Joost.VandeVondele at mat dot ethz.ch> ---
(In reply to Jerry DeLisle from comment #34)
> -Ofast does reorder execution.. 
> Opinions welcome.

That is absolutely OK for a matmul, and all techniques to get near peak
performance require that (e.g. use of fma, blocking, etc.). 

I didn't realize that one can easily put pragmas for single routines, so you
could experiment with something like 

#pragma GCC optimize ( "-Ofast -fvariable-expansion-in-unroller -funroll-loops"
)