[Bug target/94587] Intrinsics optimization bug with -O2 -march=skylake-avx512

["lopresti at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94587

--- Comment #8 from Patrick J. LoPresti <lopresti at gmail dot com> ---
(In reply to Andrew Pinski from comment #6)
> (In reply to Patrick J. LoPresti from comment #5)
> > I did not use -ffp-contract=fast nor -funsafe-math-optimizations nor
> > -ffast-math. Yet the statements were contracted. So the documentation has a
> > bug.
> 
> Not exactly because the defualt for -std=c11 is different from -std=gnu11
> (which is the default, or is the default -std=gnu17 well it depends on the
> compiler version).

Yes, I know. So the floating-point implementation chapter can only be
understood if you also search the rest of the 1000-page documentation to learn
what the defaults are. And then repeat for every release because "This is
subject to change."

(In reply to Jakub Jelinek from comment #7)
> Note, clang defaults to -ffp-contract=on which is like =fast (except when
> you use FP_CONTRACT pragma).

No, it is not like "fast". At least, not according to my experience and this
StackOverflow answer:

https://stackoverflow.com/a/43357638

Note the link to the live example on gcc.godbolt.org.