[Bug target/106902] [11/12/13 Regression] Program compiled with -O3 -mfma produces different result

[amonakov at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106902

--- Comment #19 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
(In reply to rguenther@suse.de from comment #18)
> True - but does that catch the cases people are interested and are
> allowed by the FP contraction rules?  I'm thinking of
> 
>  x = a*b + c*d + e + f;
> 
> with -fassociative-math we can form two FMAs here?

Yes; it might be reasonable to limit the match.pd rule to
-fno-associative-math, leaving mul/adds as-is for tree-ssa-math-opts to
recombine otherwise.

>  Of course with
> strict IEEE compliance but allowed FP contraction we can only
> do FMA (a, b, c*d) + e + f, right?

I think so.

>  Does that mean -ffp-contract=on
> only makes sense in absence of any other -ffast-math flags?

Well, the proposal was to make -ffp-contract=fast an '-ffast-math' flag, not
=on. I don't want to judge if '-ffp-contract=on -ffast-math' combination is
reasonable or not, because -ffast-math by itself quite nonsensical already.