Re: Should -ffp-contract=off the default on GCC?

[Andrew Pinski <pinskia@gmail.com>]

On Fri, Mar 24, 2023 at 1:14 AM Fangrui Song via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> On Wed, Mar 22, 2023 at 8:52 AM Qing Zhao via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
> >
> >
> >
> > > On Mar 22, 2023, at 9:57 AM, Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
> > >
> > > On Wed, Mar 22, 2023 at 1:26 PM Alexander Monakov <amonakov@ispras.ru> wrote:
> > >>
> > >>
> > >> On Wed, 22 Mar 2023, Richard Biener wrote:
> > >>
> > >>> I think it's even less realistic to expect users to know the details of
> > >>> floating-point math.  So I doubt any such sentence will be helpful
> > >>> besides spreading some FUD?
> > >>
> > >> I think it's closer to "fundamental notions" rather than "details". For
> > >> users who bother to read the GCC manual there's a decent chance it wouldn't
> > >> be for naught.
> > >>
> > >> For documentation, I was thinking
> > >>
> > >>  Together with -fexcess-precision=standard, -ffp-contract=off
> > >>  is necessary to ensure that rounding of intermediate results to precision
> > >>  implied by the source code and the FLT_EVAL_METHOD macro is not
> > >>  omitted by the compiler.
> > >
> > > that sounds good to me
> >
> > Shall we add such clarification to our Gcc13 doc? That should be helpful if we keep the currently default.
> >
> > Qing
> > >
> > >> Alexander
> >
>
> While updating the documentation, consider adding information that
> #pragma STDC FP_CONTRACT OFF is ignored with -ffp-contract=fast.
>
> This surprising behavior motivated Clang to add
> -Xclang=-ffp-contract=fast-honor-pragmas
> (https://discourse.llvm.org/t/fp-contract-fast-and-pragmas/58529).

`#pragma STDC FP_CONTRACT OFF` is not even implemented yet in GCC.
Rather we should document that :).
It does not matter what clang does here really since GCC does not even
implement the pragma.

Thanks,
Andrew Pinski


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