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

[Fangrui Song <maskray@google.com>]

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).



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