> On Mar 24, 2023, at 3:42 PM, Andrew Pinski <pins...@gmail.com> wrote: > > 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 <amona...@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 :).
Do we have any plan to implement this pragma? Also, do we have any plan to implement -ffp-contract=on? I am very curious on why -ffp-contract=on has not been implemented for so long time? There are some documentation on Floating Point implementation of GCC on the pragma STDC FP_CONTRACT https://gcc.gnu.org/onlinedocs/gcc-12.2.0/gcc/Floating-point-implementation.html#Floating-point-implementation " • Whether and how floating expressions are contracted when not disallowed by the FP_CONTRACT pragma (C99 and C11 6.5). Expressions are currently only contracted if -ffp-contract=fast, -funsafe-math-optimizations or -ffast-math are used. This is subject to change. • The default state for the FP_CONTRACT pragma (C99 and C11 7.12.2). This pragma is not implemented. Expressions are currently only contracted if -ffp-contract=fast, -funsafe-math-optimizations or -ffast-math are used. This is subject to change. “ So, looks like that we have documented this. But When the user read the documentation for —ffp-contract option, there is no such information. We might add a link in the documentation of -ffp-contract option to here to make it clear. Qing > It does not matter what clang does here really since GCC does not even > implement the pragma. > > Thanks, > Andrew Pinski > > >> >> >> >> -- >> 宋方睿
