On Wed, Jul 14, 2021 at 9:00 AM Hongtao Liu via Gcc-patches <gcc-patches@xxxxxxxxxxx> wrote: > > On Wed, Jul 14, 2021 at 2:39 PM Matthias Kretz <m.kretz@xxxxxx> wrote: > > > > On Wednesday, 14 July 2021 07:18:29 CEST Hongtao Liu via Gcc-help wrote: > > > On Wed, Jul 14, 2021 at 1:15 PM Hongtao Liu <crazylht@xxxxxxxxx> wrote: > > > > Hi: > > > > The original problem was that some users wanted the cmdline option > > > > > > > > -ffast-math not to act on intrinsic production code. > > > > This sounds like the users want intrinsics to map *directly* to the > Thanks for the reply. > I think the users want the mixed usage of fast-math and no-fast-math. > > corresponding instruction. If that's the case such users should use inline > > assembly, IMHO. If you compile a TU with -ffast-math then *all* floating-point > > operations are affected. Yes, more control over where to use fast-math and the > > ability to mix fast-math and no-fast-math without risking ODR violations would > > be great. But that's a larger issue, and one that would ideally be solved in > > WG14/WG21. > hmm, guess it would need a lot of work. -ffast-math decomposes to quite some flag_* and those generally are not reflected into the IL but can be different per function (and then prevent inlining). There's one "related" IL feature used by the Fortran frontend - PAREN_EXPR prevents association across it. So for Fortran (when not -fno-protect-parens which is enabled by -Ofast), (a + b) - b cannot be optimized to a. Eventually this could be used to wrap intrinsic results since most of the issues in the end require association. Note PAREN_EXPR isn't exposed to the C family frontends but we could of course add a builtin-like thing for this _Noassoc ( .... ) or so. Note PAREN_EXPR survives -Ofast so it's the frontends that would need to choose to emit or not emit it (or always emit it). Note some people do like to have their intrisic code optimized, so there's likely conflicting interest here. Richard. > > > > FWIW, this is what I'd do, i.e. turn off fast-math for the function in > > question: > > https://godbolt.org/z/3cKq5hT1o > > > > -- > > ────────────────────────────────────────────────────────────────────────── > > Dr. Matthias Kretz https://mattkretz.github.io > > GSI Helmholtz Centre for Heavy Ion Research https://gsi.de > > std::experimental::simd https://github.com/VcDevel/std-simd > > ────────────────────────────────────────────────────────────────────────── > > > > -- > BR, > Hongtao
