On Wed, Jul 14, 2021 at 10:11 AM Hongtao Liu <crazy...@gmail.com> wrote:
>
> On Wed, Jul 14, 2021 at 3:49 PM Matthias Kretz <m.kr...@gsi.de> wrote:
> >
> > On Wednesday, 14 July 2021 09:39:42 CEST Richard Biener wrote:
> > > -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).
> >
> > Is there any chance the "and then prevent inlining" can be eliminated?
> > Because
> > then I could write my own fast<float> class in C++, marking all operators
> > with
> > __attribute__((optimize("-Ofast")))...
> >
> > > 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
> after a simple grep, I see PAREN_EXPR is expanded to the common RTL
> pattern. So it doesn't prevent any reassociation at the rtl level?
We don't perform any FP reassociation on RTL (and yes, the above relies on
this). We're also expanding rint() to x + 2**52 - 2**52 (ix86_expand_rint) even
with -ffast-math so we do rely on RTL optimizations not cancelling the +-.
Richard.
>
> > > survives -Ofast so it's the frontends that would need to choose to emit or
> > > not emit it (or always emit it).
> >
> > Interesting. I want that builtin in C++. Currently I use inline asm to
> > achieve
> > a similar effect. But the inline asm hammer is really too big for the
> > problem.
> >
> >
> > --
> > ──────────────────────────────────────────────────────────────────────────
> > 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
