On Wed, Jul 14, 2021 at 4:17 PM Richard Biener
<[email protected]> wrote:
>
> On Wed, Jul 14, 2021 at 10:11 AM Hongtao Liu <[email protected]> wrote:
> >
> > On Wed, Jul 14, 2021 at 3:49 PM Matthias Kretz <[email protected]> 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
-ffast-math will imply flag_associative_math, and w/ that we do have
reassociation on RTL
/* Reassociate floating point addition only when the user
specifies associative math operations. */
if (FLOAT_MODE_P (mode)
&& flag_associative_math)
{
tem = simplify_associative_operation (code, mode, op0, op1);
if (tem)
return tem;
}
> 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
--
BR,
Hongtao
