Re: [PATCH] [x86] x86: Don't add crtfastmath.o for -shared and add a new option -mdaz-ftz to enable FTZ and DAZ flags in MXCSR.

[Richard Biener <richard.guenther@gmail.com>]

On Wed, Dec 14, 2022 at 9:16 AM Jakub Jelinek <jakub@redhat.com> wrote:
>
> On Wed, Dec 14, 2022 at 09:08:02AM +0100, Richard Biener via Gcc-patches wrote:
> > On Wed, Dec 14, 2022 at 3:21 AM liuhongt via Gcc-patches
> > <gcc-patches@gcc.gnu.org> wrote:
> > >
> > > Don't add crtfastmath.o for -shared to avoid changing the MXCSR
> > > register when loading a shared library.  crtfastmath.o will be used
> > > only when building executables.
> > >
> > > Bootstrapped and regtested on x86_64-pc-linux-gnu{-m32,}.
> > > Ok for trunk?
> >
> > You reject negative -mdaz-ftz but wouldn't that be useful with
> > -Ofast -mno-daz-ftz since there's otherwise no way to avoid that?
>
> Agreed.
> I even wonder if the best wouldn't be to make the option effectively
> three state, default, no and yes, where if the option isn't specified
> at all, then crtfastmath.o* is linked as is now except for -shared,
> if it is -mno-daz-ftz, then it is never linked in regardless of other
> options and if it is -mdaz-ftz, then it is linked even for -shared.

Possibly.  I'd also suggest to split the changed -shared handling to
a separate patch since people may want to backport this and it
should be applicable to all other targets with similar handling.

> > > --- a/gcc/config/i386/i386.opt
> > > +++ b/gcc/config/i386/i386.opt
> > > @@ -420,6 +420,10 @@ mpc80
> > >  Target RejectNegative
> > >  Set 80387 floating-point precision to 80-bit.
> > >
> > > +mdaz-ftz
> > > +Target RejectNegative
> > > +Set the FTZ and DAZ Flags.
>
> As the option is only used in the driver, shouldn't it be marked Driver
> and not Target?  It doesn't need to be saved/restored on every cfun switch
> etc.
>
> > > +@item -mdaz-ftz
> > > +@opindex mdaz-ftz
> > > +
> > > +the flush-to-zero (FTZ) and denormals-are-zero (DAZ) flags in the MXCSR register
>
> Shouldn't description start with capital letter?
>
> > > +are used to control floating-point calculations.SSE and AVX instructions
> > > +including scalar and vector instructions could benefit from enabling the FTZ
> > > +and DAZ flags when @option{-mdaz-ftz} is specified.
> >
> > Maybe say that the MXCSR register is set at program start to achieve
> > this when the
> > flag is specified at _link_ time and say this switch is ignored when
> > -shared is specified?
>
>         Jakub
>