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.

[Uros Bizjak <ubizjak@gmail.com>]

On Wed, Dec 14, 2022 at 9:52 AM Richard Biener
<richard.guenther@gmail.com> wrote:
>
> On Wed, Dec 14, 2022 at 9:34 AM Liu, Hongtao <hongtao.liu@intel.com> wrote:
> >
> >
> >
> > > -----Original Message-----
> > > From: Richard Biener <richard.guenther@gmail.com>
> > > Sent: Wednesday, December 14, 2022 4:23 PM
> > > To: Jakub Jelinek <jakub@redhat.com>
> > > Cc: Liu, Hongtao <hongtao.liu@intel.com>; gcc-patches@gcc.gnu.org;
> > > crazylht@gmail.com; hjl.tools@gmail.com; ubizjak@gmail.com
> > > Subject: 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.
> > >
> > > 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.
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55522#c26
> > So patch in the upper link is ok for trunk?
>
> It needs target maintainer approval but yes, I think that's what we want.

Yes, the one-liner is OK. Perhaps it also needs a corresponding
documentation change (haven't checked the documentation).

Uros.

>
> Richard.
>
> > I'll change -mdaz-ftz part as a separate patch.
> > >
> > > > > > --- 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
> > > >