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.
> > --- 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
