[PATCH 4/6] Support -fexcess-precision=16 which will enable FLT_EVAL_METHOD_PROMOTE_TO_FLOAT16 when backend supports _Float16.

Hongtao Liu crazylht@gmail.com
Tue Aug 17 01:53:01 GMT 2021


On Fri, Aug 6, 2021 at 2:06 PM Hongtao Liu <crazylht@gmail.com> wrote:
>
> On Tue, Aug 3, 2021 at 10:44 AM Hongtao Liu <crazylht@gmail.com> wrote:
> >
> > On Tue, Aug 3, 2021 at 3:34 AM Joseph Myers <joseph@codesourcery.com> wrote:
> > >
> > > On Mon, 2 Aug 2021, liuhongt via Gcc-patches wrote:
> > >
> > > > diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
> > > > index 7979e240426..dc673c89bc8 100644
> > > > --- a/gcc/config/i386/i386.c
> > > > +++ b/gcc/config/i386/i386.c
> > > > @@ -23352,6 +23352,8 @@ ix86_get_excess_precision (enum excess_precision_type type)
> > > >       return (type == EXCESS_PRECISION_TYPE_STANDARD
> > > >               ? FLT_EVAL_METHOD_PROMOTE_TO_FLOAT
> > > >               : FLT_EVAL_METHOD_UNPREDICTABLE);
> > > > +      case EXCESS_PRECISION_TYPE_FLOAT16:
> > > > +     return FLT_EVAL_METHOD_PROMOTE_TO_FLOAT16;
> > > >        default:
> > > >       gcc_unreachable ();
> > > >      }
> > >
> > > I'd expect an error for -fexcess-precision=16 with -mfpmath=387 (since x87
> > > doesn't do float or double arithmetic, but -fexcess-precision=16 implies
> > > that all of _Float16, float and double are represented to the range and
> > > precision of their type withou any excess precision).
> > >
> > Yes, additional changes like this.
> >
> > modified   gcc/config/i386/i386.c
> > @@ -23443,6 +23443,9 @@ ix86_get_excess_precision (enum
> > excess_precision_type type)
> >   ? FLT_EVAL_METHOD_PROMOTE_TO_FLOAT
> >   : FLT_EVAL_METHOD_UNPREDICTABLE);
> >        case EXCESS_PRECISION_TYPE_FLOAT16:
> > + if (TARGET_80387
> > +     && !(TARGET_SSE_MATH && TARGET_SSE))
> > +   error ("%<-fexcess-precision=16%> is not compatible with %<-mfpmath=387%>");
> >   return FLT_EVAL_METHOD_PROMOTE_TO_FLOAT16;
> >        default:
> >   gcc_unreachable ();
> > new file   gcc/testsuite/gcc.target/i386/float16-7.c
> > @@ -0,0 +1,9 @@
> > +/* { dg-do compile } */
> > +/* { dg-options "-O2 -mfpmath=387 -fexcess-precision=16" } */
> > +/* { dg-excess-errors "'-fexcess-precision=16' is not compatible with
> > '-mfpmath=387'" } */
> > +_Float16
> > +foo (_Float16 a, _Float16 b)
> > +{
> > +  return a + b;/* { dg-error "'-fexcess-precision=16' is not
> > compatible with '-mfpmath=387'" } */
> > +}
> > +
> >
> > > --
> > > Joseph S. Myers
> > > joseph@codesourcery.com
> >
> >
> >
> > --
> > BR,
> > Hongtao
>
>
> Updated patch and ping for it.
>
> Also for backend changes.
> 1. For backend m68k/s390 which totally don't support _Float16, backend
> will issue an error for -fexcess-precision=16, I think it should be
> fine.
> 2. For backend like arm/aarch64 which supports _Float16 , backend will
> set FLT_EVAL_METHOD_PROMOTE_TO_FLOAT16 for -fexcess-precision=16 even
> hardware instruction for fp16 is not supported. Would that be ok for
> arm?

Ping for this patch.

> --
> BR,
> Hongtao



-- 
BR,
Hongtao


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