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

[Hongtao Liu]

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?
-- 
BR,
Hongtao
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