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From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Mon, 21 Jan 2019 17:10:00 -0000
Message-ID: <CAMe9rOqq4f6mPGPb8_SQhAm_k2z2qS8E0D6RNkrrFjhkt3PQWA@mail.gmail.com>
Subject: Re: [PATCH] x86-64: Use TI->SF and TI->DF conversions in soft-fp
To: Uros Bizjak <ubizjak@gmail.com>, Wei Xiao <wei3.xiao@intel.com>, 	"Guo, Xuepeng" <xuepeng.guo@intel.com>, Hongtao Liu <hongtao.liu@intel.com>
Cc: "gcc-patches@gcc.gnu.org" <gcc-patches@gcc.gnu.org>
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On Mon, Jan 21, 2019 at 8:59 AM Uros Bizjak <ubizjak@gmail.com> wrote:
>
> On Mon, Jan 21, 2019 at 5:56 PM H.J. Lu <hjl.tools@gmail.com> wrote:
> >
> > On Mon, Jan 21, 2019 at 8:43 AM Uros Bizjak <ubizjak@gmail.com> wrote:
> > >
> > > On Mon, Jan 21, 2019 at 5:15 PM H.J. Lu <hjl.tools@gmail.com> wrote:
> > > >
> > > > TI->SF and TI->DF conversions in libgcc2.c:
> > > >
> > > > FSTYPE
> > > > FUNC (DWtype u)
> > > > {
> > > >   ...
> > > > }
> > > >
> > > > have no rounding mode support.  We should replace __floattisf, __floattidf,
> > > > __floatuntisf and __floatuntidf in libgcc2.c with these from soft-fp.
> > > >
> > > >         PR libgcc/88931
> > > >         * config/i386/64/t-softfp-compat (libgcc2-ti-functions): New.
> > > >         (LIB2FUNCS_EXCLUDE): Likewise.
> > > >         (libgcc2-ti-softp): Likewise.
> > > >         (LIB2ADD): Likewise.
> > > > ---
> > > >  libgcc/config/i386/64/t-softfp-compat | 8 ++++++++
> > > >  1 file changed, 8 insertions(+)
> > > >
> > > > diff --git a/libgcc/config/i386/64/t-softfp-compat b/libgcc/config/i386/64/t-softfp-compat
> > > > index 0978695c3a4..abb78032bf5 100644
> > > > --- a/libgcc/config/i386/64/t-softfp-compat
> > > > +++ b/libgcc/config/i386/64/t-softfp-compat
> > > > @@ -13,3 +13,11 @@ libgcc2-tf-functions = _divtc3 _multc3 _powitf2
> > > >  LIB2FUNCS_EXCLUDE += $(libgcc2-tf-functions)
> > > >  libgcc2-tf-compats = $(addsuffix .c, $(libgcc2-tf-functions))
> > > >  LIB2ADD += $(addprefix $(srcdir)/config/i386/64/, $(libgcc2-tf-compats))
> > > > +
> > > > +# Replace _floatdisf, _floatdidf, _floatundisf and _floatundidf in
> > > > +# libgcc2.c, which have no rounding mode support, with floattisf.c,
> > > > +# floattidf.c, floatundisf.c and floatundidf.c from soft-fp.
> > > > +libgcc2-ti-functions = _floatdisf _floatdidf _floatundisf _floatundidf
> > > > +LIB2FUNCS_EXCLUDE += $(libgcc2-ti-functions)
> > > > +libgcc2-ti-softp = floattisf.c floattidf.c floatuntisf.c floatuntidf.c
> > > > +LIB2ADD += $(addprefix $(srcdir)/soft-fp/, $(libgcc2-ti-softp))
> > >
> > > It is not that simple. Please note that libgcc2 functions use FP
> > > instructions in narrower mode (so, in effect still use FPU), while
> > > soft-fp functions don't even touch the FPU, and do everything using
> > > bit twiddling. I think that your change would introduce qoute
> > > noticeable runtime regressions.
> > >
> >
> > By "run-time regressions", did you mean performance or correctness?
>
> Performance.
>

We will check performance change.

-- 
H.J.