Re: [gentoo-dev] [PATCH] profiles/arch/amd64: add "-mfpmath=sse" to CFLAGS_x86

[Mike Gilbert]

On Tue, Oct 18, 2022 at 5:56 AM David Seifert <soap@g.o> wrote:
>
> On Tue, 2022-10-18 at 10:14 +0200, Ulrich Mueller wrote:
> > > > > > > On Tue, 18 Oct 2022, Mike Gilbert wrote:
> >
> > > Reference: https://gcc.gnu.org/wiki/x87note
> >
> > Which says:
> >
> > > ... the amount of worst-case error that could possibly happen using
> > > the x87 (with any amount of intermediate rounding) is at worst the
> > > same as true 64 or 32 bit arithmetic, and in practice is almost
> > > always
> > > better.
> >
> > and:
> >
> > > Note, however, that this greater repeatability comes at the cost of
> > > lost precision (i.e. SSE always gets the same precision because it
> > > always takes the equivalent of the x87's worst case: a forced round
> > > down at each step).
> >
> > So, it comes with a price, and I wonder if we shouldn't leave that
> > choice to the user, and go with the upstream GCC default?
> >
> > > -CFLAGS_x86="-m32"
> > > +CFLAGS_x86="-m32 -mfpmath=sse"
>
> -mfpmath=sse is already the default on amd64.

I have amended the first paragraph to make this more clear:

GCC uses x87 floating point instructions when building 32-bit x86
code by default. When building 64-bit code, SSE2 instructions are used
instead.