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

[David Seifert]

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.

> Also, why add the flag only to CFLAGS_x86 but not to CFLAGS_amd64?
> They should have the same single and double precision arithmetic?
>
> Ulrich