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

[Ulrich Mueller]

>>>>> 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"

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