Re: [PATCH v1] x86-64: Replace `%ah` write with `%eax` read

[Noah Goldstein <goldstein.w.n@gmail.com>]

On Mon, Mar 13, 2023 at 12:30 PM Florian Weimer <fweimer@redhat.com> wrote:
>
> * Noah Goldstein:
>
> > On Mon, Mar 13, 2023 at 3:03 AM Florian Weimer <fweimer@redhat.com> wrote:
> >>
> >> * Noah Goldstein via Libc-alpha:
> >>
> >> > High8 partial registers can incur a stall when being modified (if not
> >> > renamed seperately), or at the very least incur extra backend uops (if
> >> > renamed seperately). Either way `testl $0x0400, %eax` is preferable to
> >> > `andb $0x04, %ah`.
> >> >
> >> > Function size is unchanged when accounting for 16-byte padding.
> >> > ---
> >> >  sysdeps/x86_64/fpu/e_fmodl.S | 2 +-
> >> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >> >
> >> > diff --git a/sysdeps/x86_64/fpu/e_fmodl.S b/sysdeps/x86_64/fpu/e_fmodl.S
> >> > index d754668bce..d45f984e1a 100644
> >> > --- a/sysdeps/x86_64/fpu/e_fmodl.S
> >> > +++ b/sysdeps/x86_64/fpu/e_fmodl.S
> >> > @@ -13,7 +13,7 @@ ENTRY(__ieee754_fmodl)
> >> >       fldt    8(%rsp)
> >> >  1:   fprem
> >> >       fstsw   %ax
> >> > -     and     $04,%ah
> >> > +     testl   $0x400,%eax
> >>
> >> Why not test $0x400,%ax or test $04,%ah?
> > `test $0x400,%ax` uses imm16 which can cause length-changing-prefix
> > (`0x66` in the opcode) stalls.
> > `test $0x4,%ah` is more okay, but partial register usage has several
> > delays associated with it (even pure
> > reads), depends on arch but for example hwl/skl have 2c latency added
> > (in this case where %ah is not
> > being renamed seperately).
> > In general, if you don't need the code size, best to stick with
> > 32/64-bit instructions.
>
> Do we need to clear %eax first to avoid a false dependency?

oh  yeah, guess you're right, probably `test %ah` is best.
>
> Thanks,
> Florian
>