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

[Florian Weimer]

* 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?

Thanks,
Florian