From: Noah Goldstein <goldstein.w.n@gmail.com>
To: Florian Weimer <fweimer@redhat.com>
Cc: Noah Goldstein via Libc-alpha <libc-alpha@sourceware.org>,
hjl.tools@gmail.com, carlos@systemhalted.org
Subject: Re: [PATCH v1] x86-64: Replace `%ah` write with `%eax` read
Date: Mon, 13 Mar 2023 11:59:39 -0500 [thread overview]
Message-ID: <CAFUsyfJna9iuTNn6meWV4z3TnLrH0Abe5DukpSGCQwC53RbbCw@mail.gmail.com> (raw)
In-Reply-To: <87a60hulgi.fsf@oldenburg.str.redhat.com>
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.
>
> Thanks,
> Florian
>
next prev parent reply other threads:[~2023-03-13 16:59 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-10 2:44 Noah Goldstein
2023-03-10 16:38 ` H.J. Lu
2023-03-13 8:03 ` Florian Weimer
2023-03-13 16:59 ` Noah Goldstein [this message]
2023-03-13 17:30 ` Florian Weimer
2023-03-13 20:49 ` Noah Goldstein
2023-03-13 20:50 ` [PATCH v2] x86-64: Replace `and %ah` write with `test %ah` read Noah Goldstein
2023-03-14 2:00 ` H.J. Lu
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