From: Florian Weimer <fweimer@redhat.com>
To: Noah Goldstein via Libc-alpha <libc-alpha@sourceware.org>
Cc: Noah Goldstein <goldstein.w.n@gmail.com>,
hjl.tools@gmail.com, carlos@systemhalted.org
Subject: Re: [PATCH v1] x86-64: Replace `%ah` write with `%eax` read
Date: Mon, 13 Mar 2023 09:03:41 +0100 [thread overview]
Message-ID: <87a60hulgi.fsf@oldenburg.str.redhat.com> (raw)
In-Reply-To: <20230310024420.521941-1-goldstein.w.n@gmail.com> (Noah Goldstein via Libc-alpha's message of "Thu, 9 Mar 2023 20:44:20 -0600")
* 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?
Thanks,
Florian
next prev parent reply other threads:[~2023-03-13 8:03 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 [this message]
2023-03-13 16:59 ` Noah Goldstein
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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