From: "H.J. Lu" <hjl.tools@gmail.com>
To: Florian Weimer <fweimer@redhat.com>
Cc: "H.J. Lu via Libc-alpha" <libc-alpha@sourceware.org>
Subject: Re: V2 [PATCH] x86-64: Avoid rep movsb with short distance [BZ #27130]
Date: Mon, 4 Jan 2021 07:27:16 -0800 [thread overview]
Message-ID: <CAMe9rOqENAGBdV6Kuz+4dW5aoR3CR38-qyqiN76jcWhETUt8Cw@mail.gmail.com> (raw)
In-Reply-To: <874kjwloah.fsf@oldenburg2.str.redhat.com>
On Mon, Jan 4, 2021 at 7:22 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> * H. J. Lu via Libc-alpha:
>
> > 1:
> > +# if AVOID_SHORT_DISTANCE_REP_MOVSB
> > + movq %rsi, %rcx
> > + subq %rdi, %rcx
> > +2:
> > +/* Avoid "rep movsb" if RCX, the distance between source and destination,
> > + is N*4GB + [1..63] with N >= 0. */
> > + cmpl $63, %ecx
> > + jbe L(more_2x_vec) /* Avoid "rep movsb" if ECX <= 63. */
> > +# endif
> > mov %RDX_LP, %RCX_LP
> > rep movsb
> > L(nop):
>
> Why not use _LP names here? I think the %ecx comparison at least can
> give false results on x86-64 (64-bit).
>
This is done on purpose since we want to avoid "rep movsb" for distances of
N*4GB + [1..63] with N >= 0 which include 0x100000003.
--
H.J.
next prev parent reply other threads:[~2021-01-04 15:27 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-04 15:17 H.J. Lu
2021-01-04 15:22 ` Florian Weimer
2021-01-04 15:27 ` H.J. Lu [this message]
2021-01-04 15:47 ` Florian Weimer
2021-01-04 15:54 ` H.J. Lu
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=CAMe9rOqENAGBdV6Kuz+4dW5aoR3CR38-qyqiN76jcWhETUt8Cw@mail.gmail.com \
--to=hjl.tools@gmail.com \
--cc=fweimer@redhat.com \
--cc=libc-alpha@sourceware.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).