public inbox for gcc-bugs@sourceware.org
help / color / mirror / Atom feed
From: "rearnsha at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/54829] bad optimization: sub followed by cmp w/ zero (x86 & ARM)
Date: Sat, 13 Oct 2012 16:05:00 -0000	[thread overview]
Message-ID: <bug-54829-4-1qIe8JSq5t@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-54829-4@http.gcc.gnu.org/bugzilla/>


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54829

--- Comment #5 from Richard Earnshaw <rearnsha at gcc dot gnu.org> 2012-10-13 16:04:55 UTC ---
The result of the comparison is used in more than one instruction, so combine
cannot safely rework the branch instructions that follow to ensure that the
result of the subtract is used correctly.

In fact, on ARM there is no branch instruction that can be used for "> 0" as a
side effect of a subtract.  To get the desired effect the code would have to be
completely re-arranged to factor out the "< 0" (bmi) and then "== 0" (beq)
cases first.


  parent reply	other threads:[~2012-10-13 16:05 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-10-05 23:16 [Bug target/54829] New: " daniel.santos at pobox dot com
2012-10-05 23:40 ` [Bug target/54829] " pinskia at gcc dot gnu.org
2012-10-06 13:31 ` [Bug target/54829] bad optimization: sub followed by cmp w/ zero (ARM) hjl.tools at gmail dot com
2012-10-06 15:56 ` [Bug target/54829] bad optimization: sub followed by cmp w/ zero (x86 & ARM) daniel.santos at pobox dot com
2012-10-06 15:57 ` daniel.santos at pobox dot com
2012-10-13 16:05 ` rearnsha at gcc dot gnu.org [this message]
2012-10-13 16:18 ` rearnsha at gcc dot gnu.org
2012-11-15 21:56 ` daniel.santos at pobox dot com
2013-08-05 21:04 ` rearnsha at gcc dot gnu.org
2015-02-15  6:17 ` daniel.santos at pobox dot com

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=bug-54829-4-1qIe8JSq5t@http.gcc.gnu.org/bugzilla/ \
    --to=gcc-bugzilla@gcc.gnu.org \
    --cc=gcc-bugs@gcc.gnu.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).