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From: "svfuerst at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/47949] New: Missed optimization for -Os using xchg instead of mov.
Date: Wed, 02 Mar 2011 05:30:00 -0000	[thread overview]
Message-ID: <bug-47949-4@http.gcc.gnu.org/bugzilla/> (raw)

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47949

           Summary: Missed optimization for -Os using xchg instead of mov.
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: svfuerst@gmail.com
            Target: x86 / amd64


xchg %eax, reg is a one-byte instruction.  If reg is dead, this instruction
could replace the two-byte mov reg, %eax for a one-byte savings.

ie:

int foo(int x)
{
    return x;
}

currently compiles to

mov    %edi,%eax
retq

with -Os, whereas the following may be better:

xchg %eax, %edi
retq

(Similar cases exist with mov reg, %rax; mov reg, %ax; and mov reg, %al)

Note that xchg is slower than mov, so this is only an optimization when size is
more important than speed.


             reply	other threads:[~2011-03-02  5:30 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-03-02  5:30 svfuerst at gmail dot com [this message]
2011-03-02 10:23 ` [Bug target/47949] " rguenth at gcc dot gnu.org
2011-03-02 10:53 ` jakub at gcc dot gnu.org
2011-03-02 21:51 ` svfuerst at gmail dot com
2021-06-08  9:52 ` pinskia at gcc dot gnu.org
2022-08-03  8:11 ` cvs-commit at gcc dot gnu.org
2022-08-04 18:23 ` roger at nextmovesoftware dot com

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