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Date: Fri, 09 Jul 2004 18:05:00 -0000
From: "gcc-bugzilla at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Message-ID: <20040709175338.16458.steinmtz@us.ibm.com>
Reply-To: gcc-bugzilla@gcc.gnu.org
Subject: [Bug regression/16458] New: PowerPC - redundant compare
X-Bugzilla-Reason: CC
X-SW-Source: 2004-07/txt/msg00948.txt.bz2
List-Id: <gcc-bugs.sourceware.org>

Description:
A non-optimal compare sequence is illustrated.  Two compares are being
done, one signed and the other unsigned.  One unsigned compare would
suffice.  Duplicate using gcc 3.5 and command line:

gcc -O3 -m64 -c test.c

Testcase:
unsigned int a, b;

int foo ()
{
  if (a == b) return 1;
  if (a > b)  return 2;
  if (a < b)  return 3;
  if (a != b) return 4;
  return 0;
}

Assembly:
      ld 9,.LC1@toc(2)
      li 3,1
      ld 11,.LC0@toc(2)
      lwz 10,0(9)
      lwz 0,0(11)
      cmpw 7,0,10   - signed compare of a and b
      cmplw 6,0,10  - unsigned compare of a and b
      beqlr- 7      - (a == b uses signed compare)
      li 3,2
      bgtlr- 6      - (a > b uses unsigned compare)
      li 3,4
      bgelr- 6      - (a != b uses unsigned compare)
      li 3,3
      blr



-- 
           Summary: PowerPC - redundant compare
           Product: gcc
           Version: 3.5.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P1
         Component: regression
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: steinmtz at us dot ibm dot com
                CC: gcc-bugs at gcc dot gnu dot org,steinmtz at us dot ibm
                    dot com
 GCC build triplet: powerpc64-linux
  GCC host triplet: powerpc64-linux
GCC target triplet: powerpc64-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16458