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From: "su at cs dot ucdavis.edu" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/60229] New: wrong code at -O2 and -O3 on x86_64-linux-gnu
Date: Sun, 16 Feb 2014 20:28:00 -0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 4.7.4
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: su at cs dot ucdavis.edu
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Priority: P3
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60229

            Bug ID: 60229
           Summary: wrong code at -O2 and -O3 on x86_64-linux-gnu
           Product: gcc
           Version: 4.7.4
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: su at cs dot ucdavis.edu

The latest gcc 4.7 branch miscompiles the following code on x86_64-linux at -O2
and -O3 in both 32-bit and 64-bit modes.

It also affects 4.6.x, but not 4.8.x and the current gcc trunk. 

The issue seems to be considering signed short and signed char for overflows,
but neither overflows. 

$ gcc-trunk -O2 small.c    
$ a.out
$ gcc-4.8.2 -O2 small.c
$ a.out
$ gcc-4.7.3 -O2 small.c
$ a.out
^C
$ gcc-4.7-branch -O2 small.c
$ a.out
^C
$ gcc-4.7-branch -v
Using built-in specs.
COLLECT_GCC=gcc-4.7-branch
COLLECT_LTO_WRAPPER=/usr/local/gcc47-branch/bin/../libexec/gcc/x86_64-unknown-linux-gnu/4.7.4/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../gcc-4.7/configure --enable-languages=c,c++
Thread model: posix
gcc version 4.7.4 20140209 (prerelease) [gcc-4_7-branch revision 207641] (GCC) 
$ 


-------------------


short a;

int 
main() 
{
  for (; a >= 0; a++)
    ;

  return 0;
}