[Bug c/39231] New: Optimized code gives wrong result

[Bug c/39231] New: Optimized code gives wrong result

[gabriel dot campana at free dot fr]

The following code gives wrong result with -O2 and above :
% gcc -O0 -o example example.c && ./example
1
% gcc -O2 -o example example.c && ./example
0

The bug is triggered on i486 platform, with gcc 4.3.2 but not with gcc 4.1.2.


#include <stdio.h>

int main(void) {
    volatile int y;
    int x;

    y = 0x80000000;
    x = y;

    if (x < 0)
        x = -x;

    printf("%d\n", x == 0x80000000);

    return 0;
}


-- 
           Summary: Optimized code gives wrong result
           Product: gcc
           Version: 4.3.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: gabriel dot campana at free dot fr
 GCC build triplet: i486-linux-gnu
  GCC host triplet: i486-linux-gnu
GCC target triplet: i486-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39231

[Bug c/39231] Optimized code gives wrong result

[rguenth at gcc dot gnu dot org]

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------- Comment #1 from rguenth at gcc dot gnu dot org  2009-02-18 14:37 -------
gcc -o t t.c -O2 -Wstrict-overflow
t.c: In function ‘main’:
t.c:13: warning: assuming signed overflow does not occur when simplifying
conditional to constant

negating signed 0x80000000 invokes undefined behavior.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39231