[Bug c/48197] New: possible wrong code bug at -O0

[regehr at cs dot utah.edu]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48197

           Summary: possible wrong code bug at -O0
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: regehr@cs.utah.edu
                CC: chenyang@cs.utah.edu
              Host: x86_64-unknown-linux-gnu
            Target: x86_64-unknown-linux-gnu
             Build: x86_64-unknown-linux-gnu


Here gcc's output is the same at all optimization levels.  We think the correct
answer for this function is "x = 0" because all of the following statements are
equivalent:

   x = (long)0 > ((unsigned int)0 ^ (signed short)y);
   x = (long)0 > ((unsigned int)0 ^ (signed short)((int)0x8000));
   x = (long)0 > ((unsigned int)0 ^ (signed short)0x8000);
   x = (long)0 > ((unsigned)0 ^ (unsigned)0x8000);
   x = (long)0 > ((unsigned)0 ^ (unsigned)32768);
   x = (long)0 > (unsigned)32768;
   x = (long)0 > (long)32768;
   x = 0;

[regehr@gamow ~]$ current-gcc -O0 small.c -o small
[regehr@gamow ~]$ ./small
x = 1
[regehr@gamow ~]$ cat small.c


static int x = 0;
static int y = 0x8000;

int printf(const char *format, ...);

int main (void)
{
   x = (long)0 > ((unsigned int)0 ^ (signed short)y);
   printf("x = %d\n", x);
   return 0;
} 
[regehr@gamow ~]$ current-gcc -v

Using built-in specs.
COLLECT_GCC=current-gcc
COLLECT_LTO_WRAPPER=/uusoc/exports/scratch/regehr/z/compiler-install/gcc-r171139-install/bin/../libexec/gcc/x86_64-unknown-linux-gnu/4.7.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ../configure --with-libelf=/usr/local --enable-lto
--prefix=/home/regehr/z/compiler-install/gcc-r171139-install
--program-prefix=r171139- --enable-languages=c,c++
Thread model: posix
gcc version 4.7.0 20110318 (experimental) (GCC)