[Bug c/56250] New: Wrong constant folding on unsigned int

[ishiura-compiler at ml dot kwansei.ac.jp]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56250

             Bug #: 56250
           Summary: Wrong constant folding on unsigned int
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: ishiura-compiler@ml.kwansei.ac.jp


GCC 4.7.2 and 4.8.0 miscompile the following code where UINT_MAX ==
4294967295.

  $ cat bug.c
  int main(void)
  {
         unsigned x = 2;
         return ((unsigned) 0 - ( x / 2 ) ) / 2;
  }
  $ gcc bug.c
  $ ./a.out
  $ echo $?
  0

Unsigned (0-(x/2))/2 should evaluate to 2147483647 instead of 0.  
The following code with variable x replaced by "(unsigned) 2" is
correctly compiled.

  $ cat nobug.c
  int main(void)
  {
         return ((unsigned) 0 - ( (unsigned) 2 / 2 ) ) / 2;
  }
  $ gcc nobug.c
  $ ./a.out
  $ echo $?
  255

The miscompile seems to occur on many versions of gcc (4.2 - 4.8, and
even 4.0 and 3.1 on i686-apple-darwin, 4.4 - 4.6 on
i686-linux-gnu-gcc, etc.)