[Bug c/31075] New: 2's complement arithmetic (LLONG_MIN-1) works differently with and without optimization

["sdirkse at gams dot com" <gcc-bugzilla at gcc dot gnu dot org>]

testing if LLONG_MIN-1 == LLONG_MAX gives different results with -O3 and
without.  Here's a short example and the output I get on my machine.

mamie:/home/distrib/lang/pas$cat mini64.c
#include <stdio.h>
typedef signed long long int int64;
#define MAXINT64  9223372036854775807LL
#define MININT64 ((-9223372036854775807LL)-1LL)

int main (int argc, char **argv)
{
  int64 minint64;

  printf ("mini64: test 2's-complement arithmetic\n");
  minint64 = MININT64;
  printf ("minint64-1 = %lld\n", minint64-1);
  if (minint64 - 1 != MAXINT64) {
    printf ("minint64 - 1 <> MAXINT64\n");
  }
  else {
    printf ("minint64 - 1 == MAXINT64\n");
  }
  return 0;
} /* main */
mamie:/home/distrib/lang/pas$gcc -o mini64 mini64.c && ./mini64
mini64: test 2's-complement arithmetic
minint64-1 = 9223372036854775807
minint64 - 1 == MAXINT64
mamie:/home/distrib/lang/pas$gcc -O3 -o mini64 mini64.c && ./mini64
mini64: test 2's-complement arithmetic
minint64-1 = 9223372036854775807
minint64 - 1 <> MAXINT64
mamie:/home/distrib/lang/pas$gcc -v
Using built-in specs.
Target: i686-pc-linux-gnu
Configured with: ../configure --enable-languages=c,c++,fortran
Thread model: posix
gcc version 4.3.0 20070305 (experimental)


-- 
           Summary: 2's complement arithmetic (LLONG_MIN-1) works
                    differently with and without optimization
           Product: gcc
           Version: 4.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: sdirkse at gams dot com
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31075