[Bug tree-optimization/53806] New: Missed optimization (a<=b)&&(a>=b) with trapping

[glisse at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53806

             Bug #: 53806
           Summary: Missed optimization (a<=b)&&(a>=b) with trapping
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: gli...@gcc.gnu.org


Hello,

int f(double a,double b){
  if(a>=b) if(a<=b) return 1;
  return 0;
}

is optimized to:

  if(a==b) return 1;

only if I specify the flag -fno-trapping-math, not by default.

The test in combine_comparisons checks for any change in trapping behavior,
whereas according to PR tree-optimization/53805 we are allowed to remove traps,
just not add new ones (as would happen with ORD && UNLT -> LT for instance).