[Bug tree-optimization/71034] New: abs(f) u>= 0. is always true

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71034

            Bug ID: 71034
           Summary: abs(f) u>= 0. is always true
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: glisse at gcc dot gnu.org
  Target Milestone: ---

A very simple missed optimization, we optimize abs(x)<0 to false (in forwprop,
haven't found the exact place yet) but not abs(x) u>= 0 to true. I noticed it
because cdce now produces this comparison for sqrt, which causes a small
regression on a dead sqrt(abs(x)).

int f(double x){
  x=__builtin_fabs(x);
  // return x<0;
  return !__builtin_isless(x,0);
}