[Bug tree-optimization/70291] New: muldc3 code generation could be smarter

[ramana at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70291

            Bug ID: 70291
           Summary: muldc3 code generation could be smarter
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ramana at gcc dot gnu.org
  Target Milestone: ---

__complex double x;
__complex double y;
__complex double z;
double a, b, c, d;

int main (void)
{
  x = y * z;
  return 0; 
}

Could well be implemented as:


int main (void)
{
  x = y * z;
  if (isnan (__real x) && isnan (__imag__ x))
    x = __muldc3 (y, z);

  return 0;
}

essentially opencoding this as the standard suggests in G.5.1 => 6for C99.

spotted while looking at profiles that were reported on aarch64 with code
compiled at O2 / O3.

I note that lowering this in this form in tree-complex.c will need a bit of
book-keeping given that it's sort of bounded on the phi nodes and ssa_names
before lowering begins but this could well be another math-optimization done
later rather than munging it with the existing lowering.