[Bug rtl-optimization/55611] Operand swapping for commutative operators

["glisse at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55611

--- Comment #2 from Marc Glisse <glisse at gcc dot gnu.org> 2013-03-11 14:44:09 UTC ---
The fortran test that fails is equivalent to the following (use -Ofast -g,
surprisingly it only fails in var tracking)

float f(double*a,double*b){
  double x=a[0]*b[0];
  x+=a[1]*b[1];
  x+=a[2]*b[2];
  x+=a[3]*b[3];
  return x;
}

This happens because of an infinite loop between simplify_associative_operation
and simplify_gen_binary (and a few others) which keeps shuffling the additions
around to try and canonicalize the expression. Some well placed tests whether
GET_CODE(*)==code could probably help, although I am a bit surprised that it
fails so seldom, and only in var-tracking.

I didn't investigate the problem with the reverse order, as stage3 failures (in
middle-end code) are hard to debug...