[Bug tree-optimization/104420] New: [12 Regression] Inconsistent checks for X * 0.0 optimization

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104420

            Bug ID: 104420
           Summary: [12 Regression] Inconsistent checks for X * 0.0
                    optimization
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: glisse at gcc dot gnu.org
  Target Milestone: ---

(from a comment in PR 104389)

/* Maybe fold x * 0 to 0.  The expressions aren't the same
   when x is NaN, since x * 0 is also NaN.  Nor are they the
   same in modes with signed zeros, since multiplying a
   negative value by 0 gives -0, not +0.  Nor when x is +-Inf,
   since x * 0 is NaN.  */
(simplify
 (mult @0 real_zerop@1)
 (if (!tree_expr_maybe_nan_p (@0)
      && (!HONOR_NANS (type) || !tree_expr_maybe_infinite_p (@0))
      && !tree_expr_maybe_real_minus_zero_p (@0)
      && !tree_expr_maybe_real_minus_zero_p (@1))
  @1))

Notice how the comment talks about @0 being a "negative value" while the code
says "!tree_expr_maybe_real_minus_zero_p (@0)", which is not at all the same
thing.

Because tree_expr_maybe_real_minus_zero_p is rather weak, it does not trigger
so often, but still:

double f(int a){
  return a*0.;
}

is optimized to "return 0.;" whereas f(-42) should return -0.