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.
