[Bug middle-end/111655] [11/12/13/14 Regression] wrong code generated for __builtin_signbit and 0./0. on x86-64 -O2

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111655

--- Comment #11 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #10)
> And this conservatively has to apply to all FP divisions where we might infer
> "nonnegative" unless we can also infer !zerop?

Yes, I think the logic in tree_binary_nonnegative_warnv_p is incorrect for
floating-point division. Likewise for multiplication: it returns true for 'x *
x', but when x is a NaN, 'x * x' is also a NaN (potentially with the same
sign).


> On the side of replacing all uses I'd error on simply not folding.

Yes, as preceding transforms might have duplicated the division already. We can
only do such folding very early, when we are sure no duplication might have
taken place.


> Note 6.5.5/6 says "In both operations, if the value of the second operand is
> zero, the behavior is undefined." only remotely implying this doesn't
> apply to non-integer types (remotely by including modulo behavior in this
> sentence).
> 
> Possibly in some other place the C standard makes FP division by zero subject
> to other rules.

I think the intention is that Annex F makes it follow IEEE rules (returns an
Inf/NaN and sets FE_DIVBYZERO/FE_INVALID), but it doesn't seem to be clearly
worded, afaict.