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From: "amonakov at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/111655] [11/12/13/14 Regression] wrong code
 generated for __builtin_signbit and 0./0. on x86-64 -O2
Date: Wed, 04 Oct 2023 11:41:20 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D111655

--- 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 i=
nfer
> "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).
>=20
> Possibly in some other place the C standard makes FP division by zero sub=
ject
> 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.=