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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/111655] wrong code generated for __builtin_signbit and 0./0. on x86-64 -O2
Date: Sun, 01 Oct 2023 23:57:47 +0000 [thread overview]
Message-ID: <bug-111655-4-OaipV9C6ey@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-111655-4@http.gcc.gnu.org/bugzilla/>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111655
--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Paul Eggert from comment #5)
> > The match pattern which causes the issue:
> > (simplify
> > /* signbit(x) -> 0 if x is nonnegative. */
> > (SIGNBIT tree_expr_nonnegative_p@0)
> > { integer_zero_node; })
> I don't see anything wrong with that match pattern.
>
> I speculate that what's wrong is that GCC incorrectly thinks that 0.0/0.0 is
> nonnegative. Although it's tempting to say that the sign bit of a division
> is the exclusive OR of the sign bits of its operands, evidently this is not
> true on x86-64 when NaNs are involved.
tree_expr_nonnegative_p for divide does:
case RDIV_EXPR:
case TRUNC_DIV_EXPR:
case CEIL_DIV_EXPR:
case FLOOR_DIV_EXPR:
case ROUND_DIV_EXPR:
return RECURSE (op0) && RECURSE (op1);
Since 0.0 and 0.0 both don't have their sign bits set, GCC assumes diving them
won't produce a value with the sign bit set ...
I really think x86_64 div instruction is broken for IEEE really.
next prev parent reply other threads:[~2023-10-01 23:57 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-10-01 17:22 [Bug tree-optimization/111655] New: wrong code generated for __builtin_signbit " eggert at cs dot ucla.edu
2023-10-01 17:29 ` [Bug target/111655] " pinskia at gcc dot gnu.org
2023-10-01 17:29 ` pinskia at gcc dot gnu.org
2023-10-01 17:40 ` [Bug target/111655] wrong code generated for __builtin_signbit and 0./0. " pinskia at gcc dot gnu.org
2023-10-01 17:42 ` pinskia at gcc dot gnu.org
2023-10-01 22:08 ` eggert at cs dot ucla.edu
2023-10-01 23:57 ` pinskia at gcc dot gnu.org [this message]
2023-10-01 23:59 ` pinskia at gcc dot gnu.org
2023-10-02 0:02 ` pinskia at gcc dot gnu.org
2023-10-02 11:08 ` [Bug target/111655] [11/12/13/14 Regression] " amonakov at gcc dot gnu.org
2023-10-04 9:37 ` [Bug middle-end/111655] " rguenth at gcc dot gnu.org
2023-10-04 11:41 ` amonakov at gcc dot gnu.org
2023-11-24 9:33 ` jakub at gcc dot gnu.org
2023-11-24 9:48 ` amonakov at gcc dot gnu.org
2023-11-24 9:54 ` rguenther at suse dot de
2024-03-22 13:38 ` law at gcc dot gnu.org
2024-03-22 17:09 ` jakub at gcc dot gnu.org
2024-04-19 11:38 ` bruno at clisp dot org
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