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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/111151] [12/13/14 Regression] Wrong code at -O0 on x86_64-pc-linux-gnu
Date: Fri, 25 Aug 2023 12:35:27 +0000 [thread overview]
Message-ID: <bug-111151-4-vGYxtndvCi@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-111151-4@http.gcc.gnu.org/bugzilla/>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111151
--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #3)
> I think when overflow wraps we cannot do this transform at all, independent
> on the "sign" of 'c'.
>
> Maybe
>
> @@ -6970,8 +6972,11 @@ extract_muldiv_1 (tree t, tree c, enum tree_code
> code, tree wide_type,
>
> /* MIN (a, b) / 5 -> MIN (a / 5, b / 5) */
> sub_strict_overflow_p = false;
> - if ((t1 = extract_muldiv (op0, c, code, wide_type,
> - &sub_strict_overflow_p)) != 0
> + if ((wide_type
> + ? TYPE_OVERFLOW_UNDEFINED (wide_type)
> + : TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (op0)))
> + && (t1 = extract_muldiv (op0, c, code, wide_type,
> + &sub_strict_overflow_p)) != 0
> && (t2 = extract_muldiv (op1, c, code, wide_type,
> &sub_strict_overflow_p)) != 0)
> {
but even when overflow is undefined we don't know whether we introduce
additional overflow then. Consider MAX (INT_MIN, 0) * -1 where we compute
0 * -1 (fine) but after the transform we'd do MIN (INT_MIN * -1, 0)
which isn't valid.
And when overflow wraps consider MAX (UINT_MAX, 1) * 2 which
will compute UINT_MAX * 2 == 0 while MAX (UINT_MAX * 2, 1 * 2) will compute 2.
Unless I'm missing something.
What we'd need to know is whether the inner operations are known to not
overflow/wrap (or whether they change sign consistently). But without
range info we can't know this unless op0 and op1 are constants.
So - scrap that whole sub-rule?
next prev parent reply other threads:[~2023-08-25 12:35 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-08-25 6:39 [Bug middle-end/111151] New: " jwzeng at nuaa dot edu.cn
2023-08-25 7:16 ` [Bug middle-end/111151] " rguenth at gcc dot gnu.org
2023-08-25 7:16 ` rguenth at gcc dot gnu.org
2023-08-25 11:51 ` mikpelinux at gmail dot com
2023-08-25 12:27 ` rguenth at gcc dot gnu.org
2023-08-25 12:28 ` rguenth at gcc dot gnu.org
2023-08-25 12:35 ` rguenth at gcc dot gnu.org [this message]
2023-08-26 3:27 ` pinskia at gcc dot gnu.org
2024-03-22 13:40 ` law at gcc dot gnu.org
2024-03-22 17:29 ` jakub at gcc dot gnu.org
2024-03-22 17:37 ` jakub at gcc dot gnu.org
2024-03-22 17:41 ` jakub at gcc dot gnu.org
2024-03-22 17:57 ` jakub at gcc dot gnu.org
2024-03-22 18:48 ` jakub at gcc dot gnu.org
2024-03-22 19:15 ` jakub at gcc dot gnu.org
2024-03-25 7:23 ` rguenth at gcc dot gnu.org
2024-03-25 11:01 ` jakub at gcc dot gnu.org
2024-03-25 12:45 ` rguenth at gcc dot gnu.org
2024-03-26 10:22 ` cvs-commit at gcc dot gnu.org
2024-03-26 10:32 ` [Bug middle-end/111151] [12/13 " jakub at gcc dot gnu.org
2024-03-26 15:43 ` cvs-commit at gcc dot gnu.org
2024-03-30 3:55 ` cvs-commit at gcc dot gnu.org
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