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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/106884] ifcombine may move shift so it shifts more than bitwidth
Date: Thu, 08 Sep 2022 09:52:09 +0000 [thread overview]
Message-ID: <bug-106884-4-NwL4yDB2Qp@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-106884-4@http.gcc.gnu.org/bugzilla/>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106884
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |rguenth at gcc dot gnu.org
Keywords| |wrong-code
Status|UNCONFIRMED |NEW
Ever confirmed|0 |1
Last reconfirmed| |2022-09-08
Depends on| |106811
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed. r13-131-gc2a0d2e6f636c6 addressed some issues, but leaves others
such as shifts seen here. It's not clear if (int)1 << 33 is undefined behavior
in
GIMPLE, see PR106811. Note that ifcombine doesn't simply transform this to
(x & c) & (x & (1 << b)), thus removing the short-circuiting, instead it
computes tem = c | (1<<b) and checks (x & tem) == tem. That's probably safe
for arbitrary results of the undefined operation though.
One possibility might be to rewrite 1 << b to (1 << (b&31)) to make it always
defined, but that comes at an extra cost compared to how we treat signed
overflow.
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106811
[Bug 106811] GENERIC and GIMPLE IL undefined behavior needs documenting
next prev parent reply other threads:[~2022-09-08 9:52 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-08 0:20 [Bug tree-optimization/106884] New: " kristerw at gcc dot gnu.org
2022-09-08 9:52 ` rguenth at gcc dot gnu.org [this message]
2022-09-08 10:53 ` [Bug tree-optimization/106884] " kristerw at gcc dot gnu.org
2022-09-30 11:34 ` kristerw at gcc dot gnu.org
2023-07-11 9:45 ` rguenth at gcc dot gnu.org
2023-07-11 15:36 ` pinskia at gcc dot gnu.org
2023-08-06 0:07 ` kristerw at gcc dot gnu.org
2023-11-01 15:57 ` pinskia at gcc dot gnu.org
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