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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: Tue, 11 Jul 2023 09:45:52 +0000 [thread overview] Message-ID: <bug-106884-4-4P10ysdUoE@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 --- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> --- To clarify for the original testcase, ifcombine combines the two bit tests (x & (1<<a)) && (x & (1<<b)) to x & ((1<<a) | (1<<b)) == ((1<<a) | (1<<b)). We can rely on 1<<a being non-zero, otherwise the testcase invoked undefined behavior. That means whatever value 1<<b produced in the case it was unspecified we still get false when bit 1<<a was not set and a defined value when the value of 1<<b was well-defined. But yes, since we now unconditionally compute 1<<b after the transform GCC could assume b is in range. We are lacking a way to compute 1<<b optimally without that undefined behavior.
next prev parent reply other threads:[~2023-07-11 9:45 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 ` [Bug tree-optimization/106884] " rguenth at gcc dot gnu.org 2022-09-08 10:53 ` 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 [this message] 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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