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From: "matz at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/99101] optimization bug with -ffinite-loops
Date: Thu, 25 Feb 2021 13:31:33 +0000 [thread overview]
Message-ID: <bug-99101-4-fgh0piCXNh@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-99101-4@http.gcc.gnu.org/bugzilla/>
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99101
--- Comment #17 from Michael Matz <matz at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #16)
> Of course since -ffinite-loops and the C++ standard require forward progress
> here and all testcases expect the loop to not terminate we're in the realm
> of undefined behavior. But I'm not yet convinced the control-dependence /
> CD-DCE issue only shows up in such cases. That said, it's fully expected
> that
>
> int xx;
> int main()
> {
> int jobs_ = 1;
> int at_eof_ = 0;
> while (1)
> {
> for (int i = 0; i < jobs_; i++)
> {
> if (at_eof_)
> continue;
> at_eof_ = 1;
> if (xx)
> return 1;
> }
> jobs_ = 0;
> }
> return 0;
> }
>
> is eventually optimized to just return 1 with -ffinite-loops and we should
> try to preserve that behavior.
Just commenting on this last statement: I think that's wrong. It's provable
that 'xx' doesn't change in the loop, and that it starts out as 0 (this is main
here). So in fact we have produced an endless loop without a return, and hence
can do anything (when endless loops are undefined).
next prev parent reply other threads:[~2021-02-25 13:31 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-15 8:55 [Bug c++/99101] New: " 251078896 at qq dot com
2021-02-15 9:43 ` [Bug tree-optimization/99101] " rguenth at gcc dot gnu.org
2021-02-15 10:17 ` rguenth at gcc dot gnu.org
2021-02-15 10:25 ` 251078896 at qq dot com
2021-02-15 10:29 ` jakub at gcc dot gnu.org
2021-02-15 10:38 ` rguenth at gcc dot gnu.org
2021-02-15 10:46 ` rguenth at gcc dot gnu.org
2021-02-15 13:08 ` rguenth at gcc dot gnu.org
2021-02-24 14:00 ` rguenth at gcc dot gnu.org
2021-02-24 14:20 ` rguenth at gcc dot gnu.org
2021-02-24 14:41 ` rguenth at gcc dot gnu.org
2021-02-24 14:45 ` rguenth at gcc dot gnu.org
2021-02-24 15:09 ` rguenth at gcc dot gnu.org
2021-02-25 10:18 ` rguenth at gcc dot gnu.org
2021-02-25 11:34 ` rguenth at gcc dot gnu.org
2021-02-25 12:09 ` rguenth at gcc dot gnu.org
2021-02-25 12:40 ` rguenth at gcc dot gnu.org
2021-02-25 13:31 ` matz at gcc dot gnu.org [this message]
2021-02-25 18:29 ` matz at gcc dot gnu.org
2021-02-25 18:34 ` matz at gcc dot gnu.org
2021-02-26 9:38 ` rguenth at gcc dot gnu.org
2021-03-03 12:16 ` rguenth at gcc dot gnu.org
2021-03-03 14:00 ` matz at gcc dot gnu.org
2021-03-03 14:37 ` rguenth at gcc dot gnu.org
2021-03-03 19:52 ` hubicka at gcc dot gnu.org
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