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[Bug tree-optimization/33498] [4.2/4.3 Regression] Optimizer (-O2) may convert a normal loop to infinite
- From: "rakdver at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: 19 Sep 2007 16:56:37 -0000
- Subject: [Bug tree-optimization/33498] [4.2/4.3 Regression] Optimizer (-O2) may convert a normal loop to infinite
- References: <bug-33498-15115@http.gcc.gnu.org/bugzilla/>
- Reply-to: gcc-bugzilla at gcc dot gnu dot org
------- Comment #8 from rakdver at gcc dot gnu dot org 2007-09-19 16:56 -------
t #2)
> Technically the testcase invokes undefined behavior because 'val' overflows
> during loop execution. Practically from a QOI point of view the undefinedness
> should not propagate to the loop exit test (though GCCs behavior is certainly
> standard conforming here). A nice example for what undefined behavior can
> cause though ;)
>
> So, not sure if invalid or not.
What happens is that ivopts decide to use val as the variable to use in the
exit compare; they compute what its final value will be (67305984), and replace
the exit test by val != 67305984.
There is not much I can do with that in ivopts. I could make ivopts avoid
preserving signed variables appearing in the source code that provably
overflow; but I do not think we want to introduce this kind of hacks to handle
code with undefined behavior.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33498