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From: "hubicka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/55079] [4.8 regression] false positive -Warray-bounds
Date: Fri, 02 Nov 2012 20:51:00 -0000	[thread overview]
Message-ID: <bug-55079-4-R3pesTr8Dg@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-55079-4@http.gcc.gnu.org/bugzilla/>


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55079

--- Comment #7 from Jan Hubicka <hubicka at gcc dot gnu.org> 2012-11-02 20:51:31 UTC ---
Actually not, what happen here is that we unroll the loop 17 times based on the
fact that the array access iterates from taillen to tailen+n_iterations and the
array size is 17.

Later in compilation we prove that tailen is actually non-zero by VRP and we
work the hard way across the unrolled loop body to work out that the last
access must be out of bounds.

So this is not bug of unroller to not remove statement. Short of teaching SCEV
about the value range of initial tailen, we really can't reduce number of
iterations.

We discussed it here http://gcc.gnu.org/ml/gcc-patches/2012-10/msg01103.html

I do not think we really can solve these cases reliably short of silencing the
warning on unrolled loop copies and other duplicated statements.


  parent reply	other threads:[~2012-11-02 20:51 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-10-26  9:56 [Bug tree-optimization/55079] New: [4.8 regeression] " dimhen at gmail dot com
2012-10-26 10:09 ` [Bug tree-optimization/55079] " rguenth at gcc dot gnu.org
2012-10-30  9:34 ` [Bug tree-optimization/55079] [4.8 regression] " rguenth at gcc dot gnu.org
2012-10-30  9:35 ` rguenth at gcc dot gnu.org
2012-11-02 16:35 ` hubicka at gcc dot gnu.org
2012-11-02 16:47 ` hubicka at gcc dot gnu.org
2012-11-02 18:45 ` hubicka at gcc dot gnu.org
2012-11-02 20:51 ` hubicka at gcc dot gnu.org [this message]
2012-11-14 19:31 ` [Bug tree-optimization/55079] [4.8 regression] false positive -Warray-bounds (also seen at -O3 bootstrap) hubicka at gcc dot gnu.org
2012-11-15  1:01 ` hubicka at gcc dot gnu.org
2012-12-07 12:55 ` rguenth at gcc dot gnu.org
2012-12-10 13:23 ` rguenth at gcc dot gnu.org
2012-12-10 13:50 ` rguenth at gcc dot gnu.org
2012-12-10 14:14 ` rguenth at gcc dot gnu.org
2012-12-10 16:27 ` hubicka at ucw dot cz
2012-12-11 10:06 ` rguenth at gcc dot gnu.org
2012-12-11 10:08 ` rguenth at gcc dot gnu.org
2012-12-12  9:33 ` schwab at gcc dot gnu.org

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