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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/55731] Issue with complete innermost loop unrolling (cunrolli)
Date: Tue, 18 Dec 2012 15:35:00 -0000	[thread overview]
Message-ID: <bug-55731-4-QksLapREcL@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-55731-4@http.gcc.gnu.org/bugzilla/>


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55731

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-12-18
     Ever Confirmed|0                           |1

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> 2012-12-18 15:34:51 UTC ---
The reason is that unrolling early can be harmful to for example vectorization
and thus cunrolli restricts itself to "obviously" profitable cases.

In this case the loop is not an "inner" loop - it doesn't have a containing
loop and so growth is not allowed even with -O3 (we otherwise will fail
to vectorize if the unrolled body ends up as part of other basic-blocks).

It's a know issue that after cunroll there is no strong value-numbering
pass that handles memory (there is DOM which only has weak memory handling).

So, it's a trade-off we make, mostly for the sake of loop optimizations
that do not handle unrolled loops well.


  parent reply	other threads:[~2012-12-18 15:35 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-12-18 14:13 [Bug tree-optimization/55731] New: " ysrumyan at gmail dot com
2012-12-18 14:23 ` [Bug tree-optimization/55731] " ysrumyan at gmail dot com
2012-12-18 14:24 ` ysrumyan at gmail dot com
2012-12-18 15:35 ` rguenth at gcc dot gnu.org [this message]
2012-12-19  9:17 ` ysrumyan at gmail dot com
2012-12-19 10:27 ` rguenth at gcc dot gnu.org
2021-12-12  9:45 ` pinskia at gcc dot gnu.org

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