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From: "rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/37916] [4.2/4.3/4.4 Regression] SSA names causing register pressure; unnecessarily many simultaneously "live" names.
Date: Mon, 27 Oct 2008 16:22:00 -0000 [thread overview]
Message-ID: <20081027162103.8136.qmail@sourceware.org> (raw)
In-Reply-To: <bug-37916-507@http.gcc.gnu.org/bugzilla/>
------- Comment #13 from rguenther at suse dot de 2008-10-27 16:21 -------
Subject: Re: [4.2/4.3/4.4 Regression] SSA names
causing register pressure; unnecessarily many simultaneously "live" names.
On Mon, 27 Oct 2008, hp at gcc dot gnu dot org wrote:
> ------- Comment #12 from hp at gcc dot gnu dot org 2008-10-27 16:09 -------
> I had an "old" native x86_64-unknown-linux-gnu tree around from
> [trunk revision 139963], and I see the corresponding effect there.
>
> Of all four combinations of {,-fno-tree-reassoc} {,-fschedule-insns
> -fschedule-insns2}, together with -fno-ivopts -fno-reorder-blocks -fno-gcse, it
> was plain -fno-tree-reassoc (no scheduling) that produces the smallest
> stack-frame and *at a glance* smaller, faster code. (FWIW, I'm a bit surprised
> to see that -fschedule-insns -fschedule-insns2 is not the default for this
> arch.)
Because scheduling before register allocation tends to increase register
pressure. Of course this is not a necessary thing - in fact scheduling
could reduce register pressure, just this would probably be called
re-materialization during register allocation.
tree reassociation is not aware of register pressure as a simple matter
of fact. TER usually reduces the effect somewhat, but of course in
the end we want to get rid of TER ...
Richard.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37916
next prev parent reply other threads:[~2008-10-27 16:22 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-10-25 20:07 [Bug tree-optimization/37916] New: [4.0 " hp at gcc dot gnu dot org
2008-10-25 20:14 ` [Bug tree-optimization/37916] " hp at gcc dot gnu dot org
2008-10-25 20:23 ` hp at gcc dot gnu dot org
2008-10-25 20:58 ` hp at gcc dot gnu dot org
2008-10-25 21:08 ` hp at gcc dot gnu dot org
2008-10-25 21:11 ` hp at gcc dot gnu dot org
2008-10-25 21:22 ` hp at gcc dot gnu dot org
2008-10-25 21:25 ` hp at gcc dot gnu dot org
2008-10-27 10:46 ` rguenth at gcc dot gnu dot org
2008-10-27 11:54 ` [Bug tree-optimization/37916] [Regression] " hp at gcc dot gnu dot org
2008-10-27 12:33 ` [Bug tree-optimization/37916] [4.2/4.3/4.4 Regression] " rguenth at gcc dot gnu dot org
2008-10-27 12:44 ` jakub at gcc dot gnu dot org
2008-10-27 12:47 ` rguenth at gcc dot gnu dot org
2008-10-27 15:23 ` hp at gcc dot gnu dot org
2008-10-27 16:11 ` hp at gcc dot gnu dot org
2008-10-27 16:22 ` rguenther at suse dot de [this message]
2008-10-27 16:23 ` amacleod at redhat dot com
2008-10-27 16:37 ` hp at gcc dot gnu dot org
2009-03-31 21:00 ` [Bug tree-optimization/37916] [4.3/4.4/4.5 " jsm28 at gcc dot gnu dot org
2009-08-04 12:49 ` rguenth at gcc dot gnu dot org
2010-05-22 18:26 ` [Bug tree-optimization/37916] [4.3/4.4/4.5/4.6 " rguenth at gcc dot gnu dot org
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