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From: "rakdver at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug regression/40886] [4.3/4.4/4.5 Regression] No loop counter reversal for simple loops anymore
Date: Fri, 07 Aug 2009 08:44:00 -0000 [thread overview]
Message-ID: <20090807084421.12135.qmail@sourceware.org> (raw)
In-Reply-To: <bug-40886-7834@http.gcc.gnu.org/bugzilla/>
------- Comment #3 from rakdver at gcc dot gnu dot org 2009-08-07 08:44 -------
(In reply to comment #1)
> The tree optimizers canonicalize the loop to
>
> <bb 3>:
> # i_5 = PHI <i_3(4), 0(2)>
> # ivtmp.23_1 = PHI <ivtmp.23_4(4), 10(2)>
> f2 ();
> i_3 = i_5 + 1;
> ivtmp.23_4 = ivtmp.23_1 - 1;
> if (ivtmp.23_4 != 0)
> goto <bb 4>;
> else
> goto <bb 5>;
>
> <bb 4>:
> goto <bb 3>;
>
> But then IVOPTs chooses i as the induction variable again.
This is what I would expect it to do (I am somewhat surprised that 3.3 did
something else). Ivopts at the moment do not know that comparing with 0 is
more efficient than comparing with any other expression. In all other aspects,
i and ivtmp.23 have the same cost, so ivopts prefers to preserve the original
induction variable.
Altering determine_use_iv_cost_condition to take the cost of the comparison
into account should fix this.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40886
next prev parent reply other threads:[~2009-08-07 8:44 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-07-27 21:40 [Bug regression/40886] New: " andi-gcc at firstfloor dot org
2009-07-28 11:09 ` [Bug regression/40886] [4.3/4.4/4.5 Regression] " rguenth at gcc dot gnu dot org
2009-08-04 12:49 ` rguenth at gcc dot gnu dot org
2009-08-07 8:44 ` rakdver at gcc dot gnu dot org [this message]
2009-08-07 8:50 ` andi-gcc at firstfloor dot org
2009-08-07 8:54 ` rakdver at kam dot mff dot cuni dot cz
2009-08-07 9:39 ` andi-gcc at firstfloor dot org
2009-08-07 9:47 ` steven at gcc dot gnu dot org
2009-08-07 9:52 ` andi-gcc at firstfloor dot org
2009-08-16 21:42 ` rguenth at gcc dot gnu dot org
2010-02-09 4:57 ` spop at gcc dot gnu dot org
2010-02-09 6:00 ` spop at gcc dot gnu dot org
2010-02-09 8:30 ` rakdver at kam dot mff dot cuni dot cz
2010-02-09 17:17 ` spop at gcc dot gnu dot org
2010-02-11 15:46 ` spop at gcc dot gnu dot org
2010-02-11 15:47 ` [Bug regression/40886] [4.3/4.4 " spop at gcc dot gnu dot org
2010-05-22 18:33 ` rguenth at gcc dot gnu dot org
2010-06-09 11:21 ` andi-gcc at firstfloor dot org
2010-06-09 11:57 ` rguenth at gcc dot gnu dot org
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