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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/51879] Missed tail merging with non-const/pure calls
Date: Wed, 18 Jan 2012 10:59:00 -0000	[thread overview]
Message-ID: <bug-51879-4-JCnLJkjGHn@http.gcc.gnu.org/bugzilla/> (raw)
In-Reply-To: <bug-51879-4@http.gcc.gnu.org/bugzilla/>

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51879

--- Comment #3 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-01-18 10:45:01 UTC ---
(In reply to comment #2)
> I'm afraid a lot would break.  It really depends on what you use VN for and on
> what code.
> If you have:
>   D.12345_1 = bar (7);
>   D.12346_2 = bar (7);
> and bar isn't const/pure call, then if VN equivalences D.12345_1 and D.12346_2,
> it is wrong.  Of course if you have:
> <bb7>:
>   D.12345_1 = bar (7);
>   goto bb9;
> <bb8>:
>   D.12346_2 = bar (7);
> <bb9>:
>   D.12347_3 = PHI <D.12345_1(7), D.12346_2(8)>
> (this case), you could VN them the same.

Yes, but not for

  D.12345_1 = bar (7);
  D.12346_2 = bar (7);

so you can't really value-number the calls the same.  As we are working
on SSA SCCs and not on a CFG (and thus do not do predicated value-numbering)
that ability is useless.


  parent reply	other threads:[~2012-01-18 10:45 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2012-01-17 11:34 [Bug tree-optimization/51879] New: " jakub at gcc dot gnu.org
2012-01-18  6:14 ` [Bug tree-optimization/51879] " vries at gcc dot gnu.org
2012-01-18 10:46 ` jakub at gcc dot gnu.org
2012-01-18 10:59 ` rguenth at gcc dot gnu.org [this message]
2012-01-18 11:23 ` rguenth at gcc dot gnu.org
2012-01-23 14:51 ` vries at gcc dot gnu.org
2012-01-24 10:21 ` vries at gcc dot gnu.org
2012-01-25 10:10 ` vries at gcc dot gnu.org
2012-02-01 11:20 ` vries at gcc dot gnu.org
2012-04-27  6:13 ` vries at gcc dot gnu.org
2012-04-27  6:29 ` vries at gcc dot gnu.org
2012-04-27  6:36 ` vries at gcc dot gnu.org
2012-07-06 11:22 ` vries at gcc dot gnu.org
2012-07-06 11:22 ` vries at gcc dot gnu.org
2012-07-06 11:22 ` vries at gcc dot gnu.org
2012-07-06 11:31 ` vries at gcc dot gnu.org

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