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[Bug tree-optimization/23286] missed fully redundant expression
- From: "dberlin at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: 8 Aug 2005 18:53:35 -0000
- Subject: [Bug tree-optimization/23286] missed fully redundant expression
- References: <20050808174631.23286.bonzini@gcc.gnu.org>
- Reply-to: gcc-bugzilla at gcc dot gnu dot org
------- Additional Comments From dberlin at gcc dot gnu dot org 2005-08-08 18:53 -------
(In reply to comment #1)
> Confirmed, for some reason the following is caught though:
because it's a fully redundant expression, that is available when we go to
eliminate, as opposed to the original, which would require hoisting, and is
*not* partially redundant.
> Hmm there is only one VH for the expression:
As one would expect.
This is not a missed optimization for PRE.
PRE is not a generic hoister.
In fact, in this case, it doesn't actually save anything but size to perform
this "optimization".
All it will do is make b live over the use of a, adding another register to
allocate that has a conflict.
I don't see this as a bug at all, except maybe at -Os.
Even then, it's a a hoisting issue, using very busy expressions, not a PRE issue.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23286