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[Bug tree-optimization/26854] [4.3/4.4 Regression] Inordinate compile times on large routines
- From: "stevenb dot gcc at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: 15 Feb 2009 11:26:41 -0000
- Subject: [Bug tree-optimization/26854] [4.3/4.4 Regression] Inordinate compile times on large routines
- References: <bug-26854-271@http.gcc.gnu.org/bugzilla/>
- Reply-to: gcc-bugzilla at gcc dot gnu dot org
------- Comment #95 from stevenb dot gcc at gmail dot com 2009-02-15 11:26 -------
Subject: Re: [4.3/4.4 Regression] Inordinate
compile times on large routines
Re: Comment #94
The trouble with LCM in RTL (i.e. GCSE-PRE) is not that it is slow (or
that it is disabled -- istr it is enabled at -O2), and also not that
it is edge based. The problem is that it doesn't handle cascading
expressions, because that just doesn't fit in the LCM framework. You
have to iterate RTL GCSE-PRE to move the same invariants as what RTL
LICM (i.e. loop-invariant.c) can achieve.
(GCSE-PRE is old code from a time when GCC didn't really have a proper
CFG. It is edge based because for block based you need critical edge
splitting, which was was prohibitively expensive in the Old Days.
Nowadays, gcse.c+lcm.c works in cfglayout mode and pre-splitting
critical edges would be cheap, so it would be a good idea to
experiment with a block based GCSE-PRE rewrite...)
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26854