[Bug tree-optimization/15559] [3.3/3.4/3.5 Regression] SSAPRE misses obvious redundant expression

[dberlin at gcc dot gnu dot org]

------- Additional Comments From dberlin at gcc dot gnu dot org  2004-06-10 16:45 -------
Theoretically yes, but no PRE algorithm will do what you suggest, you need a generic code hoisting 
algorithm.  The transformation you suggest violates the optimality conditions of PRE, even if it can later 
make better code, so it wouldn't be performed.
In short, PRE is for partial redundancies, not a cost based generic code hoister, so this is not a case of 
PRE missing obvious redundant expressions. There is no redundant expression (it occurs once along 
each path) here that it could eliminate, there is simply an expression that could be hoisted one block 
up.

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=15559