[Bug tree-optimization/38785] huge performance regression on EEMBC bitmnp01

["amylaar at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #9 from amylaar at gcc dot gnu dot org  2009-01-14 18:47 -------
I think the disregard for conditional execution opportunities and the
assumption that phi nodes have no execution cost are two separate issues.
I'd like to address the latter first, because it causes exponential code and
execution time growth.

A phi node joining two constants has at least the cost of a constant load.
A phi node joining two different variables which are initialized by a graph
with constant leafs costs at least a reg-reg copy on one arm, plus the cost
of its parents if these are needed solely for this phi node.

Therefore, if an expression is only partially anticipatable, we should compare
the cost of any phi node needed to compute it early with the estimated
likelyhod that such a computatatio, once done, is actually needed, multiplied
with the cost of the replaced operation.       

Can we use edge probabilities inside tree-pre to calculate execution
probabilities?

Can we calculate the cost of replaced expressions?


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38785