Re: [patch tree-optimization 1/2]: Branch-cost optimizations

[Jeff Law <law at redhat dot com>]

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On 11/07/11 07:38, Richard Guenther wrote:
> 
> 
> The cond_chain stuff should be as we have discussed quite some
> time ago on IRC - modeled after tree-affine.c - a "vector" of
> predicates of the form [~] A op B, combined using logical AND or
> OR (thus, able to represent a disjunctive or conjunctive normal
> form). All suitably abstracted so if-conversion, the loop optimizer
> and loop number of iteration analysis can use this code (they all
> collect a controlling predicate (chain) and try to simplify against
> it).
Hmm, I've never looked at this code, does it do something similar to
simplify_plus_minus?

simplify_plus_minus takes a hunk of nested additions/subtractions,
breaks them into components (where subtractions are modeled using
addition+negation).  The components are held in an array which is
repeatedly simplified (combining constants, eliminating unnecessary
negations, etc etc).  After simpification we re-emit the new,
simplified arithmetic?

If so, then definitely, we want that kind of structure.  Interestingly
enough Kenner and I discussed this back in the early 90s before I was
capable of doing anything with trees.  There was some codegen problem
on the PA that would have been significantly helped by regrouping of
this nature at the tree level.



> You may, for example, want to pick up my (suspended) work on
> separating predicate computation from predicate use (or at least
> think about whether that's a good idea for the purpose of predicate
> optimizations).
I certainly see value in in going down this path.

jeff
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