[PATCH] D23032: Rewrite the use optimizer to be less memory intensive and 50% faster.Fixes PR28670

[Sebastian Pop via llvm-commits]

On Tue, Aug 2, 2016 at 3:01 PM, Daniel Berlin <dberlin at dberlin.org> wrote:
>> Would it be possible to merge the UseOptimizer with the renamePass?
> No, because phi nodes are not guaranteed complete until after renaming is
> done.

I'm not seeing why having incomplete phis is a problem.

>
> (more particularly, there is no ordering in which to do renaming that can
> guarantee you don't hit incomplete phi nodes).
>
> At one point, we had use optimization be part of renaming, but walking
> incomplete phi nodes is ....
> complicated and not guaranteed to get right answers.

I haven't thought enough about merging the rename with the UseOptimizer:
it seemed to me that in renamePass you have all the data you need
to decide which is the nearest def, and decide on selecting the best
name to use.