[PATCH] Select elimination pass

[Chandler Carruth]

On Thu, Sep 4, 2014 at 11:20 AM, Gerolf Hoflehner <ghoflehner at apple.com>
wrote:

> This is a good question. InstCombine is a local pattern matcher at the
> instruction level. I did not want adding a global flavor by introducing the
> dominator tree in that pass.


This is an interesting point. Despite not having a dominator tree, there
are parts of InstCombine that already approximate dominance information by
scanning instructions or checking parent basic blocks.

I think we should seriously consider adding DominatorTrees to InstCombine
rather than adding more passes. I'm not aware of anything that would make
this particularly hard or expensive...


> Also I would have to mess around with the combiner logic when inserting an
> instruction like an “or” which is not the immediate replacement instruction
> of the select.


This shouldn't be too bad. There are other places where we combine somewhat
specific uses or instruction patterns.


> I also looked at the SimplifyCFG and if-conversion code, but none seem a
> natural fit for this optimization.


I agree here.


> I’m not super happy about a new pass but it looked like the kleinstes
> Uebel among the choices.


Yea, I'm very unconvinced about adding a new pass here. Consider that this
is quite likely to expose more instruction combining opportunities, or to
be enabled by some other instruction combining. It really needs to be in
the iterative worklist IMO.
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