[PATCH] Select elimination pass

[Hal Finkel]

----- Original Message -----
> From: "Chandler Carruth" <chandlerc at gmail.com>
> To: "Gerolf Hoflehner" <ghoflehner at apple.com>
> Cc: "llvm-commits" <llvm-commits at cs.uiuc.edu>
> Sent: Thursday, September 4, 2014 2:14:53 PM
> Subject: Re: [PATCH] Select elimination pass
> 
> 
> 
> 
> 
> 
> 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...

FWIW, I've looked at this in the context of dealing with @llvm.assume. I plan to, at least, make InstCombine grab DT when it is otherwise available. As it turns out, in the standard pipeline, this happens at least once.

 -Hal

> 
> 
> 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.
> _______________________________________________
> llvm-commits mailing list
> llvm-commits at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory