[PATCH] D47385: [Polly][ForwardOpTree] Use less computationally expensive method to compute def-to-target map. NFCI.

[Tobias Grosser via llvm-commits]

This is a very good idea. I am out of office, but will review this Sunday night. 

Tobias 

On Fri, May 25, 2018, at 21:20, Michael Kruse via Phabricator wrote:
> Meinersbur created this revision.
> Meinersbur added reviewers: eli.friedman, grosser.
> Meinersbur added a project: Polly.
> Herald added a reviewer: bollu.
> 
> When forwarding a LoadInst to another statement, a map that translates 
> their domain is needed. Before this patch, is was computed by appending 
> the def-to-use map to the def-to-target of the operand tree's target. 
> This patch lets the new method `getDefToTarget` to is. This is 
> computationally less expensive due to:
> 
> - Caching of the result such that it can be used for multiple operands 
> tree to the same target.
> - The map is only computed when there is a LoadInst that needs it.
> - It is only computed for the statement requiring the translator map, 
> instead of having an intermediate result for every edge in the operand 
> tree.
> 
> The downside is that this scheme cannot handle forwarding from a 
> previous loop iteration (which would require the entire path from 
> statement to target). Since ForwardOpTree currently does not support 
> forwarding across loop iterations, this was not needed anyway.
> 
> This was written in reaction to the compilation timeout in 
> GrTestUtils.cpp by the aosp-O3-polly-before-vectorizer-unprofitable 
> buildbot 
> <http://lab.llvm.org:8011/builders/aosp-O3-polly-before-vectorizer-unprofitable>. 
> This patch may not solve the timeout itself, but would enable other 
> mitigations such as reducing the max-computations limit for 
> ForwardOptree or drastically simplifying the computations needed for the 
> translator map if the schedule is still the original from ScopInfo.
> 
> 
> Repository:
>   rPLO Polly
> 
> https://reviews.llvm.org/D47385
> 
> Files:
>   lib/Transform/ForwardOpTree.cpp
> 
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