[LLVMdev] How to broaden the SLP vectorizer's search

[Hal Finkel]

----- Original Message -----
> From: "Frank Winter" <fwinter at jlab.org>
> To: llvmdev at cs.uiuc.edu
> Sent: Thursday, August 7, 2014 10:58:47 AM
> Subject: [LLVMdev] How to broaden the SLP vectorizer's search
> 
> The BB vectorizer has an option 'bb-vectorizer-search-limit'. Is
> there a
> similar option for the SLP vectorizer? Maybe an analysis pass' scope
> that can be widen?
> 
> I have large basic blocks with instructions that should be merged
> into
> packed versions. However, the blocks are optimized independently from
> each other. Now, if the instructions to be merged aren't too far
> apart
> the SLP vectorizer successfully packs the instructions within a
> block.
> However, from a certain distance between the instructions on the SLP
> vectorizer won't detect anymore the groups of instructions that can
> be
> packed. There's no interleaved data dependency that would prevent
> vectorizing. I am almost certain that's the vectorizer must have a
> maximum distance set somewhere or indirectly through dependent
> analysis
> passes.
> 
> I tried the BB vectorizer, but for whatever reason it starts emitting
> extractvector/shufflevector instructions which I am sure aren't
> necessary. So, until further investigation I'd like to stick to SLP.

A quick comment on this point: The BB vectorizer has always required a run of InstCombine and EarlyCSE (or GVN) afterward to produce reasonable code. That having been said, the development focus is now squarely on the SLP vectorizer is this space, and I suggest you focus there regardless.

 -Hal

> 
> Ideally, I'd like the maximum distance to be infinite, thus removing
> this restriction. I am aware of the consequent search time growth.
> 
> Anyone any insights?
> 
> Frank
> 
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory