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

[Chad Rosier]

> 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.
>
> 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,
The SLP vectorizer limits the amount of recursion then building the
expression tree.  Unfortunately, there's no command-line option to control
this limit at this time.

Line 73:
http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Vectorize/SLPVectorizer.cpp?view=markup

You might consider filing a bug (llvm.org/bugs) requesting a flag, but I
don't know if the code owners want to expose such a flag.

Hope this helps.

 Regards,
  Chad