SLP/Loop vectorizer pass ordering

[Arnold Schwaighofer]

> On Jul 25, 2014, at 8:48 AM, Tobias Grosser <tobias at grosser.es> wrote:
> 
> On 25/07/2014 17:41, James Molloy wrote:
>> Hi Nadav, Arnold,
>> 
>> 
>> 
>> I've come across an interesting optimization problem in one of the SPEC
>> benchmarks. There is a loop that can be optimized by both the SLP vectorizer
>> and the loop vectorizer (when I patch the loop vectorizer to deal with fsub
>> reductions).
>> 
>> 
>> 
>> The SLP vectorizer actually makes the performance worse - I think this is
>> due to a lack of loop unrolling afterwards. The Loop vectorizer can improve
>> the performance.
>> 
>> 
>> 
>> However, the loop vectorizer runs after the SLP vectorizer, so it never gets
>> a chance. I'd have thought the ideal order would be Loop Vectorizer -> SLP
>> vectorizer -> BB vectorizer, given that the loop vectorizer if it can run
>> will probably give greater speedup than SLP.
>> 
>> 
>> 
>> The current sequence is SLP vectorizer -> BB vectorizer -> Loop vectorizer.
> 
> Even though I was not directly addressed. I still reply.
> 
> The proposed new order is what I think makes most sense. I wonder what was the reason to go for the current order. Nadav, Arnold did you choose this order?


See my answer to James. We loose noalias information during inlining (until recently - thanks Hal for moving forward with the scoped noalias feature!).

We should absolutely evaluate moving the SLPVectorizer out of the inliner PM once we have working scoped noalias information.

Inlining could obscure patterns of parallelism the SLPVectorizer can recognize so their might be some losses - Erik’s work on making the SLPVectorizer more robust with respect to recognizing what schedules prevent vectorization plus work on recognizing more (possible mid-tree) patterns  should enable us to deal with some of the fallout I think.
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