[PATCH] D47477: [VPlan] Avoid building VPlans for each VF in the ILV (NFC).

[Hideki Saito via Phabricator via llvm-commits]

hsaito added a comment.





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Comment at: lib/Transforms/Vectorize/LoopVectorize.cpp:6347
   // Select the optimal vectorization factor.
-  return CM.selectVectorizationFactor(MaxVF);
+  return BestVF;
 }
----------------
fhahn wrote:
> dcaballe wrote:
> > The rationale of building VPlans for all the VFs and then invoke CM to choose the best VF (without using those VPlans) was to eventually port CM to work on these VPlans. This hasn't happened yet but @hsaito is working on this VPlan based version of the CM. We think that this step is very important in the converge of both vectorization paths since it will allow us to move the VPlan construction in the inner loop path to earlier stages of the path.
> > 
> > Since this is a small change, my suggestion would be: 1) if this change is needed by your subsequent patches, let's go with it. Hideki could move the VPlan construction again after CM when his code is ready; 2) otherwise, let's keep the original version. Please, note that we are mostly sharing a single VPlan for all the VFs so we shouldn't be using unnecessary memory most of the times.
> > 
> > WDYT?
> > 
> I understand, but do you think we will be able to replace the legacy cost model anytime soon? I expect implementing VPlan based inner loop vectorization that does not introduce regressions over the legacy cost model will be a big task. I thought the plan was to develop this cost model in the VPlan native path (with support for inner loops). That will allow us to get an initial cost model in and iterate on that, until we match the performance of the legacy one. At that point, we can retire the legacy inner loop vectorizer.  What do you think?
I think we should be doing both, and that's probably the only way to gain enough credibility with the rest of the community. Certainly not an easy task. From my perspective of CM refactoring, if VPlan is built before CM runs, incremental work of introducing VPlan-ness is easier. If you move it to after CM computing VF/UF, the first thing I need to do, for incremental introduction of VPlan-ness, is essentially undoing your change.

First CM refactoring I plan to do is to change "BB->Instruction" traversal into "VPBB -> Recipe -> Instruction" traversal, making sure that the change is NFC. We can then proceed to change "per instruction" modeling to "per Recipe" modeling one by one and/or to "per VPInstruction" modeling, making sure cost modeling methodology and computed cost are comparable or better (we'll probably have to establish some metric there) each step. Along the way, we'll need to reflect many "behind the scenes smarts" into VPlan. All that expects VPlan to be built before CM computation. 

Does this make sense to you?


https://reviews.llvm.org/D47477