[PATCH] D131015: [LV] Track all IR blocks corresponding to VPBasicBlock

[Ayal Zaks via Phabricator via llvm-commits]

Ayal added inline comments.


================
Comment at: llvm/lib/Transforms/Vectorize/VPlan.cpp:373
 
-  State->CFG.VPBB2IRBB[this] = NewBB;
+  State->CFG.VPBB2IRBB[this].push_back(NewBB);
   State->CFG.PrevVPBB = this;
----------------
Ayal wrote:
> Hmm, VPBasicBlock::execute() is called once per VPBB when generating IR, so how could VPBB2IRBB record multiple/overwriting NewBB's here per same `this`? If there are such cases, perhaps its better to simplify them and retain a single IRBB per VPBB, adding an assert that no overwriting takes place. VPBB's contain only recipes and are free of control-flow - which is modeled using multiple VPBB's and VPRegions - so should fill a single IRBB.
> 
> A related issue are A-B-C cases above that try to reuse the same IRBB for pairs of back-to-back VPBB's (which may best be avoided for clarity and left to subsequent simplifyCFG to fold instead), but these are multiple VPBB's filling one IRBB, rather than the converse.
> 
> Another related issue is the correspondence between original IRBB's and VPBB's during VPlan construction rather than execution. There multiple replicate-and-predicate recipes that stem from the same IRBB are each assigned separate VPBB's (of a replicating region), as can be seen by trying to assign each such VPBB a unique name associated with its original IRBB (`VPBBsForBB`). Again yielding multiple VPBB's per one IRBB.
> Hmm, VPBasicBlock::execute() is called once per VPBB when generating IR, so how could VPBB2IRBB record multiple/overwriting NewBB's here per same this?

Ahh, a Replicating Region executes each of its VPBB's VF*UF times, effectively performing complete unrolling at VPlan execution time. It may indeed be clearer to either fully unroll in VPlan (when setting VF and UF?) or emit a loop instead of unrolling it.


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  https://reviews.llvm.org/D131015/new/

https://reviews.llvm.org/D131015