[PATCH] D76838: [LV][LoopInfo] Transform counting-down loops to counting-up loop

[Ayal Zaks via Phabricator via llvm-commits]

Ayal added inline comments.


================
Comment at: llvm/lib/Analysis/LoopInfo.cpp:220
+//
+PHINode *Loop::reverseInductionVariable(ScalarEvolution &SE) {
+  BasicBlock *Preheader = getLoopPreheader();
----------------
SjoerdMeijer wrote:
> Meinersbur wrote:
> >  `Loop` is part of the `LLVMAnalysis` library. Transformations should be in the `LLVMTransform` library.
> > 
> > Also, it is untypical for the analysis results to have methods for modification as well. These are typically found in the LLVMTransform library such as `LoopUtils.cpp`
> Thanks for the suggestion, I'm happy to move this to LoopUtils.cpp. Also checking with @Ayal, is this something we can all live with? 
Constructing a canonical induction if one is missing and needed for LV's FoldTail, using SCEV::getBackedgeTakenCount() instead of pattern matching, could be placed in LoopUtils.cpp (or in LoopVectorize.cpp, using ILV::createInductionVariable()).

The concern is that doing so might cause slowdowns if the loop is not vectorized, something LV has been trying hard to avoid, so far successfully, which also motivated VPlan. (BTW, doing so may also cause slowdowns even if the loop is vectorized, but that's a different story ;-). Reverting control to the original IV if the loop is not vectorized seems awkward at best.

Let's try to think of alternative (1), i.e., of having LV represent in VPlan the new PIV that it will eventually create. A new PrimaryInductionRecipe  (or VPInstruction) can be introduced and placed at the beginning of the first VPBasicBlock; its execute() will create a Phi-starting-at-zero, set ILV::Induction and possibly a PIV VPValue to it; the bump and compare could be taken care of in ILV.fixVectorizedLoop().
Interested in following this approach?


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D76838/new/

https://reviews.llvm.org/D76838