[llvm] [VPlan] Move tail folding out of VPlanPredicator. NFC (PR #176143)

[Andrei Elovikov via llvm-commits]

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@@ -964,6 +964,82 @@ void VPlanTransforms::createLoopRegions(VPlan &Plan) {
   TopRegion->getEntryBasicBlock()->setName("vector.body");
 }
 
+void VPlanTransforms::foldTailByMasking(VPlan &Plan) {
+  assert(Plan.getExitBlocks().size() == 1 &&
+         "only a single-exit block is supported currently");
+  assert(Plan.getExitBlocks().front()->getSinglePredecessor() ==
+             Plan.getMiddleBlock() &&
+         "the exit block must have middle block as single predecessor");
+
+  VPRegionBlock *LoopRegion = Plan.getVectorLoopRegion();
+  VPBasicBlock *Header = LoopRegion->getEntryBasicBlock();
+
+  Header->splitAt(Header->getFirstNonPhi());
+
+  // Create the header mask, insert it in the header and branch on it.
+  auto *IV =
+      new VPWidenCanonicalIVRecipe(Header->getParent()->getCanonicalIV());
+  VPBuilder Builder(Header, Header->getFirstNonPhi());
+  Builder.insert(IV);
+  VPValue *BTC = Plan.getOrCreateBackedgeTakenCount();
+  VPValue *HeaderMask = Builder.createICmp(CmpInst::ICMP_ULE, IV, BTC);
+  Builder.createNaryOp(VPInstruction::BranchOnCond, HeaderMask);
+
+  VPBasicBlock *Latch = LoopRegion->getExitingBasicBlock();
+  VPValue *IVInc;
+  [[maybe_unused]] bool TermBranchOnCount =
+      match(Latch->getTerminator(),
+            m_BranchOnCount(m_VPValue(IVInc),
+                            m_Specific(&Plan.getVectorTripCount())));
+  assert(TermBranchOnCount &&
+         match(IVInc, m_Add(m_Specific(LoopRegion->getCanonicalIV()),
+                            m_Specific(&Plan.getVFxUF()))) &&
+         std::next(IVInc->getDefiningRecipe()->getIterator()) ==
+             Latch->getTerminator()->getIterator() &&
+         "Unexpected canonical iv increment");
+
+  // Split the latch at the IV update, and branch to it from the header mask.
+  VPBasicBlock *LatchSplit =
+      Latch->splitAt(IVInc->getDefiningRecipe()->getIterator());
+  VPBlockUtils::connectBlocks(Header, LatchSplit);
+
+  // Insert phis for any values in the predicated body that are used outside.
+  Builder.setInsertPoint(LatchSplit, LatchSplit->begin());
+  for (VPBasicBlock *VPBB : {Header, Plan.getMiddleBlock()}) {
+    for (VPRecipeBase &R : *VPBB) {
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eas wrote:

> Probably it suffices to just iterate through those?

Maybe check that in the verifier?

https://github.com/llvm/llvm-project/pull/176143