[llvm] [VPlan] Remove loop region in optimizeForVFAndUF. (PR #108378)

[Florian Hahn via llvm-commits]

================
@@ -692,16 +692,46 @@ void VPlanTransforms::optimizeForVFAndUF(VPlan &Plan, ElementCount BestVF,
       !SE.isKnownPredicate(CmpInst::ICMP_ULE, TripCount, C))
     return;
 
-  LLVMContext &Ctx = SE.getContext();
-  auto *BOC =
-      new VPInstruction(VPInstruction::BranchOnCond,
-                        {Plan.getOrAddLiveIn(ConstantInt::getTrue(Ctx))});
-
   SmallVector<VPValue *> PossiblyDead(Term->operands());
   Term->eraseFromParent();
+  auto *Header = cast<VPBasicBlock>(Plan.getVectorLoopRegion()->getEntry());
+  if (all_of(Header->phis(), [](VPRecipeBase &R) {
+        return !isa<VPWidenIntOrFpInductionRecipe, VPReductionPHIRecipe>(&R);
+      })) {
+    for (VPRecipeBase &R : make_early_inc_range(Header->phis())) {
+      auto *P = cast<VPHeaderPHIRecipe>(&R);
+      P->replaceAllUsesWith(P->getStartValue());
+      P->eraseFromParent();
+    }
+
+    VPBlockBase *Preheader = Plan.getVectorLoopRegion()->getSinglePredecessor();
+    auto *Exiting =
+        cast<VPBasicBlock>(Plan.getVectorLoopRegion()->getExiting());
+
+    auto *LoopRegion = Plan.getVectorLoopRegion();
+    VPBlockBase *Middle = LoopRegion->getSingleSuccessor();
+    VPBlockUtils::disconnectBlocks(Preheader, LoopRegion);
+    VPBlockUtils::disconnectBlocks(LoopRegion, Middle);
+
+    Header->setParent(nullptr);
+    Exiting->setParent(nullptr);
+    VPBlockUtils::connectBlocks(Preheader, Header);
+
+    VPBlockUtils::connectBlocks(Exiting, Middle);
+    // Set LoopRegion's Entry to nullptr, as the CFG from LoopRegion shouldn't
+    // be deleted when the region is deleted.
+    LoopRegion->clearEntry();
+    delete LoopRegion;
----------------
fhahn wrote:

I'm not sure if there is a nice way to do so currently. The whole replace-and-erase logic in the block needs to happen atomically at the moment. To avoid having to do manual delete's, VPlan could track all blocks added to it and delete them all when the VPlan is deleted (at the moment it only deletes reachable blocks). But that would require some extra logic to make sure all blocks are added properly.

https://github.com/llvm/llvm-project/pull/108378