[llvm] [VPlan] Build initial VPlan 0 using HCFGBuilder for inner loops. (NFC) (PR #124432)

[Florian Hahn via llvm-commits]

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@@ -9327,26 +9328,67 @@ LoopVectorizationPlanner::tryToBuildVPlanWithVPRecipes(VFRange &Range) {
   RecipeBuilder.collectScaledReductions(Range);
 
   auto *MiddleVPBB = Plan->getMiddleBlock();
+
+  // Scan the body of the loop in a topological order to visit each basic block
+  // after having visited its predecessor basic blocks.
+  ReversePostOrderTraversal<VPBlockShallowTraversalWrapper<VPBlockBase *>> RPOT(
+      HeaderVPBB);
+
   VPBasicBlock::iterator MBIP = MiddleVPBB->getFirstNonPhi();
-  for (BasicBlock *BB : make_range(DFS.beginRPO(), DFS.endRPO())) {
-    // Relevant instructions from basic block BB will be grouped into VPRecipe
-    // ingredients and fill a new VPBasicBlock.
-    if (VPBB != HeaderVPBB)
-      VPBB->setName(BB->getName());
-    Builder.setInsertPoint(VPBB);
+  VPBlockBase *PrevVPBB = nullptr;
+  for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(RPOT)) {
+    // Handle VPBBs down to the latch.
+    if (VPBB == LoopRegion->getExiting()) {
+      assert(!HCFGBuilder.getIRBBForVPB(VPBB) &&
+             "the latch block shouldn't have a corresponding IRBB");
+      VPBlockUtils::connectBlocks(PrevVPBB, VPBB);
+      break;
+    }
 
-    if (VPBB == HeaderVPBB)
+    // Create mask based on the IR BB corresponding to VPBB.
+    // TODO: Predicate directly based on VPlan.
+    Builder.setInsertPoint(VPBB, VPBB->begin());
+    if (VPBB == HeaderVPBB) {
+      Builder.setInsertPoint(VPBB, VPBB->getFirstNonPhi());
       RecipeBuilder.createHeaderMask();
-    else if (NeedsMasks)
-      RecipeBuilder.createBlockInMask(BB);
+    } else if (NeedsMasks) {
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fhahn wrote:

Currently `createHeaderMask` itself checks if tail folding is enabled, and only then creates the mask

https://github.com/llvm/llvm-project/pull/124432