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

[via llvm-commits]

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@@ -9312,23 +9317,45 @@ LoopVectorizationPlanner::tryToBuildVPlanWithVPRecipes(VFRange &Range) {
   RecipeBuilder.collectScaledReductions(Range);
 
   auto *MiddleVPBB = Plan->getMiddleBlock();
+  ReversePostOrderTraversal<VPBlockShallowTraversalWrapper<VPBlockBase *>> RPOT(
+      Plan->getVectorLoopRegion()->getEntry());
+
   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)) {
+    // Skip VPBBs not corresponding to any input IR basic blocks.
+    if (!HCFGBuilder.getIRBBForVPB(VPBB))
+      continue;
 
-    if (VPBB == HeaderVPBB)
+    // Create mask based on the IR BB corresponding to VPBB.
+    // TODO: Predicate directly based on VPlan.
+    if (VPBB == HeaderVPBB) {
+      Builder.setInsertPoint(VPBB, VPBB->getFirstNonPhi());
       RecipeBuilder.createHeaderMask();
-    else if (NeedsMasks)
-      RecipeBuilder.createBlockInMask(BB);
+    } else if (NeedsMasks) {
+      Builder.setInsertPoint(VPBB, VPBB->begin());
+      RecipeBuilder.createBlockInMask(HCFGBuilder.getIRBBForVPB(VPBB));
----------------
ayalz wrote:

OK, let's go with HCFGBuilder.getIRBBForVPB(), for now ...
Following our roadmap, HCFGBuilder would be the VPlan-to-VPlan transformation producing "buildLoop" from "wrapInput", and tryToBuildVPlanWithRecipes would follow as a subsequent VPlan-to-VPlan transformation(s). Obtaining this VPBB-to-IRBB mapping from HCFGBuilder should be removed in one of two ways (or more):
1. If the buildLoop VPlan is to maintain its correspondence with IR basic blocks then a new type of VPBB can be introduced - one providing getIRBasicBlock() similar to VPIRBasicBlock but having an execute() similar to VPBasicBlock - that generates new basic blocks, or deemed abstract/unreachable - to be materialized into a concreate VPBB prior to codegen.
2. If, OTOH, getIRBBForVPB() is meant only as a temporary solution to support createBlockInMask() until the latter is upgraded to work directly on VPBB's, then a **FIXME** should be added.

(Recipes are created for internal conditional branches but not for unconditional branches (including early exits) - which imply a single successor. An empty block also implies a single predecessor due to lack of phi recipes. Figuring out the BasicBlock from the single predecessor and/or successor of an empty VPBB seems cumbersome; getIRBBForVPB() could record the mapping for empty VPBB's only; simplest to probably to record all VPBB's in loop, for now.)

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