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

[via llvm-commits]

================
@@ -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) {
+      RecipeBuilder.createBlockInMask(HCFGBuilder.getIRBBForVPB(VPBB));
+    }
+
+    // Convert input VPInstructions to widened recipes.
+    for (VPRecipeBase &R : make_early_inc_range(*VPBB)) {
+      auto *SingleDef = cast<VPSingleDefRecipe>(&R);
+      auto *UnderlyingValue = SingleDef->getUnderlyingValue();
+      // Skip recipes that do not need transforming, including canonical IV,
+      // wide canonical IV and VPInstructions without underlying values. The
+      // latter are added above by masking.
+      // FIXME: Migrate code relying on the underlying instruction from VPlan0
+      // to construct recipes below to not use the underlying instruction.
+      if (isa<VPCanonicalIVPHIRecipe, VPWidenCanonicalIVRecipe>(SingleDef) ||
+          (isa<VPInstruction>(&R) && !UnderlyingValue))
+        continue;
 
-    // Introduce each ingredient into VPlan.
-    // TODO: Model and preserve debug intrinsics in VPlan.
-    for (Instruction &I : drop_end(BB->instructionsWithoutDebug(false))) {
-      Instruction *Instr = &I;
+      // FIXME: VPlan0, which models a copy of the original scalar loop, should
+      // not use VPWidenPHIRecipe to model the phis.
+      assert(
+          (isa<VPWidenPHIRecipe>(SingleDef) || isa<VPInstruction>(SingleDef)) &&
+          UnderlyingValue && "unsupported recipe");
+
+      if (match(&R, m_BranchOnCond(m_VPValue())) ||
+          (isa<VPInstruction>(&R) &&
+           cast<VPInstruction>(&R)->getOpcode() == Instruction::Switch)) {
----------------
ayalz wrote:

Then perhaps more consistent / simpler to do:
```suggestion
      if (isa<VPInstruction>(&R) &&
          (cast<VPInstruction>(&R)->getOpcode() == VPInstruction::BranchOnCond ||
          (cast<VPInstruction>(&R)->getOpcode() == Instruction::Switch)) {
```
(once VPWidenPHIRecipe is turned into a VPInstruction this would be simpler)

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