[llvm] [VPlan] Scalarize to first-lane-only directly on VPlan (PR #184267)

[Ramkumar Ramachandra via llvm-commits]

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@@ -6541,3 +6541,59 @@ void VPlanTransforms::makeMemOpWideningDecisions(
     ReplaceWith(Recipe);
   }
 }
+
+void VPlanTransforms::makeScalarizationDecisions(VPlan &Plan, VFRange &Range) {
+  if (LoopVectorizationPlanner::getDecisionAndClampRange(
+          [&](ElementCount VF) { return VF.isScalar(); }, Range))
+    return;
+
+  // Extend lifetime per `llvm::PostOrderTraversal` documentation:
+  auto PO =
+      vp_post_order_shallow(Plan.getVectorLoopRegion()->getEntryBasicBlock());
+
+  for (VPBasicBlock *VPBB : VPBlockUtils::blocksOnly<VPBasicBlock>(PO)) {
+    for (VPRecipeBase &R : make_early_inc_range(reverse(*VPBB))) {
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artagnon wrote:

Why are we visiting the recipes in the reversed VPBB? I thought the objective of POT was to visit uses before defs?

https://github.com/llvm/llvm-project/pull/184267