[llvm] [VPlan] Introduce explicit broadcasts for live-ins. (PR #124644)

[Mel Chen via llvm-commits]

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@@ -2154,3 +2154,34 @@ void VPlanTransforms::handleUncountableEarlyExit(
   Builder.createNaryOp(VPInstruction::BranchOnCond, AnyExitTaken);
   LatchExitingBranch->eraseFromParent();
 }
+
+void VPlanTransforms::materializeBroadcasts(VPlan &Plan) {
+  VPDominatorTree VPDT;
+  VPDT.recalculate(Plan);
+  auto *VectorPreheader = Plan.getVectorPreheader();
+  VPBuilder Builder(VectorPreheader);
+  for (VPValue *LiveIn : Plan.getLiveIns()) {
+    if (all_of(LiveIn->users(),
+               [LiveIn](VPUser *U) {
+                 return cast<VPRecipeBase>(U)->usesScalars(LiveIn);
+               }) ||
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Mel-Chen wrote:

I saw that this patch adds many implementations of `onlyFirstLaneUsed`. Is this for the correctness of `useScalars` here?

https://github.com/llvm/llvm-project/pull/124644