[llvm] [VPlan] Introduces explicit broadcast for live-in constants. (PR #133213)

[David Sherwood via llvm-commits]

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@@ -3093,8 +3090,25 @@ void VPlanTransforms::materializeBroadcasts(VPlan &Plan) {
                "All users must be in the vector preheader or dominated by it");
     }
 
-    VPBuilder Builder(cast<VPBasicBlock>(HoistBlock), HoistPoint);
-    auto *Broadcast = Builder.createNaryOp(VPInstruction::Broadcast, {VPV});
+    VPInstruction *Broadcast;
+    if (VPV->isLiveIn() && isa_and_nonnull<Constant>(VPV->getLiveInIRValue())) {
+      // We cannot replace the constant live-ins for PHIs by broadcast in the
+      // same VPBB because it will break PHI. Also cannot replace the
+      // VPWidenGEPRecipe since it broadcasts the generated pointer instead of
+      // operands.
+      if (auto *R = dyn_cast_if_present<VPRecipeBase>(*(VPV->users().begin()));
+          R && !isa<VPHeaderPHIRecipe, VPWidenPHIRecipe, VPWidenGEPRecipe>(R) &&
+          !VPV->hasMoreThanOneUniqueUser()) {
+        Broadcast = new VPInstruction(VPInstruction::Broadcast, {VPV});
+        // Insert just before the user to reduce register pressure.
+        Broadcast->insertBefore(R);
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david-arm wrote:

Isn't there a problem here? It looks like you're assuming that the first user of VPV dominates all users of VPV, but I don't think that is always true. The `HoistPoint ` above was chosen as a location to dominate all users so why not just use that instead?

https://github.com/llvm/llvm-project/pull/133213