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

[Elvis Wang 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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ElvisWang123 wrote:

For the register pressure side, if hoisting all of constant broadcast to the preheader, the register pressure will be high in the loop body. 
The register allocated in the preheader will mark as used across entire vector body (perhaps not now, but more accurate in future), which is not that accurate. If the broadcast instruction only in loop body, the register usage can be free when it reach the usage.

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