[llvm] [VPlan] Get Addr computation cost with scalar type if it is uniform for gather/scatter. (NFC) (PR #150371)

[Elvis Wang via llvm-commits]

https://github.com/ElvisWang123 updated https://github.com/llvm/llvm-project/pull/150371

>From 04ff13c72207d5a23a8dcfffe1b74b2c1311e52f Mon Sep 17 00:00:00 2001
From: Elvis Wang <elvis.wang at sifive.com>
Date: Wed, 23 Jul 2025 17:02:40 -0700
Subject: [PATCH] [VPlan] Get Addr computation cost with scalar type if it is
 uniform for gather/scatter.

This patch query `getAddressComputationCost()` with scalar type if the
address is uniform. This can help the cost for gather/scatter more
accurate.

In current LV, non consecutive VPWidenMemoryRecipe (gather/scatter) will
account the cost of address computation. But there are some cases that
the addr is uniform accross lanes, that makes the address can be
calculated with scalar type and broadcast.

I have a follow optimization that try to converts gather/scatter with
uniform memory acces to scalar load/store + broadcast. With this
optimization, we can remove this temporary change.
---
 llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp b/llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp
index 0368f9b8dbb00..b6c902a153eba 100644
--- a/llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp
+++ b/llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp
@@ -3182,10 +3182,17 @@ InstructionCost VPWidenMemoryRecipe::computeCost(ElementCount VF,
     // TODO: Using the original IR may not be accurate.
     // Currently, ARM will use the underlying IR to calculate gather/scatter
     // instruction cost.
-    const Value *Ptr = getLoadStorePointerOperand(&Ingredient);
-    Type *PtrTy = toVectorTy(Ptr->getType(), VF);
     assert(!Reverse &&
            "Inconsecutive memory access should not have the order.");
+
+    const Value *Ptr = getLoadStorePointerOperand(&Ingredient);
+    Type *PtrTy = Ptr->getType();
+
+    // If the address value is uniform across all lanes, then the address can be
+    // calculated with scalar type and broadcast.
+    if (!vputils::isSingleScalar(getAddr()))
+      PtrTy = toVectorTy(PtrTy, VF);
+
     return Ctx.TTI.getAddressComputationCost(PtrTy, nullptr, nullptr,
                                              Ctx.CostKind) +
            Ctx.TTI.getGatherScatterOpCost(Opcode, Ty, Ptr, IsMasked, Alignment,