[llvm] [VectorCombine] Add a mask for SK_Broadcast shuffle costing (PR #85808)

[Philip Reames via llvm-commits]

https://github.com/preames created https://github.com/llvm/llvm-project/pull/85808

This is part of a series of small patches to compute shuffle masks for the couple of cases where we call getShuffleCost without one. My goal is to add an invariant that all calls to getShuffleCost for fixed length vectors have a mask.

Note that this code appears to be reachable with scalable vectors, and thus we have to only pass a non-empty mask when the number of elements is precisely known.

>From fcaba35c68e670e3e64603dd8e819f6b02c98c48 Mon Sep 17 00:00:00 2001
From: Philip Reames <preames at rivosinc.com>
Date: Tue, 19 Mar 2024 08:50:01 -0700
Subject: [PATCH] [VectorCombine] Add a mask for SK_Broadcast shuffle costing

This is part of a series of small patches to compute shuffle masks
for the couple of cases where we call getShuffleCost without one. My
goal is to add an invariant that all calls to getShuffleCost for fixed
length vectors have a mask.

Note that this code appears to be reachable with scalable vectors,
and thus we have to only pass a non-empty mask when the number of
elements is precisely known.
---
 llvm/lib/Transforms/Vectorize/VectorCombine.cpp | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/Transforms/Vectorize/VectorCombine.cpp b/llvm/lib/Transforms/Vectorize/VectorCombine.cpp
index dc669e314a0d9a..85c8d3996bba51 100644
--- a/llvm/lib/Transforms/Vectorize/VectorCombine.cpp
+++ b/llvm/lib/Transforms/Vectorize/VectorCombine.cpp
@@ -786,9 +786,12 @@ bool VectorCombine::scalarizeVPIntrinsic(Instruction &I) {
   // intrinsic
   VectorType *VecTy = cast<VectorType>(VPI.getType());
   TTI::TargetCostKind CostKind = TTI::TCK_RecipThroughput;
+  SmallVector<int> Mask;
+  if (auto *FVTy = dyn_cast<FixedVectorType>(VecTy))
+    Mask.resize(FVTy->getNumElements(), 0);
   InstructionCost SplatCost =
       TTI.getVectorInstrCost(Instruction::InsertElement, VecTy, CostKind, 0) +
-      TTI.getShuffleCost(TargetTransformInfo::SK_Broadcast, VecTy);
+      TTI.getShuffleCost(TargetTransformInfo::SK_Broadcast, VecTy, Mask);
 
   // Calculate the cost of the VP Intrinsic
   SmallVector<Type *, 4> Args;