[llvm] [SLP] Compute a shuffle mask for getGatherCost (PR #85330)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Philip Reames (preames)

<details>
<summary>Changes</summary>

This is the second 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.

---
Full diff: https://github.com/llvm/llvm-project/pull/85330.diff


1 Files Affected:

- (modified) llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp (+4-2) 


``````````diff
diff --git a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
index b4cce680e2876f..502c7a3d6bae39 100644
--- a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
+++ b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
@@ -10516,6 +10516,7 @@ InstructionCost BoUpSLP::getGatherCost(ArrayRef<Value *> VL,
           TTI->getVectorInstrCost(Instruction::InsertElement, VecTy, CostKind,
                                   I, Constant::getNullValue(VecTy), V);
   };
+  SmallVector<int> ShuffleMask(VL.size(), -1);
   for (unsigned I = 0, E = VL.size(); I < E; ++I) {
     Value *V = VL[I];
     // No need to shuffle duplicates for constants.
@@ -10526,6 +10527,7 @@ InstructionCost BoUpSLP::getGatherCost(ArrayRef<Value *> VL,
     if (!UniqueElements.insert(V).second) {
       DuplicateNonConst = true;
       ShuffledElements.setBit(I);
+      ShuffleMask[I] = I;
       continue;
     }
     EstimateInsertCost(I, V);
@@ -10535,8 +10537,8 @@ InstructionCost BoUpSLP::getGatherCost(ArrayRef<Value *> VL,
         TTI->getScalarizationOverhead(VecTy, ~ShuffledElements, /*Insert*/ true,
                                       /*Extract*/ false, CostKind);
   if (DuplicateNonConst)
-    Cost +=
-        TTI->getShuffleCost(TargetTransformInfo::SK_PermuteSingleSrc, VecTy);
+    Cost += TTI->getShuffleCost(TargetTransformInfo::SK_PermuteSingleSrc,
+                                VecTy, ShuffleMask);
   return Cost;
 }
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/85330