[clang] [llvm] [VectorCombine] Shrink loads used in shufflevector rebroadcasts (PR #128938)

[Simon Pilgrim via cfe-commits]

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@@ -3483,6 +3487,127 @@ bool VectorCombine::foldInterleaveIntrinsics(Instruction &I) {
   return true;
 }
 
+// Attempt to shrink loads that are only used by shufflevector instructions.
+bool VectorCombine::shrinkLoadForShuffles(Instruction &I) {
+  auto *OldLoad = dyn_cast<LoadInst>(&I);
+  if (!OldLoad || !OldLoad->isSimple())
+    return false;
+
+  auto *VecTy = dyn_cast<FixedVectorType>(OldLoad->getType());
+  if (!VecTy)
+    return false;
+
+  // Search all uses of load. If all uses are shufflevector instructions, and
+  // the second operands are all poison values, find the minimum and maximum
+  // indices of the vector elements referenced by all shuffle masks.
+  // Otherwise return `std::nullopt`.
+  using IndexRange = std::pair<int, int>;
+  auto GetIndexRangeInShuffles = [&]() -> std::optional<IndexRange> {
+    IndexRange OutputRange = IndexRange(VecTy->getNumElements(), -1);
+    for (auto &Use : I.uses()) {
+      // Ensure all uses match the required pattern.
+      User *Shuffle = Use.getUser();
+      Value *Op0 = nullptr;
+      ArrayRef<int> Mask;
+
+      if (!match(Shuffle, m_Shuffle(m_Value(Op0), m_Undef(), m_Mask(Mask))))
+        return std::nullopt;
+
+      // Ignore shufflevector instructions that have no uses.
+      if (Shuffle->use_empty())
+        continue;
+
+      // Find the min and max indices used by the shufflevector instruction.
+      FixedVectorType *Op0Ty = cast<FixedVectorType>(Op0->getType());
+      int NumElems = static_cast<int>(Op0Ty->getNumElements());
+
+      for (int Index : Mask) {
+        if (Index >= 0) {
+          Index %= NumElems;
+          OutputRange.first = std::min(Index, OutputRange.first);
+          OutputRange.second = std::max(Index, OutputRange.second);
+        }
+      }
+    }
+
+    if (OutputRange.second < OutputRange.first)
+      return std::nullopt;
+
+    return OutputRange;
+  };
+
+  // Get the range of vector elements used by shufflevector instructions.
+  if (auto Indices = GetIndexRangeInShuffles()) {
+    unsigned OldSize = VecTy->getNumElements();
+    unsigned NewSize = Indices->second + 1u;
+
+    // If the range of vector elements is smaller than the full load, attempt
+    // to create a smaller load.
+    if (NewSize < OldSize) {
+      auto Builder = IRBuilder(&I);
+      Builder.SetCurrentDebugLocation(I.getDebugLoc());
+
+      // Create new load of smaller vector.
+      auto *ElemTy = VecTy->getElementType();
+      auto *NewVecTy = FixedVectorType::get(ElemTy, NewSize);
+      auto *PtrOp = OldLoad->getPointerOperand();
+      auto *NewLoad = cast<LoadInst>(
+          Builder.CreateAlignedLoad(NewVecTy, PtrOp, OldLoad->getAlign()));
+      NewLoad->copyMetadata(I);
+
+      // Calculate costs of old and new ops.
+      auto OldCost = TTI.getMemoryOpCost(
+          Instruction::Load, OldLoad->getType(), OldLoad->getAlign(),
+          OldLoad->getPointerAddressSpace(), CostKind);
+      auto NewCost = TTI.getMemoryOpCost(
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RKSimon wrote:

(style) avoid auto in most cases

https://github.com/llvm/llvm-project/pull/128938