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

[Simon Pilgrim via llvm-commits]

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@@ -3483,6 +3487,131 @@ 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()) {
+      // All uses must be shufflevector instructions.
+      auto *Shuffle = dyn_cast<ShuffleVectorInst>(Use.getUser());
+      if (!Shuffle)
+        return std::nullopt;
+
+      // Ignore shufflevector instructions that have no uses.
+      if (!Shuffle->hasNUsesOrMore(1u))
+        continue;
+
+      // Ensure second operand is a poison value.
+      auto *Op0 = Shuffle->getOperand(0);
+      auto *Op1 = Shuffle->getOperand(1);
----------------
RKSimon wrote:

(style) avoid auto apart from casts

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