[llvm] [SandboxVec] Add a simple pack reuse pass (PR #141848)

[via llvm-commits]

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@@ -179,13 +197,61 @@ class VecUtils {
   /// \Returns the first integer power of 2 that is <= Num.
   static unsigned getFloorPowerOf2(unsigned Num);
 
+  /// If \p I is the last instruction of a pack pattern, then this function
+  /// returns the instructions in the pack and the operands in the pack, else
+  /// returns nullopt.
+  static std::optional<
+      std::pair<SmallVector<Instruction *>, SmallVector<Value *>>>
+  matchPack(Instruction *I) {
+    // TODO: Support vector pack patterns.
+    // TODO: Support out-of-order inserts.
+
+    // Early return if `I` is not an Insert.
+    if (!isa<InsertElementInst>(I))
+      return std::nullopt;
+    auto *BB0 = I->getParent();
+    // The pack contains as many instrs as the lanes of the bottom-most Insert
+    unsigned ExpectedNumInserts = VecUtils::getNumLanes(I);
+    assert(ExpectedNumInserts >= 2 && "Expected at least 2 inserts!");
+    SmallVector<Instruction *> PackInstrs;
+    SmallVector<Value *> PackOperands;
+    PackOperands.resize(ExpectedNumInserts);
+    // Collect the inserts by walking up the use-def chain.
+    Instruction *InsertI = I;
+    for ([[maybe_unused]] auto Cnt : seq<unsigned>(ExpectedNumInserts)) {
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vporpo wrote:

Ah yes, thanks for the suggestion, it looks a lot better now.

https://github.com/llvm/llvm-project/pull/141848