[Mlir-commits] [mlir] [mlir][Vector] add vector.insert canonicalization pattern for vectors created from ub.poison (PR #142944)

[Andrzej Warzyński]

================
@@ -3250,6 +3262,128 @@ class InsertSplatToSplat final : public OpRewritePattern<InsertOp> {
     return success();
   }
 };
+
+/// Pattern to optimize a chain of insertions.
+///
+/// This pattern identifies chains of vector.insert operations that:
+/// 1. Only insert values at static positions.
+/// 2. Completely initialize all elements in the resulting vector.
+/// 3. All intermediate insert operations have only one use.
+///
+/// When these conditions are met, the entire chain can be replaced with a
+/// single vector.from_elements operation.
+///
+/// Example transformation:
+///   %poison = ub.poison : vector<2xi32>
+///   %0 = vector.insert %c1, %poison[0] : i32 into vector<2xi32>
+///   %1 = vector.insert %c2, %0[1] : i32 into vector<2xi32>
+/// ->
+///   %result = vector.from_elements %c1, %c2 : vector<2xi32>
+class InsertChainFullyInitialized final : public OpRewritePattern<InsertOp> {
+public:
+  using OpRewritePattern::OpRewritePattern;
+  LogicalResult matchAndRewrite(InsertOp op,
+                                PatternRewriter &rewriter) const override {
+
+    VectorType destTy = op.getDestVectorType();
+    if (destTy.isScalable())
+      return failure();
+    // Check if the result is used as the dest operand of another vector.insert
+    // Only care about the last op in a chain of insertions.
+    for (Operation *user : op.getResult().getUsers())
+      if (auto insertOp = dyn_cast<InsertOp>(user))
+        if (insertOp.getDest() == op.getResult())
+          return failure();
+
+    InsertOp currentOp = op;
+    SmallVector<InsertOp> chainInsertOps;
+    while (currentOp) {
+      // Dynamic position is not supported.
+      if (currentOp.hasDynamicPosition())
+        return failure();
+
+      chainInsertOps.push_back(currentOp);
+      currentOp = currentOp.getDest().getDefiningOp<InsertOp>();
+      // Check that intermediate inserts have only one use to avoid an explosion
+      // of vectors.
----------------
banach-space wrote:

I don't quite follow what you mean by "explosion of vectors". With multiple users there simply wouldn't be a "chain" of `vector.insert`s. Could you provide an example of where such an "explosion" could happen?

https://github.com/llvm/llvm-project/pull/142944