[Mlir-commits] [mlir] [mlir][linalg] Add pattern to clean unused results after fusion (PR #158627)

[Pavel Lipskiy]

https://github.com/pavlips created https://github.com/llvm/llvm-project/pull/158627

In some cases, elementwise fusion can produce ops with multiple results, but only one of them is used in the IR. This makes the IR less readable and prevents additional fusions from being triggered.

This patch adds the `DropRedundantResultsFromGenericOps` pattern to find these outputs and convert them into inputs.

>From 60d804c7eab8f90b94a1ad6b73206521c5333e06 Mon Sep 17 00:00:00 2001
From: Pavel Lipskiy <pavel.lipskiy at arm.com>
Date: Thu, 28 Aug 2025 08:52:57 +0100
Subject: [PATCH] [mlir][linalg] Add pattern to clean unused results after
 fusion

In some cases, elementwise fusion can produce ops with multiple
results, but only one of them is used in the IR. This makes the
IR less readable and prevents additional fusions from being triggered.

This patch adds the `DropRedundantResultsFromGenericOps` pattern
to find these outputs and convert them into inputs.

Signed-off-by: Pavel Lipskiy <pavel.lipskiy at arm.com>
---
 .../Linalg/Transforms/ElementwiseOpFusion.cpp | 51 +++++++++++++++++++
 .../Linalg/fusion-elementwise-ops.mlir        | 23 ++++++++-
 2 files changed, 73 insertions(+), 1 deletion(-)

diff --git a/mlir/lib/Dialect/Linalg/Transforms/ElementwiseOpFusion.cpp b/mlir/lib/Dialect/Linalg/Transforms/ElementwiseOpFusion.cpp
index 3bd763ea00cd7..aac54327213ac 100644
--- a/mlir/lib/Dialect/Linalg/Transforms/ElementwiseOpFusion.cpp
+++ b/mlir/lib/Dialect/Linalg/Transforms/ElementwiseOpFusion.cpp
@@ -2200,6 +2200,56 @@ struct RemoveOutsDependency : public OpRewritePattern<GenericOp> {
   }
 };
 
+/// Drops an unused result from an elementwise `linalg.generic` by
+/// reclassifying its tied `outs` operand as an extra input operand.
+struct DropRedundantResultsFromGenericOps
+    : public OpRewritePattern<linalg::GenericOp> {
+  using OpRewritePattern<linalg::GenericOp>::OpRewritePattern;
+  LogicalResult matchAndRewrite(linalg::GenericOp op,
+                                PatternRewriter &rewriter) const override {
+    if (!linalg::isElementwise(op) || op.getNumResults() < 2U)
+      return failure();
+    // Given that the op has no reductions, there is no need to preserve an
+    // unused result: transform it into an input instead.
+    auto maybeUnusedRes = llvm::find_if(
+        op.getResults(), [](OpResult res) { return res.use_empty(); });
+    if (maybeUnusedRes == op.getResults().end())
+      return failure();
+    OpResult unusedRes = *maybeUnusedRes;
+    const unsigned resIdx = unusedRes.getResultNumber();
+    auto resTypes = llvm::to_vector(op.getResultTypes());
+    resTypes.erase(resTypes.begin() + resIdx);
+    SmallVector<Value> resValues = llvm::to_vector_of<Value>(op.getResults());
+    resValues.erase(resValues.begin() + resIdx);
+    const int64_t numInputs = op.getNumDpsInputs();
+    OpOperand *resOperand = op.getTiedOpOperand(unusedRes);
+    AffineMap map = op.getIndexingMapMatchingResult(unusedRes);
+    const unsigned operandIdx = resOperand->getOperandNumber();
+    // Remove the output operand and add it as an input operand with the same
+    // map.
+    SmallVector<Value> outs(op.getOutputs());
+    outs.erase(outs.begin() + resIdx);
+    SmallVector<Value> ins(op.getInputs());
+    ins.insert(ins.begin() + numInputs, resOperand->get());
+    SmallVector<AffineMap> maps = op.getIndexingMapsArray();
+    maps.erase(maps.begin() + operandIdx);
+    maps.insert(maps.begin() + numInputs, map);
+    rewriter.setInsertionPoint(op);
+    auto newGenericOp = rewriter.create<linalg::GenericOp>(
+        op.getLoc(), TypeRange(resTypes), ins, outs, maps,
+        op.getIteratorTypesArray());
+    op->setDiscardableAttrs(op->getDiscardableAttrDictionary());
+    op.getBody()->getTerminator()->eraseOperands(resIdx);
+    newGenericOp.getRegion().takeBody(op.getBodyRegion());
+    // Replace the remaining results of the old op with the results of the new
+    // op.
+    rewriter.replaceAllUsesWith(resValues, newGenericOp.getResults());
+    // Remove the old op.
+    rewriter.eraseOp(op);
+    return success();
+  }
+};
+
 /// Fold linalg.fill into linalg.generic
 struct FoldFillWithGenericOp : public OpRewritePattern<GenericOp> {
   using OpRewritePattern<GenericOp>::OpRewritePattern;
@@ -2262,6 +2312,7 @@ void mlir::linalg::populateElementwiseOpsFusionPatterns(
                RemoveOutsDependency>(context);
   // Add the patterns that clean up dead operands and results.
   populateEraseUnusedOperandsAndResultsPatterns(patterns);
+  patterns.add<DropRedundantResultsFromGenericOps>(context);
 }
 
 void mlir::linalg::populateCollapseDimensions(
diff --git a/mlir/test/Dialect/Linalg/fusion-elementwise-ops.mlir b/mlir/test/Dialect/Linalg/fusion-elementwise-ops.mlir
index bc55c12c02f29..173ec8a8a5f38 100644
--- a/mlir/test/Dialect/Linalg/fusion-elementwise-ops.mlir
+++ b/mlir/test/Dialect/Linalg/fusion-elementwise-ops.mlir
@@ -1079,4 +1079,25 @@ module {
 // CHECK-NOT:     linalg.generic
 // CHECK:         tensor.expand_shape
 // CHECK:         linalg.generic {{.*}}, iterator_types = ["parallel", "parallel", "parallel", "parallel", "parallel", "parallel", "reduction"]}
-// CHECK-SAME:     ins(%[[ARG0]], %[[FUSED]]#1 : tensor<1x1x2x1xf32>, tensor<4x1x1x1xf32>)
\ No newline at end of file
+// CHECK-SAME:     ins(%[[ARG0]], %[[FUSED]]#1 : tensor<1x1x2x1xf32>, tensor<4x1x1x1xf32>)
+
+// -----
+// CHECK-LABEL: @drop_unused_results
+// CHECK-SAME:   [[ARG0:%[a-zA-Z0-9]+]]: tensor<64xf32>, [[ARG1:%[a-zA-Z0-9]+]]: tensor<1x56x56x64xf32>
+func.func @drop_unused_results(%arg0: tensor<64xf32>, %arg1: tensor<1x56x56x64xf32>) -> tensor<1x56x56x64xf32> {
+  %cst = arith.constant 3.40282347E+38 : f32
+  %cst_0 = arith.constant 0.000000e+00 : f32
+  // CHECK: [[OUT:%[a-zA-Z0-9]+]] = tensor.empty() : tensor<1x56x56x64xf32>
+  %0 = tensor.empty() : tensor<1x56x56x64xf32>
+  // CHECK: [[RES:%[0-9]+]] = linalg.generic {{.*}} ins([[ARG0]], [[ARG1]] : tensor<64xf32>, tensor<1x56x56x64xf32>) outs([[OUT]] : tensor<1x56x56x64xf32>)
+  %1:2 = linalg.generic {indexing_maps = [affine_map<(d0, d1, d2, d3) -> (d3)>, affine_map<(d0, d1, d2, d3) -> (d0, d1, d2, d3)>, affine_map<(d0, d1, d2, d3) -> (d0, d1, d2, d3)>], iterator_types = ["parallel", "parallel", "parallel", "parallel"]} ins(%arg0 : tensor<64xf32>) outs(%arg1, %0 : tensor<1x56x56x64xf32>, tensor<1x56x56x64xf32>) {
+  ^bb0(%in: f32, %out: f32, %out_1: f32):
+    %2 = arith.addf %in, %out : f32
+    %3 = arith.minimumf %2, %cst : f32
+    %4 = arith.maximumf %3, %cst_0 : f32
+    linalg.yield %2, %4 : f32, f32
+  } -> (tensor<1x56x56x64xf32>, tensor<1x56x56x64xf32>)
+  // CHECK: -> tensor<1x56x56x64xf32>
+  // CHECK: return [[RES]] : tensor<1x56x56x64xf32>
+  return %1#1 : tensor<1x56x56x64xf32>
+}