Add a fold-add-into-dest pattern and pass

Achieves the sum of linalg.add by relying on the accumulation semantics
of contraction ops w.r.t. their out/dest argument. Main value is likely
to come from that a buffer allocation can be elided, in case the
contraction op had its own buffer.

include/TPP/Passes.td

@@ -221,6 +221,16 @@ def ConstantFoldPack : Pass<"constant-fold-pack", "ModuleOp"> {
                            "arith::ArithDialect"];
 }
 
+def FoldAddIntoDest : Pass<"fold-add-into-dest", "ModuleOp"> {
+  let summary = "TODO";
+  let description = [{
+    TODO.
+  }];
+  let dependentDialects = ["linalg::LinalgDialect",
+                           "tensor::TensorDialect",
+                           "arith::ArithDialect"];
+}
+
 def ElementWiseFusion : Pass<"element-wise-fusion", "func::FuncOp"> {
   let summary = "Run linalg element-wise fusion";
 }

include/TPP/Transforms/Utils/ValueUtils.h

@@ -12,6 +12,7 @@
 namespace mlir {
 class Value;
 class OpBuilder;
+class Operation;
 namespace utils {
 
 // Returns true if the value is a constant float or integer.
@@ -20,6 +21,9 @@ bool isValConstZero(Value val);
 // Returns true if the op defining `val` represents a zero filled tensor.
 bool isZeroTensor(Value val);
 
+// Returns true if the operation represents a zero filled tensor.
+bool isZeroOp(Operation *);
+
 // Returns the strides of `val`. The method returns something usefull
 // only if the `val` type is a strided memref and the strides are statically
 // known.

lib/TPP/Transforms/CMakeLists.txt

@@ -22,6 +22,7 @@ add_mlir_library(TPPTransforms
   LinalgConvertCompareSelectToMaximumfPass.cpp
   ConvertLinalgToInplace.cpp
   FoldIntoEltwise.cpp
+  FoldAddIntoDest.cpp
 
   ADDITIONAL_HEADER_DIRS
     ${PROJECT_SOURCE_DIR}/include/TPP

lib/TPP/Transforms/FoldAddIntoDest.cpp

@@ -0,0 +1,101 @@
+//===- FoldAddIntoDest.cpp ---------------------------------------*- C++-*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "TPP/Passes.h"
+#include "TPP/Transforms/Utils/ValueUtils.h"
+#include "mlir/Dialect/Linalg/IR/Linalg.h"
+#include "mlir/Dialect/Linalg/Transforms/Transforms.h"
+#include "mlir/IR/Dominance.h"
+#include "mlir/Transforms/GreedyPatternRewriteDriver.h"
+using namespace mlir;
+
+namespace mlir {
+namespace tpp {
+#define GEN_PASS_DEF_FOLDADDINTODEST
+#include "TPP/Passes.h.inc"
+} // namespace tpp
+} // namespace mlir
+
+namespace {
+
+/// Replace a linalg.add where its linalg-contraction operand - with a
+/// zero-filled destination - is dominated by the `other` linalg operand,
+/// by passing `other` as the destination of the contraction.
+struct FoldAddIntoDestRewrite : public OpRewritePattern<linalg::AddOp> {
+  using OpRewritePattern<linalg::AddOp>::OpRewritePattern;
+
+  LogicalResult matchAndRewrite(linalg::AddOp addOp,
+                                PatternRewriter &rewriter) const override {
+    Value dominatingOperand = nullptr;
+    linalg::LinalgOp dominatedOp = nullptr;
+    {
+      auto firstOperand = addOp.getOperand(0);
+      auto secondOperand = addOp.getOperand(1);
+
+      // Can only put one of addOp's operands in the dest/out arg of the other's
+      // defining op based on suitable dominance.
+      if (auto secondOp = secondOperand.getDefiningOp<linalg::LinalgOp>()) {
+        DominanceInfo domInfo(secondOp);
+        if (domInfo.properlyDominates(firstOperand, secondOp)) {
+          dominatingOperand = firstOperand;
+          dominatedOp = secondOp;
+        }
+      }
+      if (auto firstOp = firstOperand.getDefiningOp<linalg::LinalgOp>()) {
+        DominanceInfo domInfo(firstOp);
+        if (domInfo.properlyDominates(secondOperand, firstOp)) {
+          dominatingOperand = secondOperand;
+          dominatedOp = firstOp;
+        }
+      }
+      if (!dominatingOperand || !dominatedOp)
+        return failure();
+      // NB: As linalg.add's generalisation ignores the out argument in its
+      //     region there is no need to perform checks on addOp's out argument.
+    }
+
+    // When the dominated op has a single-result and is a contraction, the op
+    // accumulates on the out argument, starting from the supplied out argument.
+    // E.g., AddOp is not a contraction and hence ignores its out arg's value.
+    if (dominatedOp->getNumResults() != 1 ||
+        !linalg::isaContractionOpInterface(dominatedOp))
+      return rewriter.notifyMatchFailure(
+          dominatedOp, "expected dominated op to be single-result contraction");
+
+    // As the dominated op was already accumulating on its out argument, it is
+    // only safe to discard its current out arg when it is the additive zero.
+    auto *dominatedDest = dominatedOp->getOperand(2).getDefiningOp();
+    if (!mlir::utils::isZeroOp(dominatedDest))
+      return rewriter.notifyMatchFailure(
+          dominatedOp, "expected dominated op's dest to be additive zero");
+
+    // Replace the additive-zero out argument of the dominated op by the
+    // dominating summand. This makes the dominated op's result the sum of both
+    // of addOp's arguments - therefore we replace addOp and it uses by it.
+    rewriter.modifyOpInPlace(
+        dominatedOp, [&]() { dominatedOp->setOperand(2, dominatingOperand); });
+    rewriter.replaceAllOpUsesWith(addOp, dominatedOp->getResult(0));
+    return success();
+  }
+};
+
+/// Replace linalg.add when destination passing suffices for achieving the sum.
+struct FoldAddIntoDest
+    : public tpp::impl::FoldAddIntoDestBase<FoldAddIntoDest> {
+
+  void runOnOperation() override {
+    auto *ctx = &getContext();
+
+    RewritePatternSet patterns(ctx);
+    patterns.add<FoldAddIntoDestRewrite>(ctx);
+
+    (void)applyPatternsAndFoldGreedily(getOperation(), std::move(patterns));
+  }
+};
+
+} // namespace

lib/TPP/Transforms/Utils/ValueUtils.cpp

@@ -40,7 +40,7 @@ static bool isZeroAttr(Attribute attribute) {
 }
 
 // Prototypes
-static bool isZeroOp(Operation *);
+bool isZeroOp(Operation *);
 
 // Returns true if the value represents a zero filled tensor.
 // Recurse into isZeroOp for defining ops if not immediately obvious
@@ -70,7 +70,7 @@ bool isZeroTensor(Value val) {
 
 // Returns true if the operation represents a zero filled tensor
 // Recurses into isZeroTensor for operands and isZeroAttr for attributes
-static bool isZeroOp(Operation *defOp) {
+bool isZeroOp(Operation *defOp) {
   if (!defOp)
     return false;