[Mlir-commits] [mlir] [mlir][bufferization] Empty tensor elimination for materialize_in_destination (PR #65468)

[llvmlistbot at llvm.org]

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+//===-- InferDestinationOpInterface.td - Infer destination -*- tablegen -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef INFER_DESTINATION_OP_INTERFACE
+#define INFER_DESTINATION_OP_INTERFACE
+
+include "mlir/IR/OpBase.td"
+
+def InferDestinationOpInterface : OpInterface<"InferDestinationOpInterface"> {
+  let description = [{
+    This interface can be implemented by ops that read data from an "input"
+    tensor and store the result into an "output"/"init" tensor (i.e., the
+    "destination" tensor). It drives the empty tensor elimination pass.
+
+    The `getOrBuildDestination` interface method returns the destination tensor
+    (or a slice thereof). Assuming that the op does not bufferize to a memory
+    read on the destination tensor (or the aforementioned slice), if the source
+    originates from a "tensor.empty", that "tensor.empty" can be replaced with
+    the result of `getOrBuildDestination`. This can reduce the number of
+    allocations and memcpys during bufferization: instead of computing data in
+    a temporary buffer (the result of "tensor.empty") and then copying it into
+    the destination buffer, the result will be computed in the destination
+    buffer directly.
+
+    Example:
+    ```
+    %0 = tensor.empty() : tensor<5xf32>
+    %1 = linalg.fill ins(%cst : f32) outs(%0 : tensor<5xf32>) -> tensor<5xf32>
+    %2 = tensor.insert_slice %1 into %dst [%offset][5][1]
+        : tensor<5xf32> into tensor<?xf32>
+    ```
+
+    "tensor.insert_slice" transfers %1 into a slice of %dst.
+    `getOrBuildDestination` should the slice. `getNeededValues` should return
+    %dst and %offset, because these values are needed to build the destination.
+
+    After empty tensor elimination:
+    ```
+    %0 = tensor.extract_slice %dst [%offset][5][1]
+        : tensor<?xf32> to tensor<5xf32>
+    %1 = linalg.fill ins(%cst : f32) outs(%0 : tensor<5xf32>) -> tensor<5xf32>
+    %2 = tensor.insert_slice %1 into %dst [%offset][5][1]
+        : tensor<5xf32> into tensor<?xf32>
+    ```
+
+    This IR will bufferize without an allocation and without a memcpy.
+  }];
+  let cppNamespace = "::mlir::bufferization";
+  let methods = [
+      InterfaceMethod<
+        /*desc=*/[{
+          Build or return the destination tensor (or a slice thereof) into which
+          the given operand (or an element-wise computation thereof) will be
+          stored. Only values returned by `getNeededValues` may be used to build
+          the destination. This interface method will be called only on tensor
+          operands for which `isAnchor` returns "true".
+        }],
+        /*retType=*/"::mlir::Value",
+        /*methodName=*/"getOrBuildDestination",
+        /*args=*/(ins "::mlir::OpBuilder &":$builder,
+                      "::mlir::Location":$loc,
+                      "::mlir::OpOperand &":$operand),
+        /*methodBody=*/"",
+        /*defaultImplementation=*/[{
+          llvm_unreachable("getOrBuildDestination not implemented");
+        }]
+      >,
+      InterfaceMethod<
+        /*desc=*/[{
+          Return the SSA values that are needed to build the destination tensor.
+          This interface method will be called only on tensor operands for which
+          `isAnchor` returns "true"
----------------
srcarroll wrote:

maybe it's just me, but it's unclear what "needed to build" means. Are these all the SSA values that would be required to build whatever op is being built in `getOrBuildDestination`?  I must be thinking about that incorrectly because I see no dependence between the two in the `InsertSliceLikeOpInferDestinationInterface` changes.

https://github.com/llvm/llvm-project/pull/65468