[mlir][bufferization] Add BufferViewFlowOpInterface

This commit adds the `BufferViewFlowOpInterface` to the bufferization dialect. This interface can be implemented by ops that operate on buffers to indicate that a buffer op result and/or region entry block argument may be the same buffer as a buffer operand (or a view thereof). This interface is queried by the `BufferViewFlowAnalysis`.

There are currently no ops that implement this interface. The first op implementations will be added in a consecutive commit.

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Author: matthias-springer

mlir/include/mlir/Dialect/Arith/Transforms/BufferViewFlowOpInterfaceImpl.h

@@ -0,0 +1,20 @@
+//===- BufferViewFlowOpInterfaceImpl.h - Buffer View Analysis ---*- 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef MLIR_DIALECT_ARITH_BUFFERVIEWFLOWOPINTERFACEIMPL_H
+#define MLIR_DIALECT_ARITH_BUFFERVIEWFLOWOPINTERFACEIMPL_H
+
+namespace mlir {
+class DialectRegistry;
+
+namespace arith {
+void registerBufferViewFlowOpInterfaceExternalModels(DialectRegistry &registry);
+} // namespace arith
+} // namespace mlir
+
+#endif // MLIR_DIALECT_ARITH_BUFFERVIEWFLOWOPINTERFACEIMPL_H

mlir/include/mlir/Dialect/Bufferization/IR/BufferViewFlowOpInterface.h

@@ -0,0 +1,27 @@
+//===- BufferViewFlowOpInterface.h - Buffer View Flow Analysis --*- 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef MLIR_DIALECT_BUFFERIZATION_IR_BUFFERVIEWFLOWOPINTERFACE_H_
+#define MLIR_DIALECT_BUFFERIZATION_IR_BUFFERVIEWFLOWOPINTERFACE_H_
+
+#include "mlir/IR/OpDefinition.h"
+#include "mlir/Support/LLVM.h"
+
+namespace mlir {
+class ValueRange;
+
+namespace bufferization {
+
+using RegisterDependenciesFn = std::function<void(ValueRange, ValueRange)>;
+
+} // namespace bufferization
+} // namespace mlir
+
+#include "mlir/Dialect/Bufferization/IR/BufferViewFlowOpInterface.h.inc"
+
+#endif // MLIR_DIALECT_BUFFERIZATION_IR_BUFFERVIEWFLOWOPINTERFACE_H_

mlir/include/mlir/Dialect/Bufferization/IR/BufferViewFlowOpInterface.td

@@ -0,0 +1,67 @@
+//===-- BufferViewFlowOpInterface.td - Buffer View Flow ----*- 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 BUFFER_VIEW_FLOW_OP_INTERFACE
+#define BUFFER_VIEW_FLOW_OP_INTERFACE
+
+include "mlir/IR/OpBase.td"
+
+def BufferViewFlowOpInterface :
+    OpInterface<"BufferViewFlowOpInterface"> {
+  let description = [{
+    An op interface for the buffer view flow analysis. This interface describes
+    buffer dependencies between operands and op results/region entry block
+    arguments.
+  }];
+  let cppNamespace = "::mlir::bufferization";
+  let methods = [
+      InterfaceMethod<
+        /*desc=*/[{
+          Populate buffer dependencies between operands and op results/region
+          entry block arguments.
+
+          Implementations should register dependencies between an operand ("X")
+          and an op result/region entry block argument ("Y") if Y may depend
+          on X. Y depends on X if Y and X are the same buffer or if Y is a
+          subview of X.
+
+          Example:
+          ```
+          %r = arith.select %c, %m1, %m2 : memref<5xf32>
+          ```
+          In the above example, %0 may depend on %m1 or %m2 and a correct
+          interface implementation should call:
+          - "registerDependenciesFn(%m1, %r)".
+          - "registerDependenciesFn(%m2, %r)"
+        }],
+        /*retType=*/"void",
+        /*methodName=*/"populateDependencies",
+        /*args=*/(ins
+            "::mlir::bufferization::RegisterDependenciesFn"
+                :$registerDependenciesFn)
+      >,
+      InterfaceMethod<
+        /*desc=*/[{
+          Return "true" if the given value is a terminal buffer. A buffer value
+          is "terminal" if it cannot be traced back any further in the buffer
+          view flow analysis. E.g., because the value is a newly allocated
+          buffer or because there is not enough information available.
+
+          The given SSA value is guaranteed to be an OpResult of this operation
+          or a region entry block argument of this operation.
+        }],
+        /*retType=*/"bool",
+        /*methodName=*/"isTerminalBuffer",
+        /*args=*/(ins "Value":$value),
+        /*methodBody=*/"",
+        /*defaultImplementation=*/"return false;"
+      >,
+  ];
+}
+
+#endif  // BUFFER_VIEW_FLOW_OP_INTERFACE

mlir/include/mlir/Dialect/Bufferization/IR/CMakeLists.txt

@@ -3,6 +3,7 @@ add_mlir_doc(BufferizationOps BufferizationOps Dialects/ -gen-dialect-doc)
 add_mlir_interface(AllocationOpInterface)
 add_mlir_interface(BufferDeallocationOpInterface)
 add_mlir_interface(BufferizableOpInterface)
+add_mlir_interface(BufferViewFlowOpInterface)
 
 set(LLVM_TARGET_DEFINITIONS BufferizationEnums.td)
 mlir_tablegen(BufferizationEnums.h.inc -gen-enum-decls)

mlir/include/mlir/Dialect/Bufferization/Transforms/BufferViewFlowAnalysis.h

@@ -63,13 +63,19 @@ class BufferViewFlowAnalysis {
   /// results have to be changed.
   void rename(Value from, Value to);
 
+  /// Return "true" if the given value is a terminal.
+  bool isTerminalBuffer(Value value) const;
+
 private:
   /// This function constructs a mapping from values to its immediate
   /// dependencies.
   void build(Operation *op);
 
   /// Maps values to all immediate dependencies this value can have.
   ValueMapT dependencies;
+
+  /// A set of all terminal values. I.e., values at which the analysis stopped.
+  DenseSet<Value> terminals;
 };
 
 } // namespace mlir

mlir/include/mlir/Dialect/MemRef/Transforms/BufferViewFlowOpInterfaceImpl.h

@@ -0,0 +1,20 @@
+//===- BufferViewFlowOpInterfaceImpl.h - Buffer View Analysis ---*- 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef MLIR_DIALECT_MEMREF_BUFFERVIEWFLOWOPINTERFACEIMPL_H
+#define MLIR_DIALECT_MEMREF_BUFFERVIEWFLOWOPINTERFACEIMPL_H
+
+namespace mlir {
+class DialectRegistry;
+
+namespace memref {
+void registerBufferViewFlowOpInterfaceExternalModels(DialectRegistry &registry);
+} // namespace memref
+} // namespace mlir
+
+#endif // MLIR_DIALECT_MEMREF_BUFFERVIEWFLOWOPINTERFACEIMPL_H

mlir/include/mlir/InitAllDialects.h

@@ -21,6 +21,7 @@
 #include "mlir/Dialect/Arith/IR/Arith.h"
 #include "mlir/Dialect/Arith/IR/ValueBoundsOpInterfaceImpl.h"
 #include "mlir/Dialect/Arith/Transforms/BufferDeallocationOpInterfaceImpl.h"
+#include "mlir/Dialect/Arith/Transforms/BufferViewFlowOpInterfaceImpl.h"
 #include "mlir/Dialect/Arith/Transforms/BufferizableOpInterfaceImpl.h"
 #include "mlir/Dialect/ArmNeon/ArmNeonDialect.h"
 #include "mlir/Dialect/ArmSME/IR/ArmSME.h"
@@ -53,6 +54,7 @@
 #include "mlir/Dialect/MemRef/IR/MemRefMemorySlot.h"
 #include "mlir/Dialect/MemRef/IR/ValueBoundsOpInterfaceImpl.h"
 #include "mlir/Dialect/MemRef/Transforms/AllocationOpInterfaceImpl.h"
+#include "mlir/Dialect/MemRef/Transforms/BufferViewFlowOpInterfaceImpl.h"
 #include "mlir/Dialect/MemRef/Transforms/RuntimeOpVerification.h"
 #include "mlir/Dialect/Mesh/IR/MeshOps.h"
 #include "mlir/Dialect/NVGPU/IR/NVGPUDialect.h"
@@ -145,6 +147,7 @@ inline void registerAllDialects(DialectRegistry &registry) {
   affine::registerValueBoundsOpInterfaceExternalModels(registry);
   arith::registerBufferDeallocationOpInterfaceExternalModels(registry);
   arith::registerBufferizableOpInterfaceExternalModels(registry);
+  arith::registerBufferViewFlowOpInterfaceExternalModels(registry);
   arith::registerValueBoundsOpInterfaceExternalModels(registry);
   bufferization::func_ext::registerBufferizableOpInterfaceExternalModels(
       registry);
@@ -157,6 +160,7 @@ inline void registerAllDialects(DialectRegistry &registry) {
   linalg::registerTilingInterfaceExternalModels(registry);
   linalg::registerValueBoundsOpInterfaceExternalModels(registry);
   memref::registerAllocationOpInterfaceExternalModels(registry);
+  memref::registerBufferViewFlowOpInterfaceExternalModels(registry);
   memref::registerRuntimeVerifiableOpInterfaceExternalModels(registry);
   memref::registerValueBoundsOpInterfaceExternalModels(registry);
   memref::registerMemorySlotExternalModels(registry);

mlir/lib/Dialect/Arith/Transforms/BufferViewFlowOpInterfaceImpl.cpp

@@ -0,0 +1,45 @@
+//===- BufferViewFlowOpInterfaceImpl.cpp - Buffer View Flow Analysis ------===//
+//
+// 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 "mlir/Dialect/Arith/Transforms/BufferViewFlowOpInterfaceImpl.h"
+
+#include "mlir/Dialect/Arith/IR/Arith.h"
+#include "mlir/Dialect/Bufferization/IR/BufferViewFlowOpInterface.h"
+
+using namespace mlir;
+using namespace mlir::bufferization;
+using namespace mlir::arith;
+
+namespace mlir {
+namespace arith {
+namespace {
+
+struct SelectOpInterface
+    : public BufferViewFlowOpInterface::ExternalModel<SelectOpInterface,
+                                                      SelectOp> {
+  void
+  populateDependencies(Operation *op,
+                       RegisterDependenciesFn registerDependenciesFn) const {
+    auto selectOp = cast<SelectOp>(op);
+
+    // Either one of the true/false value may be selected at runtime.
+    registerDependenciesFn(selectOp.getTrueValue(), selectOp.getResult());
+    registerDependenciesFn(selectOp.getFalseValue(), selectOp.getResult());
+  }
+};
+
+} // namespace
+} // namespace arith
+} // namespace mlir
+
+void arith::registerBufferViewFlowOpInterfaceExternalModels(
+    DialectRegistry &registry) {
+  registry.addExtension(+[](MLIRContext *ctx, arith::ArithDialect *dialect) {
+    SelectOp::attachInterface<SelectOpInterface>(*ctx);
+  });
+}

mlir/lib/Dialect/Arith/Transforms/CMakeLists.txt

@@ -2,6 +2,7 @@ add_mlir_dialect_library(MLIRArithTransforms
   BufferDeallocationOpInterfaceImpl.cpp
   BufferizableOpInterfaceImpl.cpp
   Bufferize.cpp
+  BufferViewFlowOpInterfaceImpl.cpp
   EmulateUnsupportedFloats.cpp
   EmulateWideInt.cpp
   EmulateNarrowType.cpp

mlir/lib/Dialect/Bufferization/IR/BufferViewFlowOpInterface.cpp

@@ -0,0 +1,18 @@
+//===- BufferViewFlowOpInterface.cpp - Buffer View Flow Analysis ----------===//
+//
+// 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 "mlir/Dialect/Bufferization/IR/BufferViewFlowOpInterface.h"
+#include "mlir/Dialect/Bufferization/IR/Bufferization.h"
+
+namespace mlir {
+namespace bufferization {
+
+#include "mlir/Dialect/Bufferization/IR/BufferViewFlowOpInterface.cpp.inc"
+
+} // namespace bufferization
+} // namespace mlir

mlir/lib/Dialect/Bufferization/IR/CMakeLists.txt

@@ -4,6 +4,7 @@ add_mlir_dialect_library(MLIRBufferizationDialect
   BufferDeallocationOpInterface.cpp
   BufferizationOps.cpp
   BufferizationDialect.cpp
+  BufferViewFlowOpInterface.cpp
   UnstructuredControlFlow.cpp
 
   ADDITIONAL_HEADER_DIRS

mlir/lib/Dialect/Bufferization/Transforms/BufferViewFlowAnalysis.cpp

@@ -8,12 +8,16 @@
 
 #include "mlir/Dialect/Bufferization/Transforms/BufferViewFlowAnalysis.h"
 
+#include "mlir/Dialect/Bufferization/IR/BufferViewFlowOpInterface.h"
+#include "mlir/Interfaces/CallInterfaces.h"
 #include "mlir/Interfaces/ControlFlowInterfaces.h"
+#include "mlir/Interfaces/FunctionInterfaces.h"
 #include "mlir/Interfaces/ViewLikeInterface.h"
 #include "llvm/ADT/SetOperations.h"
 #include "llvm/ADT/SetVector.h"
 
 using namespace mlir;
+using namespace mlir::bufferization;
 
 /// Constructs a new alias analysis using the op provided.
 BufferViewFlowAnalysis::BufferViewFlowAnalysis(Operation *op) { build(op); }
@@ -65,18 +69,44 @@ void BufferViewFlowAnalysis::rename(Value from, Value to) {
 void BufferViewFlowAnalysis::build(Operation *op) {
   // Registers all dependencies of the given values.
   auto registerDependencies = [&](ValueRange values, ValueRange dependencies) {
-    for (auto [value, dep] : llvm::zip(values, dependencies))
+    for (auto [value, dep] : llvm::zip_equal(values, dependencies))
       this->dependencies[value].insert(dep);
   };
 
+  // Mark all buffer results and buffer region entry block arguments of the
+  // given op as terminals.
+  auto populateTerminalValues = [&](Operation *op) {
+    for (Value v : op->getResults())
+      if (isa<BaseMemRefType>(v.getType()))
+        this->terminals.insert(v);
+    for (Region &r : op->getRegions())
+      for (BlockArgument v : r.getArguments())
+        if (isa<BaseMemRefType>(v.getType()))
+          this->terminals.insert(v);
+  };
+
   op->walk([&](Operation *op) {
-    // TODO: We should have an op interface instead of a hard-coded list of
-    // interfaces/ops.
+    // Query BufferViewFlowOpInterface. If the op does not implement that
+    // interface, try to infer the dependencies from other interfaces that the
+    // op may implement.
+    if (auto bufferViewFlowOp = dyn_cast<BufferViewFlowOpInterface>(op)) {
+      bufferViewFlowOp.populateDependencies(registerDependencies);
+      for (Value v : op->getResults())
+        if (isa<BaseMemRefType>(v.getType()) &&
+            bufferViewFlowOp.isTerminalBuffer(v))
+          this->terminals.insert(v);
+      for (Region &r : op->getRegions())
+        for (BlockArgument v : r.getArguments())
+          if (isa<BaseMemRefType>(v.getType()) &&
+              bufferViewFlowOp.isTerminalBuffer(v))
+            this->terminals.insert(v);
+      return WalkResult::advance();
+    }
 
     // Add additional dependencies created by view changes to the alias list.
     if (auto viewInterface = dyn_cast<ViewLikeOpInterface>(op)) {
-      dependencies[viewInterface.getViewSource()].insert(
-          viewInterface->getResult(0));
+      registerDependencies(viewInterface.getViewSource(),
+                           viewInterface->getResult(0));
       return WalkResult::advance();
     }
 
@@ -131,16 +161,30 @@ void BufferViewFlowAnalysis::build(Operation *op) {
       return WalkResult::advance();
     }
 
-    // Unknown op: Assume that all operands alias with all results.
-    for (Value operand : op->getOperands()) {
-      if (!isa<BaseMemRefType>(operand.getType()))
-        continue;
-      for (Value result : op->getResults()) {
-        if (!isa<BaseMemRefType>(result.getType()))
-          continue;
-        registerDependencies({operand}, {result});
-      }
+    // Region terminators are handled together with RegionBranchOpInterface.
+    if (isa<RegionBranchTerminatorOpInterface>(op))
+      return WalkResult::advance();
+
+    if (isa<CallOpInterface>(op)) {
+      // This is an intra-function analysis. We have no information about other
+      // functions. Conservatively assume that each operand may alias with each
+      // result. Also mark the results are terminals because the function could
+      // return newly allocated buffers.
+      populateTerminalValues(op);
+      for (Value operand : op->getOperands())
+        for (Value result : op->getResults())
+          registerDependencies({operand}, {result});
+      return WalkResult::advance();
     }
+
+    // We have no information about unknown ops.
+    populateTerminalValues(op);
+
     return WalkResult::advance();
   });
 }
+
+bool BufferViewFlowAnalysis::isTerminalBuffer(Value value) const {
+  assert(isa<BaseMemRefType>(value.getType()) && "expected memref");
+  return terminals.contains(value);
+}

mlir/lib/Dialect/Bufferization/Transforms/CMakeLists.txt

@@ -26,6 +26,7 @@ add_mlir_dialect_library(MLIRBufferizationTransforms
   LINK_LIBS PUBLIC
   MLIRArithDialect
   MLIRBufferizationDialect
+  MLIRBufferizationTransforms
   MLIRControlFlowInterfaces
   MLIRFuncDialect
   MLIRFunctionInterfaces