Add initial liveness analysis including test cases.

Closes tensorflow/mlir#255

PiperOrigin-RevId: 284935454

mlir/include/mlir/Analysis/Liveness.h

@@ -0,0 +1,157 @@
+//===- Liveness.h - Liveness analysis for MLIR ------------------*- C++ -*-===//
+//
+// Copyright 2019 The MLIR Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+// =============================================================================
+//
+// This file contains an analysis for computing liveness information from a
+// given top-level operation. The current version of the analysis uses a
+// traditional algorithm to resolve detailed live-range information about all
+// values within the specified regions. It is also possible to query liveness
+// information on block level.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef MLIR_ANALYSIS_LIVENESS_H
+#define MLIR_ANALYSIS_LIVENESS_H
+
+#include <vector>
+
+#include "mlir/Support/LLVM.h"
+#include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/SmallPtrSet.h"
+
+namespace mlir {
+
+class Block;
+class LivenessBlockInfo;
+class Operation;
+class Region;
+class Value;
+
+/// Represents an analysis for computing liveness information from a
+/// given top-level operation. The analysis iterates over all associated
+/// regions that are attached to the given top-level operation. It
+/// computes liveness information for every value and block that are
+/// included in the mentioned regions. It relies on a fixpoint iteration
+/// to compute all live-in and live-out values of all included blocks.
+/// Sample usage:
+///   Liveness liveness(topLevelOp);
+///   auto &allInValues = liveness.getLiveIn(block);
+///   auto &allOutValues = liveness.getLiveOut(block);
+///   auto allOperationsInWhichValueIsLive = liveness.resolveLiveness(value);
+///   bool lastUse = liveness.isLastUse(value, operation);
+class Liveness {
+public:
+  using OperationListT = std::vector<Operation *>;
+  using BlockMapT = DenseMap<Block *, LivenessBlockInfo>;
+  using ValueSetT = SmallPtrSet<Value *, 16>;
+
+public:
+  /// Creates a new Liveness analysis that computes liveness
+  /// information for all associated regions.
+  Liveness(Operation *op);
+
+  /// Returns the operation this analysis was constructed from.
+  Operation *getOperation() const { return operation; }
+
+  /// Gets liveness info (if any) for the given value.
+  /// This includes all operations in which the given value is live.
+  /// Note that the operations in this list are not ordered and the current
+  /// implementation is computationally expensive (as it iterates over all
+  /// blocks in which the given value is live).
+  OperationListT resolveLiveness(Value *value) const;
+
+  /// Gets liveness info (if any) for the block.
+  const LivenessBlockInfo *getLiveness(Block *block) const;
+
+  /// Returns a reference to a set containing live-in values (unordered).
+  const ValueSetT &getLiveIn(Block *block) const;
+
+  /// Returns a reference to a set containing live-out values (unordered).
+  const ValueSetT &getLiveOut(Block *block) const;
+
+  /// Returns true if the given operation represent the last use of the
+  /// given value.
+  bool isLastUse(Value *value, Operation *operation) const;
+
+  /// Dumps the liveness information in a human readable format.
+  void dump() const;
+
+  /// Dumps the liveness information to the given stream.
+  void print(raw_ostream &os) const;
+
+private:
+  /// Initializes the internal mappings.
+  void build(MutableArrayRef<Region> regions);
+
+private:
+  /// The operation this analysis was constructed from.
+  Operation *operation;
+
+  /// Maps blocks to internal liveness information.
+  BlockMapT blockMapping;
+};
+
+/// This class represents liveness information on block level.
+class LivenessBlockInfo {
+public:
+  /// A typedef declaration of a value set.
+  using ValueSetT = Liveness::ValueSetT;
+
+public:
+  /// Returns the underlying block.
+  Block *getBlock() const { return block; }
+
+  /// Returns all values that are live at the beginning
+  /// of the block (unordered).
+  const ValueSetT &in() const { return inValues; }
+
+  /// Returns all values that are live at the end
+  /// of the block (unordered).
+  const ValueSetT &out() const { return outValues; }
+
+  /// Returns true if the given value is in the live-in set.
+  bool isLiveIn(Value *value) const;
+
+  /// Returns true if the given value is in the live-out set.
+  bool isLiveOut(Value *value) const;
+
+  /// Gets the start operation for the given value. This is the first operation
+  /// the given value is considered to be live. This could either be the start
+  /// operation of the current block (in case the value is live-in) or the
+  /// operation that defines the given value (must be referenced in this block).
+  Operation *getStartOperation(Value *value) const;
+
+  /// Gets the end operation for the given value using the start operation
+  /// provided (must be referenced in this block).
+  Operation *getEndOperation(Value *value, Operation *startOperation) const;
+
+private:
+  /// The underlying block.
+  Block *block;
+
+  /// The set of all live in values.
+  ValueSetT inValues;
+
+  /// The set of all live out values.
+  ValueSetT outValues;
+
+  friend class Liveness;
+};
+
+} // end namespace mlir
+
+#endif // MLIR_ANALYSIS_LIVENESS_H

mlir/lib/Analysis/CMakeLists.txt

@@ -4,6 +4,7 @@ add_llvm_library(MLIRAnalysis STATIC
   CallGraph.cpp
   Dominance.cpp
   InferTypeOpInterface.cpp
+  Liveness.cpp
   LoopAnalysis.cpp
   MemRefBoundCheck.cpp
   NestedMatcher.cpp