lib/Transforms/CSE.cpp

//===- CSE.cpp - Common Sub-expression Elimination ------------------------===//
//
// 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 transformation pass performs a simple common sub-expression elimination
// algorithm on operations within a function.
//
//===----------------------------------------------------------------------===//

#include "mlir/Analysis/Dominance.h"
#include "mlir/IR/Attributes.h"
#include "mlir/IR/Builders.h"
#include "mlir/IR/Function.h"
#include "mlir/Pass/Pass.h"
#include "mlir/Support/Functional.h"
#include "mlir/Transforms/Passes.h"
#include "mlir/Transforms/Utils.h"
#include "llvm/ADT/DenseMapInfo.h"
#include "llvm/ADT/Hashing.h"
#include "llvm/ADT/ScopedHashTable.h"
#include "llvm/Support/Allocator.h"
#include "llvm/Support/RecyclingAllocator.h"
#include <deque>
using namespace mlir;

namespace {
// TODO(riverriddle) Handle commutative operations.
struct SimpleOperationInfo : public llvm::DenseMapInfo<Instruction *> {
  static unsigned getHashValue(const Instruction *op) {
    // Hash the operations based upon their:
    //   - Instruction Name
    //   - Attributes
    //   - Result Types
    //   - Operands
    return hash_combine(
        op->getName(), op->getAttrs(),
        hash_combine_range(op->result_type_begin(), op->result_type_end()),
        hash_combine_range(op->operand_begin(), op->operand_end()));
  }
  static bool isEqual(const Instruction *lhs, const Instruction *rhs) {
    if (lhs == rhs)
      return true;
    if (lhs == getTombstoneKey() || lhs == getEmptyKey() ||
        rhs == getTombstoneKey() || rhs == getEmptyKey())
      return false;

    // Compare the operation name.
    if (lhs->getName() != rhs->getName())
      return false;
    // Check operand and result type counts.
    if (lhs->getNumOperands() != rhs->getNumOperands() ||
        lhs->getNumResults() != rhs->getNumResults())
      return false;
    // Compare attributes.
    if (lhs->getAttrs() != rhs->getAttrs())
      return false;
    // Compare operands.
    if (!std::equal(lhs->operand_begin(), lhs->operand_end(),
                    rhs->operand_begin()))
      return false;
    // Compare result types.
    return std::equal(lhs->result_type_begin(), lhs->result_type_end(),
                      rhs->result_type_begin());
  }
};
} // end anonymous namespace

namespace {
/// Simple common sub-expression elimination.
struct CSE : public FunctionPass {
  CSE() : FunctionPass(&CSE::passID) {}

  static char passID;

  /// Shared implementation of operation elimination and scoped map definitions.
  using AllocatorTy = llvm::RecyclingAllocator<
      llvm::BumpPtrAllocator,
      llvm::ScopedHashTableVal<Instruction *, Instruction *>>;
  using ScopedMapTy = llvm::ScopedHashTable<Instruction *, Instruction *,
                                            SimpleOperationInfo, AllocatorTy>;

  /// Represents a single entry in the depth first traversal of a CFG.
  struct CFGStackNode {
    CFGStackNode(ScopedMapTy &knownValues, DominanceInfoNode *node)
        : scope(knownValues), node(node), childIterator(node->begin()),
          processed(false) {}

    /// Scope for the known values.
    ScopedMapTy::ScopeTy scope;

    DominanceInfoNode *node;
    DominanceInfoNode::iterator childIterator;

    /// If this node has been fully processed yet or not.
    bool processed;
  };

  /// Attempt to eliminate a redundant operation. Returns true if the operation
  /// was marked for removal, false otherwise.
  bool simplifyOperation(Instruction *op);

  void simplifyBlock(Block *bb);

  PassResult runOnFunction(Function *f) override;

private:
  /// A scoped hash table of defining operations within a function.
  ScopedMapTy knownValues;

  /// Operations marked as dead and to be erased.
  std::vector<Instruction *> opsToErase;
};
} // end anonymous namespace

char CSE::passID = 0;

/// Attempt to eliminate a redundant operation.
bool CSE::simplifyOperation(Instruction *op) {
  // TODO(riverriddle) We currently only eliminate non side-effecting
  // operations.
  if (!op->hasNoSideEffect())
    return false;

  // If the operation is already trivially dead just add it to the erase list.
  if (op->use_empty()) {
    opsToErase.push_back(op);
    return true;
  }

  // Look for an existing definition for the operation.
  if (auto *existing = knownValues.lookup(op)) {
    // If we find one then replace all uses of the current operation with the
    // existing one and mark it for deletion.
    for (unsigned i = 0, e = existing->getNumResults(); i != e; ++i)
      op->getResult(i)->replaceAllUsesWith(existing->getResult(i));
    opsToErase.push_back(op);

    // If the existing operation has an unknown location and the current
    // operation doesn't, then set the existing op's location to that of the
    // current op.
    if (existing->getLoc().isa<UnknownLoc>() &&
        !op->getLoc().isa<UnknownLoc>()) {
      existing->setLoc(op->getLoc());
    }
    return true;
  }

  // Otherwise, we add this operation to the known values map.
  knownValues.insert(op, op);
  return false;
}

void CSE::simplifyBlock(Block *bb) {
  for (auto &i : *bb) {
    // If the operation is simplified, we don't process any held block lists.
    if (simplifyOperation(&i))
      continue;

    // Simplify any held blocks.
    for (auto &blockList : i.getBlockLists()) {
      for (auto &b : blockList) {
        ScopedMapTy::ScopeTy scope(knownValues);
        simplifyBlock(&b);
      }
    }
  }
}
PassResult CSE::runOnFunction(Function *f) {
  // Note, deque is being used here because there was significant performance
  // gains over vector when the container becomes very large due to the
  // specific access patterns. If/when these performance issues are no
  // longer a problem we can change this to vector. For more information see
  // the llvm mailing list discussion on this:
  // http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20120116/135228.html
  std::deque<std::unique_ptr<CFGStackNode>> stack;

  // Process the nodes of the dom tree.
  DominanceInfo domInfo(f);
  stack.emplace_back(
      std::make_unique<CFGStackNode>(knownValues, domInfo.getRootNode()));

  while (!stack.empty()) {
    auto &currentNode = stack.back();

    // Check to see if we need to process this node.
    if (!currentNode->processed) {
      currentNode->processed = true;
      simplifyBlock(currentNode->node->getBlock());
    }

    // Otherwise, check to see if we need to process a child node.
    if (currentNode->childIterator != currentNode->node->end()) {
      auto *childNode = *(currentNode->childIterator++);
      stack.emplace_back(
          std::make_unique<CFGStackNode>(knownValues, childNode));
    } else {
      // Finally, if the node and all of its children have been processed
      // then we delete the node.
      stack.pop_back();
    }
  }

  /// Erase any operations that were marked as dead during simplification.
  for (auto *op : opsToErase)
    op->erase();
  opsToErase.clear();

  return success();
}

FunctionPass *mlir::createCSEPass() { return new CSE(); }

static PassRegistration<CSE>
    pass("cse", "Eliminate common sub-expressions in functions");