lib/Transforms/CSE.cpp

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

#include "PassDetail.h"
#include "mlir/IR/Dominance.h"
#include "mlir/Pass/Pass.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;

/// Return true if the specified region is known to follow SSA dominance
/// properties, i.e. it isn't a graph region.
static bool regionHasSSADominance(Operation &op, size_t regionNo,
                                  RegionKindInterface regionKindItf) {
  // If the op is unregistered, then we don't know if it has SSADominance or
  // not, so assume not.
  if (!op.isRegistered())
    return false;

  // If the op is registered but has no RegionKindInterface, then it defaults to
  // SSADominance.
  if (!regionKindItf)
    return true;

  // Otherwise, ask the interface.
  return regionKindItf.hasSSADominance(regionNo);
}

namespace {
struct SimpleOperationInfo : public llvm::DenseMapInfo<Operation *> {
  static unsigned getHashValue(const Operation *opC) {
    return OperationEquivalence::computeHash(const_cast<Operation *>(opC));
  }
  static bool isEqual(const Operation *lhsC, const Operation *rhsC) {
    auto *lhs = const_cast<Operation *>(lhsC);
    auto *rhs = const_cast<Operation *>(rhsC);
    if (lhs == rhs)
      return true;
    if (lhs == getTombstoneKey() || lhs == getEmptyKey() ||
        rhs == getTombstoneKey() || rhs == getEmptyKey())
      return false;
    return OperationEquivalence::isEquivalentTo(const_cast<Operation *>(lhsC),
                                                const_cast<Operation *>(rhsC));
  }
};
} // end anonymous namespace

namespace {
/// Simple common sub-expression elimination.
struct CSE : public CSEBase<CSE> {
  /// Shared implementation of operation elimination and scoped map definitions.
  using AllocatorTy = llvm::RecyclingAllocator<
      llvm::BumpPtrAllocator,
      llvm::ScopedHashTableVal<Operation *, Operation *>>;
  using ScopedMapTy = llvm::ScopedHashTable<Operation *, Operation *,
                                            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::const_iterator childIterator;

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

  /// Attempt to eliminate a redundant operation. Returns success if the
  /// operation was marked for removal, failure otherwise.
  LogicalResult simplifyOperation(ScopedMapTy &knownValues, Operation *op,
                                  bool hasSSADominance);
  void simplifyBlock(ScopedMapTy &knownValues, Block *bb, bool hasSSADominance);
  void simplifyRegion(ScopedMapTy &knownValues, Region &region,
                      bool hasSSADominance);

  void runOnOperation() override;

private:
  /// Operations marked as dead and to be erased.
  std::vector<Operation *> opsToErase;
  DominanceInfo *domInfo = nullptr;
};
} // end anonymous namespace

/// Attempt to eliminate a redundant operation.
LogicalResult CSE::simplifyOperation(ScopedMapTy &knownValues, Operation *op,
                                     bool hasSSADominance) {
  // Don't simplify terminator operations.
  if (op->hasTrait<OpTrait::IsTerminator>())
    return failure();

  // If the operation is already trivially dead just add it to the erase list.
  if (isOpTriviallyDead(op)) {
    opsToErase.push_back(op);
    ++numDCE;
    return success();
  }

  // Don't simplify operations with nested blocks. We don't currently model
  // equality comparisons correctly among other things. It is also unclear
  // whether we would want to CSE such operations.
  if (op->getNumRegions() != 0)
    return failure();

  // TODO: We currently only eliminate non side-effecting
  // operations.
  if (!MemoryEffectOpInterface::hasNoEffect(op))
    return failure();

  // 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. We can only replace an operand in
    // an operation if it has not been visited yet.
    if (hasSSADominance) {
      // If the region has SSA dominance, then we are guaranteed to have not
      // visited any use of the current operation.
      op->replaceAllUsesWith(existing);
      opsToErase.push_back(op);
    } else {
      // When the region does not have SSA dominance, we need to check if we
      // have visited a use before replacing any use.
      for (auto it : llvm::zip(op->getResults(), existing->getResults())) {
        std::get<0>(it).replaceUsesWithIf(
            std::get<1>(it), [&](OpOperand &operand) {
              return !knownValues.count(operand.getOwner());
            });
      }

      // There may be some remaining uses of the operation.
      if (op->use_empty())
        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());
    }

    ++numCSE;
    return success();
  }

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

void CSE::simplifyBlock(ScopedMapTy &knownValues, Block *bb,
                        bool hasSSADominance) {
  for (auto &op : *bb) {
    // If the operation is simplified, we don't process any held regions.
    if (succeeded(simplifyOperation(knownValues, &op, hasSSADominance)))
      continue;

    // Most operations don't have regions, so fast path that case.
    if (op.getNumRegions() == 0)
      continue;

    auto regionKindItf = dyn_cast<RegionKindInterface>(op);

    // If this operation is isolated above, we can't process nested regions with
    // the given 'knownValues' map. This would cause the insertion of implicit
    // captures in explicit capture only regions.
    if (op.mightHaveTrait<OpTrait::IsIsolatedFromAbove>()) {
      ScopedMapTy nestedKnownValues;
      for (size_t i = 0, e = op.getNumRegions(); i != e; ++i) {
        simplifyRegion(nestedKnownValues, op.getRegion(i),
                       regionHasSSADominance(op, i, regionKindItf));
      }
      continue;
    }

    // Otherwise, process nested regions normally.
    for (size_t i = 0, e = op.getNumRegions(); i != e; ++i) {
      simplifyRegion(knownValues, op.getRegion(i),
                     regionHasSSADominance(op, i, regionKindItf));
    }
  }
}

void CSE::simplifyRegion(ScopedMapTy &knownValues, Region &region,
                         bool hasSSADominance) {
  // If the region is empty there is nothing to do.
  if (region.empty())
    return;

  // If the region only contains one block, then simplify it directly.
  if (std::next(region.begin()) == region.end()) {
    ScopedMapTy::ScopeTy scope(knownValues);
    simplifyBlock(knownValues, &region.front(), hasSSADominance);
    return;
  }

  // If the region does not have dominanceInfo, then skip it.
  // TODO: Regions without SSA dominance should define a different
  // traversal order which is appropriate and can be used here.
  if (!hasSSADominance)
    return;

  // 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 for this region.
  stack.emplace_back(std::make_unique<CFGStackNode>(
      knownValues, domInfo->getRootNode(&region)));

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

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

    // 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();
    }
  }
}

void CSE::runOnOperation() {
  /// A scoped hash table of defining operations within a region.
  ScopedMapTy knownValues;

  domInfo = &getAnalysis<DominanceInfo>();
  Operation *rootOp = getOperation();

  auto regionKindItf = dyn_cast<RegionKindInterface>(getOperation());
  for (size_t i = 0, e = rootOp->getNumRegions(); i != e; ++i) {
    simplifyRegion(knownValues, rootOp->getRegion(i),
                   regionHasSSADominance(*rootOp, i, regionKindItf));
  }

  // If no operations were erased, then we mark all analyses as preserved.
  if (opsToErase.empty())
    return markAllAnalysesPreserved();

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

  // We currently don't remove region operations, so mark dominance as
  // preserved.
  markAnalysesPreserved<DominanceInfo, PostDominanceInfo>();
  domInfo = nullptr;
}

std::unique_ptr<Pass> mlir::createCSEPass() { return std::make_unique<CSE>(); }