llvm/mlir/lib/Analysis/Dominance.cpp

//===- Dominance.cpp - Dominator analysis for CFGs ------------------------===//
//
// 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.
// =============================================================================
//
// Implementation of dominance related classes and instantiations of extern
// templates.
//
//===----------------------------------------------------------------------===//

#include "mlir/Analysis/Dominance.h"
#include "mlir/IR/Operation.h"
#include "llvm/Support/GenericDomTreeConstruction.h"

using namespace mlir;
using namespace mlir::detail;

template class llvm::DominatorTreeBase<Block, /*IsPostDom=*/false>;
template class llvm::DominatorTreeBase<Block, /*IsPostDom=*/true>;
template class llvm::DomTreeNodeBase<Block>;

//===----------------------------------------------------------------------===//
// DominanceInfoBase
//===----------------------------------------------------------------------===//

template <bool IsPostDom>
void DominanceInfoBase<IsPostDom>::recalculate(Operation *op) {
  dominanceInfos.clear();

  /// Build the dominance for each of the operation regions.
  op->walk([&](Operation *op) {
    for (auto &region : op->getRegions()) {
      // Don't compute dominance if the region is empty.
      if (region.empty())
        continue;
      auto opDominance = std::make_unique<base>();
      opDominance->recalculate(region);
      dominanceInfos.try_emplace(&region, std::move(opDominance));
    }
  });
}

/// Return true if the specified block A properly dominates block B.
template <bool IsPostDom>
bool DominanceInfoBase<IsPostDom>::properlyDominates(Block *a, Block *b) {
  // A block dominates itself but does not properly dominate itself.
  if (a == b)
    return false;

  // If either a or b are null, then conservatively return false.
  if (!a || !b)
    return false;

  // If both blocks are not in the same region, 'a' properly dominates 'b' if
  // 'b' is defined in an operation region that (recursively) ends up being
  // dominated by 'a'. Walk up the list of containers enclosing B.
  auto *regionA = a->getParent(), *regionB = b->getParent();
  if (regionA != regionB) {
    Operation *bAncestor;
    do {
      bAncestor = regionB->getParentOp();
      // If 'bAncestor' is the top level region, then 'a' is a block that post
      // dominates 'b'.
      if (!bAncestor || !bAncestor->getBlock())
        return IsPostDom;

      regionB = bAncestor->getBlock()->getParent();
    } while (regionA != regionB);

    // Check to see if the ancestor of 'b' is the same block as 'a'.
    b = bAncestor->getBlock();
    if (a == b)
      return true;
  }

  // Otherwise, use the standard dominance functionality.

  // If we don't have a dominance information for this region, assume that b is
  // dominated by anything.
  auto baseInfoIt = dominanceInfos.find(regionA);
  if (baseInfoIt == dominanceInfos.end())
    return true;
  return baseInfoIt->second->properlyDominates(a, b);
}

template class mlir::detail::DominanceInfoBase</*IsPostDom=*/true>;
template class mlir::detail::DominanceInfoBase</*IsPostDom=*/false>;

//===----------------------------------------------------------------------===//
// DominanceInfo
//===----------------------------------------------------------------------===//

/// Return true if operation A properly dominates operation B.
bool DominanceInfo::properlyDominates(Operation *a, Operation *b) {
  auto *aBlock = a->getBlock(), *bBlock = b->getBlock();

  // If a or b are not within a block, then a does not dominate b.
  if (!aBlock || !bBlock)
    return false;

  // If the blocks are the same, then check if b is before a in the block.
  if (aBlock == bBlock)
    return a->isBeforeInBlock(b);

  // Traverse up b's hierarchy to check if b's block is contained in a's.
  if (auto *bAncestor = aBlock->findAncestorInstInBlock(*b)) {
    // Since we already know that aBlock != bBlock, here bAncestor != b.
    // a and bAncestor are in the same block; check if 'a' dominates
    // bAncestor.
    return dominates(a, bAncestor);
  }

  // If the blocks are different, check if a's block dominates b's.
  return properlyDominates(aBlock, bBlock);
}

/// Return true if value A properly dominates operation B.
bool DominanceInfo::properlyDominates(Value *a, Operation *b) {
  if (auto *aInst = a->getDefiningOp()) {
    // The values defined by an operation do *not* dominate any nested
    // operations.
    if (aInst->getParentRegion() != b->getParentRegion() &&
        aInst->isAncestor(b))
      return false;
    return properlyDominates(aInst, b);
  }

  // block arguments properly dominate all operations in their own block, so
  // we use a dominates check here, not a properlyDominates check.
  return dominates(cast<BlockArgument>(a)->getOwner(), b->getBlock());
}

DominanceInfoNode *DominanceInfo::getNode(Block *a) {
  auto *region = a->getParent();
  assert(dominanceInfos.count(region) != 0);
  return dominanceInfos[region]->getNode(a);
}

void DominanceInfo::updateDFSNumbers() {
  for (auto &iter : dominanceInfos)
    iter.second->updateDFSNumbers();
}

//===----------------------------------------------------------------------===//
// PostDominanceInfo
//===----------------------------------------------------------------------===//

/// Returns true if statement 'a' properly postdominates statement b.
bool PostDominanceInfo::properlyPostDominates(Operation *a, Operation *b) {
  auto *aBlock = a->getBlock(), *bBlock = b->getBlock();

  // If a or b are not within a block, then a does not post dominate b.
  if (!aBlock || !bBlock)
    return false;

  // If the blocks are the same, check if b is before a in the block.
  if (aBlock == bBlock)
    return b->isBeforeInBlock(a);

  // Traverse up b's hierarchy to check if b's block is contained in a's.
  if (auto *bAncestor = a->getBlock()->findAncestorInstInBlock(*b))
    // Since we already know that aBlock != bBlock, here bAncestor != b.
    // a and bAncestor are in the same block; check if 'a' postdominates
    // bAncestor.
    return postDominates(a, bAncestor);

  // If the blocks are different, check if a's block post dominates b's.
  return properlyDominates(aBlock, bBlock);
}