compiler/optimizing/control_flow_simplifier.cc

/*
 * Copyright (C) 2016 The Android Open Source Project
 *
 * 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.
 */

#include "control_flow_simplifier.h"

#include "optimizing/nodes.h"
#include "reference_type_propagation.h"

namespace art HIDDEN {

static constexpr size_t kMaxInstructionsInBranch = 1u;

HControlFlowSimplifier::HControlFlowSimplifier(HGraph* graph,
                                               OptimizingCompilerStats* stats,
                                               const char* name)
    : HOptimization(graph, name, stats) {
}

// Returns true if `block` has only one predecessor, ends with a Goto
// or a Return and contains at most `kMaxInstructionsInBranch` other
// movable instruction with no side-effects.
static bool IsSimpleBlock(HBasicBlock* block) {
  if (block->GetPredecessors().size() != 1u) {
    return false;
  }
  DCHECK(block->GetPhis().IsEmpty());

  size_t num_instructions = 0u;
  for (HInstructionIterator it(block->GetInstructions()); !it.Done(); it.Advance()) {
    HInstruction* instruction = it.Current();
    if (instruction->IsControlFlow()) {
      return instruction->IsGoto() || instruction->IsReturn();
    } else if (instruction->CanBeMoved() &&
               !instruction->HasSideEffects() &&
               !instruction->CanThrow()) {
      if (instruction->IsSelect() && instruction->AsSelect()->GetCondition()->GetBlock() == block) {
        // Count one HCondition and HSelect in the same block as a single instruction.
        // This enables finding nested selects.
        continue;
      } else if (++num_instructions > kMaxInstructionsInBranch) {
        return false;  // bail as soon as we exceed number of allowed instructions
      }
    } else {
      return false;
    }
  }

  LOG(FATAL) << "Unreachable";
  UNREACHABLE();
}

// Returns true if 'block1' and 'block2' are empty and merge into the
// same single successor.
static bool BlocksMergeTogether(HBasicBlock* block1, HBasicBlock* block2) {
  return block1->GetSingleSuccessor() == block2->GetSingleSuccessor();
}

// Search `block` for phis that have different inputs at `index1` and `index2`.
// If none is found, returns `{true, nullptr}`.
// If exactly one such `phi` is found, returns `{true, phi}`.
// Otherwise (if more than one such phi is found), returns `{false, nullptr}`.
static std::pair<bool, HPhi*> HasAtMostOnePhiWithDifferentInputs(HBasicBlock* block,
                                                                 size_t index1,
                                                                 size_t index2) {
  DCHECK_NE(index1, index2);

  HPhi* select_phi = nullptr;
  for (HInstructionIterator it(block->GetPhis()); !it.Done(); it.Advance()) {
    HPhi* phi = it.Current()->AsPhi();
    auto&& inputs = phi->GetInputs();
    if (inputs[index1] == inputs[index2]) {
      continue;
    }
    if (select_phi == nullptr) {
      // First phi found.
      select_phi = phi;
    } else {
      // More than one phi found, return null.
      return {false, nullptr};
    }
  }
  return {true, select_phi};
}

bool HControlFlowSimplifier::TryGenerateSelectSimpleDiamondPattern(
    HBasicBlock* block, ScopedArenaSafeMap<HInstruction*, HSelect*>* cache) {
  DCHECK(block->GetLastInstruction()->IsIf());
  HIf* if_instruction = block->GetLastInstruction()->AsIf();
  HBasicBlock* true_block = if_instruction->IfTrueSuccessor();
  HBasicBlock* false_block = if_instruction->IfFalseSuccessor();
  DCHECK_NE(true_block, false_block);

  if (!IsSimpleBlock(true_block) ||
      !IsSimpleBlock(false_block) ||
      !BlocksMergeTogether(true_block, false_block)) {
    return false;
  }
  HBasicBlock* merge_block = true_block->GetSingleSuccessor();

  // If the branches are not empty, move instructions in front of the If.
  // TODO(dbrazdil): This puts an instruction between If and its condition.
  //                 Implement moving of conditions to first users if possible.
  while (!true_block->IsSingleGoto() && !true_block->IsSingleReturn()) {
    HInstruction* instr = true_block->GetFirstInstruction();
    DCHECK(!instr->CanThrow());
    instr->MoveBefore(if_instruction);
  }
  while (!false_block->IsSingleGoto() && !false_block->IsSingleReturn()) {
    HInstruction* instr = false_block->GetFirstInstruction();
    DCHECK(!instr->CanThrow());
    instr->MoveBefore(if_instruction);
  }
  DCHECK(true_block->IsSingleGoto() || true_block->IsSingleReturn());
  DCHECK(false_block->IsSingleGoto() || false_block->IsSingleReturn());

  // Find the resulting true/false values.
  size_t predecessor_index_true = merge_block->GetPredecessorIndexOf(true_block);
  size_t predecessor_index_false = merge_block->GetPredecessorIndexOf(false_block);
  DCHECK_NE(predecessor_index_true, predecessor_index_false);

  bool both_successors_return = true_block->IsSingleReturn() && false_block->IsSingleReturn();
  // TODO(solanes): Extend to support multiple phis? e.g.
  //   int a, b;
  //   if (bool) {
  //     a = 0; b = 1;
  //   } else {
  //     a = 1; b = 2;
  //   }
  //   // use a and b
  bool at_most_one_phi_with_different_inputs = false;
  HPhi* phi = nullptr;
  HInstruction* true_value = nullptr;
  HInstruction* false_value = nullptr;
  if (both_successors_return) {
    // Note: This can create a select with the same then-value and else-value.
    true_value = true_block->GetFirstInstruction()->InputAt(0);
    false_value = false_block->GetFirstInstruction()->InputAt(0);
  } else {
    std::tie(at_most_one_phi_with_different_inputs, phi) = HasAtMostOnePhiWithDifferentInputs(
        merge_block, predecessor_index_true, predecessor_index_false);
    if (!at_most_one_phi_with_different_inputs) {
      return false;
    }
    if (phi != nullptr) {
      true_value = phi->InputAt(predecessor_index_true);
      false_value = phi->InputAt(predecessor_index_false);
    }  // else we don't need to create a `HSelect` at all.
  }
  DCHECK(both_successors_return || at_most_one_phi_with_different_inputs);

  // Create the Select instruction and insert it in front of the If.
  HInstruction* condition = if_instruction->InputAt(0);
  HSelect* select = nullptr;
  if (both_successors_return || phi != nullptr) {
    select = new (graph_->GetAllocator()) HSelect(condition,
                                                  true_value,
                                                  false_value,
                                                  if_instruction->GetDexPc());
    block->InsertInstructionBefore(select, if_instruction);
    if (both_successors_return) {
      if (true_value->GetType() == DataType::Type::kReference) {
        DCHECK(false_value->GetType() == DataType::Type::kReference);
        ReferenceTypePropagation::FixUpSelectType(select, graph_->GetHandleCache());
      }
      false_block->GetFirstInstruction()->ReplaceInput(select, 0);
    } else {
      if (phi->GetType() == DataType::Type::kReference) {
        select->SetReferenceTypeInfoIfValid(phi->GetReferenceTypeInfo());
      }
      phi->ReplaceInput(select, predecessor_index_false);  // We'll remove the true branch below.
    }
  }

  // Remove the true branch which removes the corresponding Phi input if needed.
  // If left only with the false branch, the Phi is automatically removed.
  true_block->DisconnectAndDelete();

  // Merge remaining blocks which are now connected with Goto.
  DCHECK_EQ(block->GetSingleSuccessor(), false_block);
  block->MergeWith(false_block);
  if (!both_successors_return && merge_block->GetPredecessors().size() == 1u) {
    DCHECK_IMPLIES(phi != nullptr, phi->GetBlock() == nullptr);
    DCHECK(merge_block->GetPhis().IsEmpty());
    DCHECK_EQ(block->GetSingleSuccessor(), merge_block);
    block->MergeWith(merge_block);
  }

  MaybeRecordStat(stats_, select != nullptr ? MethodCompilationStat::kControlFlowSelectGenerated
                                            : MethodCompilationStat::kControlFlowDiamondRemoved);

  // Very simple way of finding common subexpressions in the generated HSelect statements
  // (since this runs after GVN). Lookup by condition, and reuse latest one if possible
  // (due to post order, latest select is most likely replacement). If needed, we could
  // improve this by e.g. using the operands in the map as well.
  if (select != nullptr) {
    auto it = cache->find(condition);
    if (it == cache->end()) {
      cache->Put(condition, select);
    } else {
      // Found cached value. See if latest can replace cached in the HIR.
      HSelect* cached_select = it->second;
      DCHECK_EQ(cached_select->GetCondition(), select->GetCondition());
      if (cached_select->GetTrueValue() == select->GetTrueValue() &&
          cached_select->GetFalseValue() == select->GetFalseValue() &&
          select->StrictlyDominates(cached_select)) {
        cached_select->ReplaceWith(select);
        cached_select->GetBlock()->RemoveInstruction(cached_select);
      }
      it->second = select;  // always cache latest
    }
  }

  // No need to update dominance information, as we are simplifying
  // a simple diamond shape, where the join block is merged with the
  // entry block. Any following blocks would have had the join block
  // as a dominator, and `MergeWith` handles changing that to the
  // entry block
  return true;
}

HBasicBlock* HControlFlowSimplifier::TryFixupDoubleDiamondPattern(HBasicBlock* block) {
  DCHECK(block->GetLastInstruction()->IsIf());
  HIf* if_instruction = block->GetLastInstruction()->AsIf();
  HBasicBlock* true_block = if_instruction->IfTrueSuccessor();
  HBasicBlock* false_block = if_instruction->IfFalseSuccessor();
  DCHECK_NE(true_block, false_block);

  // One branch must be a single goto, and the other one the inner if.
  if (true_block->IsSingleGoto() == false_block->IsSingleGoto()) {
    return nullptr;
  }

  HBasicBlock* single_goto = true_block->IsSingleGoto() ? true_block : false_block;
  HBasicBlock* inner_if_block = true_block->IsSingleGoto() ? false_block : true_block;

  // The innner if branch has to be a block with just a comparison and an if.
  if (!inner_if_block->EndsWithIf() ||
      inner_if_block->GetLastInstruction()->AsIf()->InputAt(0) !=
          inner_if_block->GetFirstInstruction() ||
      inner_if_block->GetLastInstruction()->GetPrevious() !=
          inner_if_block->GetFirstInstruction() ||
      !inner_if_block->GetFirstInstruction()->IsCondition()) {
    return nullptr;
  }

  HIf* inner_if_instruction = inner_if_block->GetLastInstruction()->AsIf();
  HBasicBlock* inner_if_true_block = inner_if_instruction->IfTrueSuccessor();
  HBasicBlock* inner_if_false_block = inner_if_instruction->IfFalseSuccessor();
  if (!inner_if_true_block->IsSingleGoto() || !inner_if_false_block->IsSingleGoto()) {
    return nullptr;
  }

  // One must merge into the outer condition and the other must not.
  if (BlocksMergeTogether(single_goto, inner_if_true_block) ==
      BlocksMergeTogether(single_goto, inner_if_false_block)) {
    return nullptr;
  }

  // First merge merges the outer if with one of the inner if branches. The block must be a Phi and
  // a Goto.
  HBasicBlock* first_merge = single_goto->GetSingleSuccessor();
  if (first_merge->GetNumberOfPredecessors() != 2 ||
      first_merge->GetPhis().CountSize() != 1 ||
      !first_merge->GetLastInstruction()->IsGoto() ||
      first_merge->GetFirstInstruction() != first_merge->GetLastInstruction()) {
    return nullptr;
  }

  HPhi* first_phi = first_merge->GetFirstPhi()->AsPhi();

  // Second merge is first_merge and the remainder branch merging. It must be phi + goto, or phi +
  // return. Depending on the first merge, we define the second merge.
  HBasicBlock* merges_into_second_merge =
    BlocksMergeTogether(single_goto, inner_if_true_block)
      ? inner_if_false_block
      : inner_if_true_block;
  if (!BlocksMergeTogether(first_merge, merges_into_second_merge)) {
    return nullptr;
  }

  HBasicBlock* second_merge = merges_into_second_merge->GetSingleSuccessor();
  if (second_merge->GetNumberOfPredecessors() != 2 ||
      second_merge->GetPhis().CountSize() != 1 ||
      !(second_merge->GetLastInstruction()->IsGoto() ||
        second_merge->GetLastInstruction()->IsReturn()) ||
      second_merge->GetFirstInstruction() != second_merge->GetLastInstruction()) {
    return nullptr;
  }

  size_t index = second_merge->GetPredecessorIndexOf(merges_into_second_merge);
  HPhi* second_phi = second_merge->GetFirstPhi()->AsPhi();

  // Merge the phis.
  first_phi->AddInput(second_phi->InputAt(index));
  merges_into_second_merge->ReplaceSuccessor(second_merge, first_merge);
  second_phi->ReplaceWith(first_phi);
  second_merge->RemovePhi(second_phi);

  // Sort out the new domination before merging the blocks
  DCHECK_EQ(second_merge->GetSinglePredecessor(), first_merge);
  second_merge->GetDominator()->RemoveDominatedBlock(second_merge);
  second_merge->SetDominator(first_merge);
  first_merge->AddDominatedBlock(second_merge);
  first_merge->MergeWith(second_merge);

  // No need to update dominance information. There's a chance that `merges_into_second_merge`
  // doesn't come before `first_merge` but we don't need to fix it since `merges_into_second_merge`
  // will disappear from the graph altogether when doing the follow-up
  // TryGenerateSelectSimpleDiamondPattern.

  return inner_if_block;
}

bool HControlFlowSimplifier::Run() {
  bool did_select = false;
  // Select cache with local allocator.
  ScopedArenaAllocator allocator(graph_->GetArenaStack());
  ScopedArenaSafeMap<HInstruction*, HSelect*> cache(
      std::less<HInstruction*>(), allocator.Adapter(kArenaAllocControlFlowSimplifier));

  // Iterate in post order in the unlikely case that removing one occurrence of
  // the selection pattern empties a branch block of another occurrence.
  for (HBasicBlock* block : graph_->GetPostOrder()) {
    if (!block->EndsWithIf()) {
      continue;
    }

    if (TryGenerateSelectSimpleDiamondPattern(block, &cache)) {
      did_select = true;
    } else {
      // Try to fix up the odd version of the double diamond pattern. If we could do it, it means
      // that we can generate two selects.
      HBasicBlock* inner_if_block = TryFixupDoubleDiamondPattern(block);
      if (inner_if_block != nullptr) {
        // Generate the selects now since `inner_if_block` should be after `block` in PostOrder.
        bool result = TryGenerateSelectSimpleDiamondPattern(inner_if_block, &cache);
        DCHECK(result);
        result = TryGenerateSelectSimpleDiamondPattern(block, &cache);
        DCHECK(result);
        did_select = true;
      }
    }
  }

  return did_select;
}

}  // namespace art