compiler/optimizing/boolean_simplifier.cc

/*
 * Copyright (C) 2015 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 "boolean_simplifier.h"

namespace art {

static bool EndsWithAnIf(HBasicBlock* block) {
  return block->GetLastInstruction()->IsIf();
}

static bool HasSinglePhi(HBasicBlock* block) {
  return !block->GetPhis().IsEmpty()
      && block->GetFirstPhi()->GetNext() == nullptr;
}

// Returns true if 'block1' and 'block2' are empty, merge into the same single
// successor and the successor can only be reached from them.
static bool BlocksDoMergeTogether(HBasicBlock* block1, HBasicBlock* block2) {
  if (!block1->IsSingleGoto() || !block2->IsSingleGoto()) return false;
  HBasicBlock* succ1 = block1->GetSuccessors().Get(0);
  HBasicBlock* succ2 = block2->GetSuccessors().Get(0);
  return succ1 == succ2 && succ1->GetPredecessors().Size() == 2u;
}

// Returns true if the outcome of the branching matches the boolean value of
// the branching condition.
static bool PreservesCondition(HInstruction* input_true, HInstruction* input_false) {
  return input_true->IsIntConstant() && input_true->AsIntConstant()->GetValue() == 1
         && input_false->IsIntConstant() && input_false->AsIntConstant()->GetValue() == 0;
}

// Returns true if the outcome of the branching is exactly opposite of the
// boolean value of the branching condition.
static bool NegatesCondition(HInstruction* input_true, HInstruction* input_false) {
  return input_true->IsIntConstant() && input_true->AsIntConstant()->GetValue() == 0
         && input_false->IsIntConstant() && input_false->AsIntConstant()->GetValue() == 1;
}

// Returns an instruction with the opposite boolean value from 'cond'.
static HInstruction* GetOppositeCondition(HInstruction* cond) {
  HGraph* graph = cond->GetBlock()->GetGraph();
  ArenaAllocator* allocator = graph->GetArena();

  if (cond->IsCondition()) {
    HInstruction* lhs = cond->InputAt(0);
    HInstruction* rhs = cond->InputAt(1);
    if (cond->IsEqual()) {
      return new (allocator) HNotEqual(lhs, rhs);
    } else if (cond->IsNotEqual()) {
      return new (allocator) HEqual(lhs, rhs);
    } else if (cond->IsLessThan()) {
      return new (allocator) HGreaterThanOrEqual(lhs, rhs);
    } else if (cond->IsLessThanOrEqual()) {
      return new (allocator) HGreaterThan(lhs, rhs);
    } else if (cond->IsGreaterThan()) {
      return new (allocator) HLessThanOrEqual(lhs, rhs);
    } else if (cond->IsGreaterThanOrEqual()) {
      return new (allocator) HLessThan(lhs, rhs);
    }
  } else if (cond->IsIntConstant()) {
    int32_t value = cond->AsIntConstant()->GetValue();
    if (value == 0) {
      return graph->GetIntConstant1();
    } else {
      DCHECK_EQ(value, 1);
      return graph->GetIntConstant0();
    }
  }

  LOG(FATAL) << "Instruction " << cond->DebugName() << " used as a condition";
  UNREACHABLE();
}

void HBooleanSimplifier::Run() {
  // Iterate in post order in the unlikely case that removing one occurrence of
  // the pattern empties a branch block of another occurrence. Otherwise the
  // order does not matter.
  for (HPostOrderIterator it(*graph_); !it.Done(); it.Advance()) {
    HBasicBlock* block = it.Current();
    if (!EndsWithAnIf(block)) continue;

    // Find elements of the pattern.
    HIf* if_instruction = block->GetLastInstruction()->AsIf();
    HBasicBlock* true_block = if_instruction->IfTrueSuccessor();
    HBasicBlock* false_block = if_instruction->IfFalseSuccessor();
    if (!BlocksDoMergeTogether(true_block, false_block)) {
      continue;
    }
    HBasicBlock* merge_block = true_block->GetSuccessors().Get(0);
    if (!HasSinglePhi(merge_block)) {
      continue;
    }
    HPhi* phi = merge_block->GetFirstPhi()->AsPhi();
    HInstruction* true_value = phi->InputAt(merge_block->GetPredecessorIndexOf(true_block));
    HInstruction* false_value = phi->InputAt(merge_block->GetPredecessorIndexOf(false_block));

    // Check if the selection negates/preserves the value of the condition and
    // if so, generate a suitable replacement instruction.
    HInstruction* if_condition = if_instruction->InputAt(0);
    HInstruction* replacement;
    if (NegatesCondition(true_value, false_value)) {
      replacement = GetOppositeCondition(if_condition);
      if (replacement->GetBlock() == nullptr) {
        block->InsertInstructionBefore(replacement, if_instruction);
      }
    } else if (PreservesCondition(true_value, false_value)) {
      replacement = if_condition;
    } else {
      continue;
    }

    // Replace the selection outcome with the new instruction.
    phi->ReplaceWith(replacement);
    merge_block->RemovePhi(phi);

    // Link the start/end blocks and remove empty branches.
    graph_->MergeEmptyBranches(block, merge_block);

    // Remove the original condition if it is now unused.
    if (!if_condition->HasUses()) {
      if_condition->GetBlock()->RemoveInstruction(if_condition);
    }
  }
}

}  // namespace art