compiler/optimizing/dead_code_elimination.cc

/*
 * Copyright (C) 2014 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 "dead_code_elimination.h"

#include "base/array_ref.h"
#include "base/bit_vector-inl.h"
#include "base/scoped_arena_allocator.h"
#include "base/scoped_arena_containers.h"
#include "base/stl_util.h"
#include "ssa_phi_elimination.h"

namespace art {

static void MarkReachableBlocks(HGraph* graph, ArenaBitVector* visited) {
  // Use local allocator for allocating memory.
  ScopedArenaAllocator allocator(graph->GetArenaStack());

  ScopedArenaVector<HBasicBlock*> worklist(allocator.Adapter(kArenaAllocDCE));
  constexpr size_t kDefaultWorlistSize = 8;
  worklist.reserve(kDefaultWorlistSize);
  visited->SetBit(graph->GetEntryBlock()->GetBlockId());
  worklist.push_back(graph->GetEntryBlock());

  while (!worklist.empty()) {
    HBasicBlock* block = worklist.back();
    worklist.pop_back();
    int block_id = block->GetBlockId();
    DCHECK(visited->IsBitSet(block_id));

    ArrayRef<HBasicBlock* const> live_successors(block->GetSuccessors());
    HInstruction* last_instruction = block->GetLastInstruction();
    if (last_instruction->IsIf()) {
      HIf* if_instruction = last_instruction->AsIf();
      HInstruction* condition = if_instruction->InputAt(0);
      if (condition->IsIntConstant()) {
        if (condition->AsIntConstant()->IsTrue()) {
          live_successors = live_successors.SubArray(0u, 1u);
          DCHECK_EQ(live_successors[0], if_instruction->IfTrueSuccessor());
        } else {
          DCHECK(condition->AsIntConstant()->IsFalse()) << condition->AsIntConstant()->GetValue();
          live_successors = live_successors.SubArray(1u, 1u);
          DCHECK_EQ(live_successors[0], if_instruction->IfFalseSuccessor());
        }
      }
    } else if (last_instruction->IsPackedSwitch()) {
      HPackedSwitch* switch_instruction = last_instruction->AsPackedSwitch();
      HInstruction* switch_input = switch_instruction->InputAt(0);
      if (switch_input->IsIntConstant()) {
        int32_t switch_value = switch_input->AsIntConstant()->GetValue();
        int32_t start_value = switch_instruction->GetStartValue();
        // Note: Though the spec forbids packed-switch values to wrap around, we leave
        // that task to the verifier and use unsigned arithmetic with it's "modulo 2^32"
        // semantics to check if the value is in range, wrapped or not.
        uint32_t switch_index =
            static_cast<uint32_t>(switch_value) - static_cast<uint32_t>(start_value);
        if (switch_index < switch_instruction->GetNumEntries()) {
          live_successors = live_successors.SubArray(switch_index, 1u);
          DCHECK_EQ(live_successors[0], block->GetSuccessors()[switch_index]);
        } else {
          live_successors = live_successors.SubArray(switch_instruction->GetNumEntries(), 1u);
          DCHECK_EQ(live_successors[0], switch_instruction->GetDefaultBlock());
        }
      }
    }

    for (HBasicBlock* successor : live_successors) {
      // Add only those successors that have not been visited yet.
      if (!visited->IsBitSet(successor->GetBlockId())) {
        visited->SetBit(successor->GetBlockId());
        worklist.push_back(successor);
      }
    }
  }
}

void HDeadCodeElimination::MaybeRecordDeadBlock(HBasicBlock* block) {
  if (stats_ != nullptr) {
    stats_->RecordStat(MethodCompilationStat::kRemovedDeadInstruction,
                       block->GetPhis().CountSize() + block->GetInstructions().CountSize());
  }
}

void HDeadCodeElimination::MaybeRecordSimplifyIf() {
  if (stats_ != nullptr) {
    stats_->RecordStat(MethodCompilationStat::kSimplifyIf);
  }
}

static bool HasInput(HCondition* instruction, HInstruction* input) {
  return (instruction->InputAt(0) == input) ||
         (instruction->InputAt(1) == input);
}

static bool HasEquality(IfCondition condition) {
  switch (condition) {
    case kCondEQ:
    case kCondLE:
    case kCondGE:
    case kCondBE:
    case kCondAE:
      return true;
    case kCondNE:
    case kCondLT:
    case kCondGT:
    case kCondB:
    case kCondA:
      return false;
  }
}

static HConstant* Evaluate(HCondition* condition, HInstruction* left, HInstruction* right) {
  if (left == right && !DataType::IsFloatingPointType(left->GetType())) {
    return condition->GetBlock()->GetGraph()->GetIntConstant(
        HasEquality(condition->GetCondition()) ? 1 : 0);
  }

  if (!left->IsConstant() || !right->IsConstant()) {
    return nullptr;
  }

  if (left->IsIntConstant()) {
    return condition->Evaluate(left->AsIntConstant(), right->AsIntConstant());
  } else if (left->IsNullConstant()) {
    return condition->Evaluate(left->AsNullConstant(), right->AsNullConstant());
  } else if (left->IsLongConstant()) {
    return condition->Evaluate(left->AsLongConstant(), right->AsLongConstant());
  } else if (left->IsFloatConstant()) {
    return condition->Evaluate(left->AsFloatConstant(), right->AsFloatConstant());
  } else {
    DCHECK(left->IsDoubleConstant());
    return condition->Evaluate(left->AsDoubleConstant(), right->AsDoubleConstant());
  }
}

// Simplify the pattern:
//
//           B1
//          /  \
//          |   foo()  // always throws
//          \   goto B2
//           \ /
//            B2
//
// Into:
//
//           B1
//          /  \
//          |  foo()
//          |  goto Exit
//          |   |
//         B2  Exit
//
// Rationale:
// Removal of the never taken edge to B2 may expose
// other optimization opportunities, such as code sinking.
bool HDeadCodeElimination::SimplifyAlwaysThrows() {
  // Make sure exceptions go to exit.
  if (graph_->HasTryCatch()) {
    return false;
  }
  HBasicBlock* exit = graph_->GetExitBlock();
  if (exit == nullptr) {
    return false;
  }

  bool rerun_dominance_and_loop_analysis = false;

  // Order does not matter, just pick one.
  for (HBasicBlock* block : graph_->GetReversePostOrder()) {
    HInstruction* first = block->GetFirstInstruction();
    HInstruction* last = block->GetLastInstruction();
    // Ensure only one throwing instruction appears before goto.
    if (first->AlwaysThrows() &&
        first->GetNext() == last &&
        last->IsGoto() &&
        block->GetPhis().IsEmpty() &&
        block->GetPredecessors().size() == 1u) {
      DCHECK_EQ(block->GetSuccessors().size(), 1u);
      HBasicBlock* pred = block->GetSinglePredecessor();
      HBasicBlock* succ = block->GetSingleSuccessor();
      // Ensure no computations are merged through throwing block.
      // This does not prevent the optimization per se, but would
      // require an elaborate clean up of the SSA graph.
      if (succ != exit &&
          !block->Dominates(pred) &&
          pred->Dominates(succ) &&
          succ->GetPredecessors().size() > 1u &&
          succ->GetPhis().IsEmpty()) {
        block->ReplaceSuccessor(succ, exit);
        rerun_dominance_and_loop_analysis = true;
        MaybeRecordStat(stats_, MethodCompilationStat::kSimplifyThrowingInvoke);
      }
    }
  }

  // We need to re-analyze the graph in order to run DCE afterwards.
  if (rerun_dominance_and_loop_analysis) {
    graph_->ClearLoopInformation();
    graph_->ClearDominanceInformation();
    graph_->BuildDominatorTree();
    return true;
  }
  return false;
}

// Simplify the pattern:
//
//        B1    B2    ...
//       goto  goto  goto
//         \    |    /
//          \   |   /
//             B3
//     i1 = phi(input, input)
//     (i2 = condition on i1)
//        if i1 (or i2)
//          /     \
//         /       \
//        B4       B5
//
// Into:
//
//       B1      B2    ...
//        |      |      |
//       B4      B5    B?
//
// Note that individual edges can be redirected (for example B2->B3
// can be redirected as B2->B5) without applying this optimization
// to other incoming edges.
//
// This simplification cannot be applied to catch blocks, because
// exception handler edges do not represent normal control flow.
// Though in theory this could still apply to normal control flow
// going directly to a catch block, we cannot support it at the
// moment because the catch Phi's inputs do not correspond to the
// catch block's predecessors, so we cannot identify which
// predecessor corresponds to a given statically evaluated input.
//
// We do not apply this optimization to loop headers as this could
// create irreducible loops. We rely on the suspend check in the
// loop header to prevent the pattern match.
//
// Note that we rely on the dead code elimination to get rid of B3.
bool HDeadCodeElimination::SimplifyIfs() {
  bool simplified_one_or_more_ifs = false;
  bool rerun_dominance_and_loop_analysis = false;

  for (HBasicBlock* block : graph_->GetReversePostOrder()) {
    HInstruction* last = block->GetLastInstruction();
    HInstruction* first = block->GetFirstInstruction();
    if (!block->IsCatchBlock() &&
        last->IsIf() &&
        block->HasSinglePhi() &&
        block->GetFirstPhi()->HasOnlyOneNonEnvironmentUse()) {
      bool has_only_phi_and_if = (last == first) && (last->InputAt(0) == block->GetFirstPhi());
      bool has_only_phi_condition_and_if =
          !has_only_phi_and_if &&
          first->IsCondition() &&
          HasInput(first->AsCondition(), block->GetFirstPhi()) &&
          (first->GetNext() == last) &&
          (last->InputAt(0) == first) &&
          first->HasOnlyOneNonEnvironmentUse();

      if (has_only_phi_and_if || has_only_phi_condition_and_if) {
        DCHECK(!block->IsLoopHeader());
        HPhi* phi = block->GetFirstPhi()->AsPhi();
        bool phi_input_is_left = (first->InputAt(0) == phi);

        // Walk over all inputs of the phis and update the control flow of
        // predecessors feeding constants to the phi.
        // Note that phi->InputCount() may change inside the loop.
        for (size_t i = 0; i < phi->InputCount();) {
          HInstruction* input = phi->InputAt(i);
          HInstruction* value_to_check = nullptr;
          if (has_only_phi_and_if) {
            if (input->IsIntConstant()) {
              value_to_check = input;
            }
          } else {
            DCHECK(has_only_phi_condition_and_if);
            if (phi_input_is_left) {
              value_to_check = Evaluate(first->AsCondition(), input, first->InputAt(1));
            } else {
              value_to_check = Evaluate(first->AsCondition(), first->InputAt(0), input);
            }
          }
          if (value_to_check == nullptr) {
            // Could not evaluate to a constant, continue iterating over the inputs.
            ++i;
          } else {
            HBasicBlock* predecessor_to_update = block->GetPredecessors()[i];
            HBasicBlock* successor_to_update = nullptr;
            if (value_to_check->AsIntConstant()->IsTrue()) {
              successor_to_update = last->AsIf()->IfTrueSuccessor();
            } else {
              DCHECK(value_to_check->AsIntConstant()->IsFalse())
                  << value_to_check->AsIntConstant()->GetValue();
              successor_to_update = last->AsIf()->IfFalseSuccessor();
            }
            predecessor_to_update->ReplaceSuccessor(block, successor_to_update);
            phi->RemoveInputAt(i);
            simplified_one_or_more_ifs = true;
            if (block->IsInLoop()) {
              rerun_dominance_and_loop_analysis = true;
            }
            // For simplicity, don't create a dead block, let the dead code elimination
            // pass deal with it.
            if (phi->InputCount() == 1) {
              break;
            }
          }
        }
        if (block->GetPredecessors().size() == 1) {
          phi->ReplaceWith(phi->InputAt(0));
          block->RemovePhi(phi);
          if (has_only_phi_condition_and_if) {
            // Evaluate here (and not wait for a constant folding pass) to open
            // more opportunities for DCE.
            HInstruction* result = first->AsCondition()->TryStaticEvaluation();
            if (result != nullptr) {
              first->ReplaceWith(result);
              block->RemoveInstruction(first);
            }
          }
        }
        if (simplified_one_or_more_ifs) {
          MaybeRecordSimplifyIf();
        }
      }
    }
  }
  // We need to re-analyze the graph in order to run DCE afterwards.
  if (simplified_one_or_more_ifs) {
    if (rerun_dominance_and_loop_analysis) {
      graph_->ClearLoopInformation();
      graph_->ClearDominanceInformation();
      graph_->BuildDominatorTree();
    } else {
      graph_->ClearDominanceInformation();
      // We have introduced critical edges, remove them.
      graph_->SimplifyCFG();
      graph_->ComputeDominanceInformation();
      graph_->ComputeTryBlockInformation();
    }
  }

  return simplified_one_or_more_ifs;
}

void HDeadCodeElimination::ConnectSuccessiveBlocks() {
  // Order does not matter. Skip the entry block by starting at index 1 in reverse post order.
  for (size_t i = 1u, size = graph_->GetReversePostOrder().size(); i != size; ++i) {
    HBasicBlock* block  = graph_->GetReversePostOrder()[i];
    DCHECK(!block->IsEntryBlock());
    while (block->GetLastInstruction()->IsGoto()) {
      HBasicBlock* successor = block->GetSingleSuccessor();
      if (successor->IsExitBlock() || successor->GetPredecessors().size() != 1u) {
        break;
      }
      DCHECK_LT(i, IndexOfElement(graph_->GetReversePostOrder(), successor));
      block->MergeWith(successor);
      --size;
      DCHECK_EQ(size, graph_->GetReversePostOrder().size());
      DCHECK_EQ(block, graph_->GetReversePostOrder()[i]);
      // Reiterate on this block in case it can be merged with its new successor.
    }
  }
}

bool HDeadCodeElimination::RemoveDeadBlocks() {
  // Use local allocator for allocating memory.
  ScopedArenaAllocator allocator(graph_->GetArenaStack());

  // Classify blocks as reachable/unreachable.
  ArenaBitVector live_blocks(&allocator, graph_->GetBlocks().size(), false, kArenaAllocDCE);
  live_blocks.ClearAllBits();

  MarkReachableBlocks(graph_, &live_blocks);
  bool removed_one_or_more_blocks = false;
  bool rerun_dominance_and_loop_analysis = false;

  // Remove all dead blocks. Iterate in post order because removal needs the
  // block's chain of dominators and nested loops need to be updated from the
  // inside out.
  for (HBasicBlock* block : graph_->GetPostOrder()) {
    int id = block->GetBlockId();
    if (!live_blocks.IsBitSet(id)) {
      MaybeRecordDeadBlock(block);
      block->DisconnectAndDelete();
      removed_one_or_more_blocks = true;
      if (block->IsInLoop()) {
        rerun_dominance_and_loop_analysis = true;
      }
    }
  }

  // If we removed at least one block, we need to recompute the full
  // dominator tree and try block membership.
  if (removed_one_or_more_blocks) {
    if (rerun_dominance_and_loop_analysis) {
      graph_->ClearLoopInformation();
      graph_->ClearDominanceInformation();
      graph_->BuildDominatorTree();
    } else {
      graph_->ClearDominanceInformation();
      graph_->ComputeDominanceInformation();
      graph_->ComputeTryBlockInformation();
    }
  }
  return removed_one_or_more_blocks;
}

void HDeadCodeElimination::RemoveDeadInstructions() {
  // Process basic blocks in post-order in the dominator tree, so that
  // a dead instruction depending on another dead instruction is removed.
  for (HBasicBlock* block : graph_->GetPostOrder()) {
    // Traverse this block's instructions in backward order and remove
    // the unused ones.
    HBackwardInstructionIterator i(block->GetInstructions());
    // Skip the first iteration, as the last instruction of a block is
    // a branching instruction.
    DCHECK(i.Current()->IsControlFlow());
    for (i.Advance(); !i.Done(); i.Advance()) {
      HInstruction* inst = i.Current();
      DCHECK(!inst->IsControlFlow());
      if (inst->IsDeadAndRemovable()) {
        block->RemoveInstruction(inst);
        MaybeRecordStat(stats_, MethodCompilationStat::kRemovedDeadInstruction);
      }
    }
  }
}

void HDeadCodeElimination::Run() {
  // Do not eliminate dead blocks if the graph has irreducible loops. We could
  // support it, but that would require changes in our loop representation to handle
  // multiple entry points. We decided it was not worth the complexity.
  if (!graph_->HasIrreducibleLoops()) {
    // Simplify graph to generate more dead block patterns.
    ConnectSuccessiveBlocks();
    bool did_any_simplification = false;
    did_any_simplification |= SimplifyAlwaysThrows();
    did_any_simplification |= SimplifyIfs();
    did_any_simplification |= RemoveDeadBlocks();
    if (did_any_simplification) {
      // Connect successive blocks created by dead branches.
      ConnectSuccessiveBlocks();
    }
  }
  SsaRedundantPhiElimination(graph_).Run();
  RemoveDeadInstructions();
}

}  // namespace art