compiler/optimizing/ssa_phi_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 "ssa_phi_elimination.h"

#include "base/arena_containers.h"

namespace art {

void SsaDeadPhiElimination::Run() {
  MarkDeadPhis();
  EliminateDeadPhis();
}

void SsaDeadPhiElimination::MarkDeadPhis() {
  // Phis are constructed live and should not be revived if previously marked
  // dead. This algorithm temporarily breaks that invariant but we DCHECK that
  // only phis which were initially live are revived.
  ArenaSet<HPhi*> initially_live(graph_->GetArena()->Adapter());

  // Add to the worklist phis referenced by non-phi instructions.
  for (HReversePostOrderIterator it(*graph_); !it.Done(); it.Advance()) {
    HBasicBlock* block = it.Current();
    for (HInstructionIterator inst_it(block->GetPhis()); !inst_it.Done(); inst_it.Advance()) {
      HPhi* phi = inst_it.Current()->AsPhi();
      if (phi->IsDead()) {
        continue;
      }

      bool keep_alive = (graph_->IsDebuggable() && phi->HasEnvironmentUses());
      if (!keep_alive) {
        for (HUseIterator<HInstruction*> use_it(phi->GetUses()); !use_it.Done(); use_it.Advance()) {
          if (!use_it.Current()->GetUser()->IsPhi()) {
            keep_alive = true;
            break;
          }
        }
      }

      if (keep_alive) {
        worklist_.push_back(phi);
      } else {
        phi->SetDead();
        if (kIsDebugBuild) {
          initially_live.insert(phi);
        }
      }
    }
  }

  // Process the worklist by propagating liveness to phi inputs.
  while (!worklist_.empty()) {
    HPhi* phi = worklist_.back();
    worklist_.pop_back();
    for (HInputIterator it(phi); !it.Done(); it.Advance()) {
      HPhi* input = it.Current()->AsPhi();
      if (input != nullptr && input->IsDead()) {
        // Input is a dead phi. Revive it and add to the worklist. We make sure
        // that the phi was not dead initially (see definition of `initially_live`).
        DCHECK(ContainsElement(initially_live, input));
        input->SetLive();
        worklist_.push_back(input);
      }
    }
  }
}

void SsaDeadPhiElimination::EliminateDeadPhis() {
  // Remove phis that are not live. Visit in post order so that phis
  // that are not inputs of loop phis can be removed when they have
  // no users left (dead phis might use dead phis).
  for (HPostOrderIterator it(*graph_); !it.Done(); it.Advance()) {
    HBasicBlock* block = it.Current();
    HInstruction* current = block->GetFirstPhi();
    HInstruction* next = nullptr;
    HPhi* phi;
    while (current != nullptr) {
      phi = current->AsPhi();
      next = current->GetNext();
      if (phi->IsDead()) {
        // Make sure the phi is only used by other dead phis.
        if (kIsDebugBuild) {
          for (HUseIterator<HInstruction*> use_it(phi->GetUses()); !use_it.Done();
               use_it.Advance()) {
            HInstruction* user = use_it.Current()->GetUser();
            DCHECK(user->IsLoopHeaderPhi());
            DCHECK(user->AsPhi()->IsDead());
          }
        }
        // Remove the phi from use lists of its inputs.
        for (size_t i = 0, e = phi->InputCount(); i < e; ++i) {
          phi->RemoveAsUserOfInput(i);
        }
        // Remove the phi from environments that use it.
        for (HUseIterator<HEnvironment*> use_it(phi->GetEnvUses()); !use_it.Done();
             use_it.Advance()) {
          HUseListNode<HEnvironment*>* user_node = use_it.Current();
          HEnvironment* user = user_node->GetUser();
          user->SetRawEnvAt(user_node->GetIndex(), nullptr);
        }
        // Delete it from the instruction list.
        block->RemovePhi(phi, /*ensure_safety=*/ false);
      }
      current = next;
    }
  }
}

void SsaRedundantPhiElimination::Run() {
  // Add all phis in the worklist. Order does not matter for correctness, and
  // neither will necessarily converge faster.
  for (HReversePostOrderIterator it(*graph_); !it.Done(); it.Advance()) {
    HBasicBlock* block = it.Current();
    for (HInstructionIterator inst_it(block->GetPhis()); !inst_it.Done(); inst_it.Advance()) {
      worklist_.push_back(inst_it.Current()->AsPhi());
    }
  }

  while (!worklist_.empty()) {
    HPhi* phi = worklist_.back();
    worklist_.pop_back();

    // If the phi has already been processed, continue.
    if (!phi->IsInBlock()) {
      continue;
    }

    if (phi->InputCount() == 0) {
      DCHECK(phi->IsDead());
      continue;
    }

    // Find if the inputs of the phi are the same instruction.
    HInstruction* candidate = phi->InputAt(0);
    // A loop phi cannot have itself as the first phi. Note that this
    // check relies on our simplification pass ensuring the pre-header
    // block is first in the list of predecessors of the loop header.
    DCHECK(!phi->IsLoopHeaderPhi() || phi->GetBlock()->IsLoopPreHeaderFirstPredecessor());
    DCHECK_NE(phi, candidate);

    for (size_t i = 1; i < phi->InputCount(); ++i) {
      HInstruction* input = phi->InputAt(i);
      // For a loop phi, if the input is the phi, the phi is still candidate for
      // elimination.
      if (input != candidate && input != phi) {
        candidate = nullptr;
        break;
      }
    }

    // If the inputs are not the same, continue.
    if (candidate == nullptr) {
      continue;
    }

    // The candidate may not dominate a phi in a catch block.
    if (phi->IsCatchPhi() && !candidate->StrictlyDominates(phi)) {
      continue;
    }

    // Because we're updating the users of this phi, we may have new candidates
    // for elimination. Add phis that use this phi to the worklist.
    for (HUseIterator<HInstruction*> it(phi->GetUses()); !it.Done(); it.Advance()) {
      HUseListNode<HInstruction*>* current = it.Current();
      HInstruction* user = current->GetUser();
      if (user->IsPhi()) {
        worklist_.push_back(user->AsPhi());
      }
    }

    phi->ReplaceWith(candidate);
    phi->GetBlock()->RemovePhi(phi);
  }
}

}  // namespace art