compiler/optimizing/ssa_liveness_analysis.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_liveness_analysis.h"
#include "nodes.h"

namespace art {

void SsaLivenessAnalysis::Analyze() {
  NumberInstructions();
  ComputeSets();
}

void SsaLivenessAnalysis::NumberInstructions() {
  int ssa_index = 0;
  for (HReversePostOrderIterator it(graph_); !it.Done(); it.Advance()) {
    HBasicBlock* block = it.Current();

    for (HInstructionIterator it(*block->GetPhis()); !it.Done(); it.Advance()) {
      HInstruction* current = it.Current();
      if (current->HasUses()) {
        current->SetSsaIndex(ssa_index++);
      }
    }

    for (HInstructionIterator it(*block->GetInstructions()); !it.Done(); it.Advance()) {
      HInstruction* current = it.Current();
      if (current->HasUses()) {
        current->SetSsaIndex(ssa_index++);
      }
    }
  }
  number_of_ssa_values_ = ssa_index;
}

void SsaLivenessAnalysis::ComputeSets() {
  for (HReversePostOrderIterator it(graph_); !it.Done(); it.Advance()) {
    HBasicBlock* block = it.Current();
    block_infos_.Put(
        block->GetBlockId(),
        new (graph_.GetArena()) BlockInfo(graph_.GetArena(), *block, number_of_ssa_values_));
  }

  // Compute the initial live_in, live_out, and kill sets. This method does not handle
  // backward branches, therefore live_in and live_out sets are not yet correct.
  ComputeInitialSets();

  // Do a fixed point calculation to take into account backward branches,
  // that will update live_in of loop headers, and therefore live_out and live_in
  // of blocks in the loop.
  ComputeLiveInAndLiveOutSets();
}

void SsaLivenessAnalysis::ComputeInitialSets() {
  // Do a post orderr visit, adding inputs of instructions live in the block where
  // that instruction is defined, and killing instructions that are being visited.
  for (HPostOrderIterator it(graph_); !it.Done(); it.Advance()) {
    HBasicBlock* block = it.Current();

    BitVector* kill = GetKillSet(*block);
    BitVector* live_in = GetLiveInSet(*block);

    for (HBackwardInstructionIterator it(*block->GetInstructions()); !it.Done(); it.Advance()) {
      HInstruction* current = it.Current();
      if (current->HasSsaIndex()) {
        kill->SetBit(current->GetSsaIndex());
        live_in->ClearBit(current->GetSsaIndex());
      }

      // All inputs of an instruction must be live.
      for (size_t i = 0, e = current->InputCount(); i < e; ++i) {
        DCHECK(current->InputAt(i)->HasSsaIndex());
        live_in->SetBit(current->InputAt(i)->GetSsaIndex());
      }

      if (current->HasEnvironment()) {
        // All instructions in the environment must be live.
        GrowableArray<HInstruction*>* environment = current->GetEnvironment()->GetVRegs();
        for (size_t i = 0, e = environment->Size(); i < e; ++i) {
          HInstruction* instruction = environment->Get(i);
          if (instruction != nullptr) {
            DCHECK(instruction->HasSsaIndex());
            live_in->SetBit(instruction->GetSsaIndex());
          }
        }
      }
    }

    for (HInstructionIterator it(*block->GetPhis()); !it.Done(); it.Advance()) {
      HInstruction* current = it.Current();
      if (current->HasSsaIndex()) {
        kill->SetBit(current->GetSsaIndex());
        live_in->ClearBit(current->GetSsaIndex());
      }

      // Mark a phi input live_in for its corresponding predecessor.
      for (size_t i = 0, e = current->InputCount(); i < e; ++i) {
        HInstruction* input = current->InputAt(i);

        HBasicBlock* predecessor = block->GetPredecessors().Get(i);
        size_t ssa_index = input->GetSsaIndex();
        BitVector* predecessor_kill = GetKillSet(*predecessor);
        BitVector* predecessor_live_in = GetLiveInSet(*predecessor);

        // Phi inputs from a back edge have already been visited. If the back edge
        // block defines that input, we should not add it to its live_in.
        if (!predecessor_kill->IsBitSet(ssa_index)) {
          predecessor_live_in->SetBit(ssa_index);
        }
      }
    }
  }
}

void SsaLivenessAnalysis::ComputeLiveInAndLiveOutSets() {
  bool changed;
  do {
    changed = false;

    for (HPostOrderIterator it(graph_); !it.Done(); it.Advance()) {
      const HBasicBlock& block = *it.Current();

      // The live_in set depends on the kill set (which does not
      // change in this loop), and the live_out set.  If the live_out
      // set does not change, there is no need to update the live_in set.
      if (UpdateLiveOut(block) && UpdateLiveIn(block)) {
        changed = true;
      }
    }
  } while (changed);
}

bool SsaLivenessAnalysis::UpdateLiveOut(const HBasicBlock& block) {
  BitVector* live_out = GetLiveOutSet(block);
  bool changed = false;
  // The live_out set of a block is the union of live_in sets of its successors.
  for (size_t i = 0, e = block.GetSuccessors().Size(); i < e; ++i) {
    HBasicBlock* successor = block.GetSuccessors().Get(i);
    if (live_out->Union(GetLiveInSet(*successor))) {
      changed = true;
    }
  }
  return changed;
}


bool SsaLivenessAnalysis::UpdateLiveIn(const HBasicBlock& block) {
  BitVector* live_out = GetLiveOutSet(block);
  BitVector* kill = GetKillSet(block);
  BitVector* live_in = GetLiveInSet(block);
  // If live_out is updated (because of backward branches), we need to make
  // sure instructions in live_out are also in live_in, unless they are killed
  // by this block.
  return live_in->UnionIfNotIn(live_out, kill);
}

}  // namespace art