src/main/java/com/android/tools/r8/ir/optimize/DeadCodeRemover.java

// Copyright (c) 2016, the R8 project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.
package com.android.tools.r8.ir.optimize;

import com.android.tools.r8.graph.DexType;
import com.android.tools.r8.graph.GraphLense;
import com.android.tools.r8.ir.code.BasicBlock;
import com.android.tools.r8.ir.code.CatchHandlers;
import com.android.tools.r8.ir.code.IRCode;
import com.android.tools.r8.ir.code.Instruction;
import com.android.tools.r8.ir.code.InstructionListIterator;
import com.android.tools.r8.ir.code.Phi;
import com.android.tools.r8.ir.code.Value;
import com.android.tools.r8.utils.InternalOptions;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;
import java.util.Set;

public class DeadCodeRemover {

  public static void removeDeadCode(
      IRCode code, CodeRewriter codeRewriter, GraphLense graphLense, InternalOptions options) {
    removeUnneededCatchHandlers(code, graphLense, options);
    Queue<BasicBlock> worklist = new LinkedList<>();
    worklist.addAll(code.blocks);
    for (BasicBlock block = worklist.poll(); block != null; block = worklist.poll()) {
      removeDeadInstructions(worklist, code, block, options);
      removeDeadPhis(worklist, block, options);
    }
    assert code.isConsistentSSA();
    codeRewriter.rewriteMoveResult(code);
  }

  // Add the block from where the value originates to the worklist.
  private static void updateWorklist(Queue<BasicBlock> worklist, Value value) {
    BasicBlock block = null;
    if (value.isPhi()) {
      block = value.asPhi().getBlock();
    } else if (value.definition.hasBlock()) {
      block = value.definition.getBlock();
    }
    if (block != null) {
      worklist.add(block);
    }
  }

  // Add all blocks from where the in/debug-values to the instruction originates.
  private static void updateWorklist(
      Queue<BasicBlock> worklist, Instruction instruction) {
    for (Value inValue : instruction.inValues()) {
      updateWorklist(worklist, inValue);
    }
    for (Value debugValue : instruction.getDebugValues()) {
      updateWorklist(worklist, debugValue);
    }
  }

  private static void removeDeadPhis(Queue<BasicBlock> worklist, BasicBlock block,
      InternalOptions options) {
    Iterator<Phi> phiIt = block.getPhis().iterator();
    while (phiIt.hasNext()) {
      Phi phi = phiIt.next();
      if (phi.isDead(options)) {
        phiIt.remove();
        for (Value operand : phi.getOperands()) {
          operand.removePhiUser(phi);
          updateWorklist(worklist, operand);
        }
      }
    }
  }

  private static void removeDeadInstructions(
      Queue<BasicBlock> worklist, IRCode code, BasicBlock block, InternalOptions options) {
    InstructionListIterator iterator = block.listIterator(block.getInstructions().size());
    while (iterator.hasPrevious()) {
      Instruction current = iterator.previous();
      // Remove unused invoke results.
      if (current.isInvoke()
          && current.outValue() != null
          && !current.outValue().isUsed()) {
        current.setOutValue(null);
      }
      if (!current.canBeDeadCode(code, options)) {
        continue;
      }
      Value outValue = current.outValue();
      // Instructions with no out value cannot be dead code by the current definition
      // (unused out value). They typically side-effect input values or deals with control-flow.
      assert outValue != null;
      if (!outValue.isDead(options)) {
        continue;
      }
      updateWorklist(worklist, current);
      // All users will be removed for this instruction. Eagerly clear them so further inspection
      // of this instruction during dead code elimination will terminate here.
      outValue.clearUsers();
      iterator.removeOrReplaceByDebugLocalRead();
    }
  }

  private static void removeUnneededCatchHandlers(
      IRCode code, GraphLense graphLense, InternalOptions options) {
    for (BasicBlock block : code.blocks) {
      if (block.hasCatchHandlers()) {
        if (block.canThrow()) {
          if (options.enableVerticalClassMerging) {
            // Handle the case where an exception class has been merged into its sub class.
            block.renameGuardsInCatchHandlers(graphLense);
            unlinkDeadCatchHandlers(block, graphLense);
          }
        } else {
          CatchHandlers<BasicBlock> handlers = block.getCatchHandlers();
          for (BasicBlock target : handlers.getUniqueTargets()) {
            target.unlinkCatchHandler();
          }
        }
      }
    }
    code.removeUnreachableBlocks();
  }

  // Due to class merging, it is possible that two exception classes have been merged into one. This
  // function removes catch handlers where the guards ended up being the same as a previous one.
  private static void unlinkDeadCatchHandlers(BasicBlock block, GraphLense graphLense) {
    assert block.hasCatchHandlers();
    CatchHandlers<BasicBlock> catchHandlers = block.getCatchHandlers();
    List<DexType> guards = catchHandlers.getGuards();
    List<BasicBlock> targets = catchHandlers.getAllTargets();

    Set<DexType> previouslySeenGuards = new HashSet<>();
    List<BasicBlock> deadCatchHandlers = new ArrayList<>();
    for (int i = 0; i < guards.size(); i++) {
      // The type may have changed due to class merging.
      DexType guard = graphLense.lookupType(guards.get(i));
      boolean guardSeenBefore = !previouslySeenGuards.add(guard);
      if (guardSeenBefore) {
        deadCatchHandlers.add(targets.get(i));
      }
    }
    // Remove the guards that are guaranteed to be dead.
    for (BasicBlock deadCatchHandler : deadCatchHandlers) {
      deadCatchHandler.unlinkCatchHandler();
    }
    assert block.consistentCatchHandlers();
  }
}