src/main/java/com/android/tools/r8/shaking/AbstractMethodRemover.java

// Copyright (c) 2017, 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.shaking;

import com.android.tools.r8.graph.AppView;
import com.android.tools.r8.graph.DexClass;
import com.android.tools.r8.graph.DexEncodedMethod;
import com.android.tools.r8.graph.DexType;
import com.android.tools.r8.graph.SubtypingInfo;
import com.android.tools.r8.logging.Log;
import com.android.tools.r8.shaking.ScopedDexMethodSet.AddMethodIfMoreVisibleResult;
import com.android.tools.r8.utils.IterableUtils;
import java.util.ArrayList;
import java.util.List;

/**
 * Removes abstract methods if they only shadow methods of the same signature in a superclass.
 * <p>
 * We do not consider classes from the library for this optimization, as the program might run
 * against a different version of the library where methods are missing.
 * <p>
 * This optimization is beneficial mostly as it removes superfluous abstract methods that are
 * created by the {@link TreePruner}.
 */
public class AbstractMethodRemover {

  private final AppView<AppInfoWithLiveness> appView;
  private final SubtypingInfo subtypingInfo;
  private ScopedDexMethodSet scope = new ScopedDexMethodSet();

  public AbstractMethodRemover(AppView<AppInfoWithLiveness> appView, SubtypingInfo subtypingInfo) {
    this.appView = appView;
    this.subtypingInfo = subtypingInfo;
  }

  public void run() {
    assert scope.getParent() == null;
    processClass(appView.dexItemFactory().objectType);
  }

  private void processClass(DexType type) {
    DexClass holder = appView.definitionFor(type);
    scope = scope.newNestedScope();
    if (holder != null && holder.isProgramClass()) {
      DexEncodedMethod[] newVirtualMethods =
          processMethods(IterableUtils.ensureUnmodifiableList(holder.virtualMethods()));
      if (newVirtualMethods != null) {
        holder.setVirtualMethods(newVirtualMethods);
      }
    }
    // TODO(b/154881041): Does this need the full subtype hierarchy of referenced types!?
    subtypingInfo.forAllImmediateExtendsSubtypes(type, this::processClass);
    scope = scope.getParent();
  }

  private DexEncodedMethod[] processMethods(List<DexEncodedMethod> virtualMethods) {
    if (virtualMethods == null) {
      return null;
    }
    // Removal of abstract methods is rare, so avoid copying the array until we find one.
    List<DexEncodedMethod> methods = null;
    for (int i = 0; i < virtualMethods.size(); i++) {
      DexEncodedMethod method = virtualMethods.get(i);
      if (scope.addMethodIfMoreVisible(method) != AddMethodIfMoreVisibleResult.NOT_ADDED
          || !method.accessFlags.isAbstract()
          || appView.appInfo().isPinned(method.method)) {
        if (methods != null) {
          methods.add(method);
        }
      } else {
        if (methods == null) {
          methods = new ArrayList<>(virtualMethods.size() - 1);
          for (int j = 0; j < i; j++) {
            methods.add(virtualMethods.get(j));
          }
        }
        if (Log.ENABLED) {
          Log.debug(getClass(), "Removing abstract method %s.", method.method);
        }
      }
    }
    if (methods != null) {
      return methods.toArray(DexEncodedMethod.EMPTY_ARRAY);
    }
    return null;
  }

}