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/*
* Copyright (C) 2007 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.
*/
package com.android.dx.ssa.back;
import com.android.dx.rop.code.BasicBlock;
import com.android.dx.rop.code.BasicBlockList;
import com.android.dx.rop.code.CstInsn;
import com.android.dx.rop.code.Insn;
import com.android.dx.rop.code.InsnList;
import com.android.dx.rop.code.RopMethod;
import com.android.dx.rop.code.SwitchInsn;
import com.android.dx.util.IntList;
import java.util.BitSet;
/**
* Searches for basic blocks that all have the same successor and insns
* but different predecessors. These blocks are then combined into a single
* block and the now-unused blocks are deleted. These identical blocks
* frequently are created when catch blocks are edge-split.
*/
public class IdenticalBlockCombiner {
private final RopMethod ropMethod;
private final BasicBlockList blocks;
private final BasicBlockList newBlocks;
/**
* Constructs instance. Call {@code process()} to run.
*
* @param rm {@code non-null;} instance to process
*/
public IdenticalBlockCombiner(RopMethod rm) {
ropMethod = rm;
blocks = ropMethod.getBlocks();
newBlocks = blocks.getMutableCopy();
}
/**
* Runs algorithm. TODO: This is n^2, and could be made linear-ish with
* a hash. In particular, hash the contents of each block and only
* compare blocks with the same hash.
*
* @return {@code non-null;} new method that has been processed
*/
public RopMethod process() {
int szBlocks = blocks.size();
// indexed by label
BitSet toDelete = new BitSet(blocks.getMaxLabel());
// For each non-deleted block...
for (int bindex = 0; bindex < szBlocks; bindex++) {
BasicBlock b = blocks.get(bindex);
if (toDelete.get(b.getLabel())) {
// doomed block
continue;
}
IntList preds = ropMethod.labelToPredecessors(b.getLabel());
// ...look at all of it's predecessors that have only one succ...
int szPreds = preds.size();
for (int i = 0; i < szPreds; i++) {
int iLabel = preds.get(i);
BasicBlock iBlock = blocks.labelToBlock(iLabel);
if (toDelete.get(iLabel)
|| iBlock.getSuccessors().size() > 1) {
continue;
}
IntList toCombine = new IntList();
// ...and see if they can be combined with any other preds...
for (int j = i + 1; j < szPreds; j++) {
int jLabel = preds.get(j);
BasicBlock jBlock = blocks.labelToBlock(jLabel);
if (jBlock.getSuccessors().size() == 1
&& compareInsns(iBlock, jBlock)) {
toCombine.add(jLabel);
toDelete.set(jLabel);
}
}
combineBlocks(iLabel, toCombine);
}
}
for (int i = szBlocks - 1; i >= 0; i--) {
if (toDelete.get(newBlocks.get(i).getLabel())) {
newBlocks.set(i, null);
}
}
newBlocks.shrinkToFit();
newBlocks.setImmutable();
return new RopMethod(newBlocks, ropMethod.getFirstLabel());
}
/**
* Helper method to compare the contents of two blocks.
*
* @param a {@code non-null;} a block to compare
* @param b {@code non-null;} another block to compare
* @return {@code true} iff the two blocks' instructions are the same
*/
private static boolean compareInsns(BasicBlock a, BasicBlock b) {
return a.getInsns().contentEquals(b.getInsns());
}
/**
* Combines blocks proven identical into one alpha block, re-writing
* all of the successor links that point to the beta blocks to point
* to the alpha block instead.
*
* @param alphaLabel block that will replace all the beta block
* @param betaLabels label list of blocks to combine
*/
private void combineBlocks(int alphaLabel, IntList betaLabels) {
int szBetas = betaLabels.size();
for (int i = 0; i < szBetas; i++) {
int betaLabel = betaLabels.get(i);
BasicBlock bb = blocks.labelToBlock(betaLabel);
IntList preds = ropMethod.labelToPredecessors(bb.getLabel());
int szPreds = preds.size();
for (int j = 0; j < szPreds; j++) {
BasicBlock predBlock = newBlocks.labelToBlock(preds.get(j));
replaceSucc(predBlock, betaLabel, alphaLabel);
}
}
}
/**
* Replaces one of a block's successors with a different label. Constructs
* an updated BasicBlock instance and places it in {@code newBlocks}.
*
* @param block block to replace
* @param oldLabel label of successor to replace
* @param newLabel label of new successor
*/
private void replaceSucc(BasicBlock block, int oldLabel, int newLabel) {
IntList newSuccessors = block.getSuccessors().mutableCopy();
int newPrimarySuccessor;
newSuccessors.set(newSuccessors.indexOf(oldLabel), newLabel);
newPrimarySuccessor = block.getPrimarySuccessor();
if (newPrimarySuccessor == oldLabel) {
newPrimarySuccessor = newLabel;
}
newSuccessors.setImmutable();
BasicBlock newBB = new BasicBlock(block.getLabel(),
block.getInsns(), newSuccessors, newPrimarySuccessor);
newBlocks.set(newBlocks.indexOfLabel(block.getLabel()), newBB);
}
}