sootup.java.bytecode.interceptors.ConditionalBranchFolder Maven / Gradle / Ivy
package sootup.java.bytecode.interceptors;
/*-
* #%L
* Soot - a J*va Optimization Framework
* %%
* Copyright (C) 1997-2020 Raja Vallée-Rai, Marcus Nachtigall, Markus Schmidt and others
* %%
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU Lesser General Public License as
* published by the Free Software Foundation, either version 2.1 of the
* License, or (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Lesser Public License for more details.
*
* You should have received a copy of the GNU General Lesser Public
* License along with this program. If not, see
* .
* #L%
*/
import com.google.common.collect.Lists;
import java.util.*;
import javax.annotation.Nonnull;
import sootup.core.graph.MutableStmtGraph;
import sootup.core.graph.StmtGraph;
import sootup.core.jimple.common.constant.Constant;
import sootup.core.jimple.common.constant.IntConstant;
import sootup.core.jimple.common.stmt.FallsThroughStmt;
import sootup.core.jimple.common.stmt.JIfStmt;
import sootup.core.jimple.common.stmt.Stmt;
import sootup.core.model.Body;
import sootup.core.transform.BodyInterceptor;
import sootup.core.views.View;
/**
* Statically evaluates the conditional expression of Jimple if statements. If the condition is
* identically true or false, the Folder replaces the conditional branch statement with an
* unconditional goto statement
*
* @author Marcus Nachtigall
* @author Markus Schmidt
*/
public class ConditionalBranchFolder implements BodyInterceptor {
@Override
public void interceptBody(@Nonnull Body.BodyBuilder builder, @Nonnull View view) {
final MutableStmtGraph stmtGraph = builder.getStmtGraph();
for (Stmt stmt : Lists.newArrayList(stmtGraph.getNodes())) {
if (!(stmt instanceof JIfStmt)) {
continue;
}
JIfStmt ifStmt = (JIfStmt) stmt;
// check for constant-valued conditions
Constant evaluatedCondition = Evaluator.getConstantValueOf(ifStmt.getCondition());
if (evaluatedCondition == null) {
// not or not "easy" evaluatable
continue;
}
final List ifSuccessors = stmtGraph.successors(ifStmt);
final Stmt tautologicSuccessor;
final Stmt neverReachedSucessor;
if (((IntConstant) evaluatedCondition).getValue() == 1) {
// the evaluated if evaluatedCondition is always true: redirect all predecessors to the
// successor
// of this if-statement and prune the "true"-block stmt tree until another branch flows
// to a Stmt
tautologicSuccessor = ifSuccessors.get(0);
neverReachedSucessor = ifSuccessors.get(1);
} else if (((IntConstant) evaluatedCondition).getValue() == 0) {
// the evaluated evaluatedCondition is always false remove the fallsthrough successor etc.
tautologicSuccessor = ifSuccessors.get(1);
neverReachedSucessor = ifSuccessors.get(0);
} else {
// should not occur?
continue;
}
// link previous stmt with always-reached successor of the if-Stmt
for (Stmt predecessor : stmtGraph.predecessors(ifStmt)) {
stmtGraph.removeEdge(predecessor, ifStmt);
stmtGraph.putEdge((FallsThroughStmt) predecessor, tautologicSuccessor);
}
// removeFlow calls should be obsolete as of following removeStmt
stmtGraph.removeEdge(ifStmt, tautologicSuccessor);
stmtGraph.removeEdge(ifStmt, neverReachedSucessor);
stmtGraph.removeNode(ifStmt);
pruneExclusivelyReachableStmts(stmtGraph, neverReachedSucessor);
}
}
private void pruneExclusivelyReachableStmts(
@Nonnull MutableStmtGraph stmtGraph, @Nonnull Stmt fallsThroughStmt) {
Set reachedBranchingStmts = new HashSet<>();
Deque q = new ArrayDeque<>();
q.addFirst(fallsThroughStmt);
// stmts we want to remove
// remove all now unreachable stmts from "true"-block
while (!q.isEmpty()) {
Stmt itStmt = q.pollFirst();
if (itStmt.branches()) {
// reachable branching stmts that may or may not branch to another reachable stmt is all we
// are actually interested in
reachedBranchingStmts.add(itStmt);
}
if (stmtGraph.containsNode(itStmt)) {
final List predecessors = stmtGraph.predecessors(itStmt);
if (predecessors.size() <= 1) {
q.addAll(stmtGraph.successors(itStmt));
}
}
}
// now iterate again and remove if possible: ie predecessor.size() < 1
q.addFirst(fallsThroughStmt);
while (!q.isEmpty()) {
Stmt itStmt = q.pollFirst();
if (stmtGraph.containsNode(itStmt)) {
// hint: predecessor could also be already removed
if (isExclusivelyReachable(stmtGraph, itStmt, reachedBranchingStmts)) {
q.addAll(stmtGraph.successors(itStmt));
stmtGraph.removeNode(itStmt);
}
}
}
}
/** reachedStmts contains all reached Stmts from entrypoint which ALSO do branch! */
private boolean isExclusivelyReachable(
@Nonnull StmtGraph graph, @Nonnull Stmt stmt, @Nonnull Set reachedStmts) {
final List predecessors = graph.predecessors(stmt);
final int predecessorSize = predecessors.size();
int amount = predecessorSize;
if (predecessorSize <= 1) {
// we already reached this stmt somehow via reachable stmts so at least one predecessor was
// reachable which makes it exclusively reachable if there are no other ingoing flows
// hint: <= because a predecessor could already be removed
return true;
}
for (Stmt predecessor : predecessors) {
if (predecessor.fallsThrough()) {
if (predecessor instanceof JIfStmt) {
final List predsSuccessors = graph.successors(predecessor);
if (predsSuccessors.size() > 0 && predsSuccessors.get(0) == stmt) {
// TODO: hint: possible problem occurs with partial removed targets as they change the
// idx positions..
amount--;
continue;
}
} else {
// "usual" fallsthrough
amount--;
continue;
}
}
// was a branching predecessor reachable?
if (reachedStmts.contains(predecessor)) {
amount--;
continue;
}
}
return amount == 0;
}
}