Compute topological sort using post-order traversal

* fixes #2080

src/main/java/soot/jimple/spark/solver/TopoSorter.java

@@ -22,9 +22,9 @@
  * #L%
  */
 
-import java.util.ArrayList;
 import java.util.HashSet;
-import java.util.List;
+import java.util.Set;
+import java.util.Stack;
 
 import soot.jimple.spark.pag.Node;
 import soot.jimple.spark.pag.PAG;
@@ -61,26 +61,31 @@ public TopoSorter(PAG pag, boolean ignoreTypes) {
   protected HashSet<VarNode> visited;
 
   protected void dfsVisit(VarNode n) {
-    if (visited.contains(n)) {
+    if (!visited.add(n)) {
       return;
     }
-    List<VarNode> stack = new ArrayList<>();
-    List<VarNode> all = new ArrayList<>();
+
+    Stack<VarNode> stack = new Stack<>();
+    Set<VarNode> visitedSuccessors = new HashSet<>();
     stack.add(n);
+
     while (!stack.isEmpty()) {
-      VarNode s = stack.remove(stack.size() - 1);
-      if (visited.add(s)) {
-        all.add(s);
+      VarNode s = stack.peek();
+
+      if (visitedSuccessors.add(s)) {
         Node[] succs = pag.simpleLookup(s);
         for (Node element : succs) {
           if (ignoreTypes || pag.getTypeManager().castNeverFails(n.getType(), element.getType())) {
-            stack.add((VarNode) element);
+            if (visited.add((VarNode) element)) {
+              stack.push((VarNode) element);
+            }
           }
         }
+
+      } else {
+        stack.pop();
+        s.setFinishingNumber(nextFinishNumber++);
       }
     }
-    for (int i = all.size() - 1; i >= 0; i--) {
-      all.get(i).setFinishingNumber(nextFinishNumber++);
-    }
   }
 }

src/test/java/soot/jimple/spark/solver/SCCCollapserTest.java

@@ -0,0 +1,62 @@
+package soot.jimple.spark.solver;
+
+/*-
+ * #%L
+ * Soot - a J*va Optimization Framework
+ * %%
+ * Copyright (C) 1997 - 2018 Raja Vallée-Rai 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
+ * <http://www.gnu.org/licenses/lgpl-2.1.html>.
+ * #L%
+ */
+
+import java.util.Collections;
+
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+import soot.Scene;
+import soot.Type;
+import soot.jimple.spark.pag.PAG;
+import soot.jimple.spark.pag.VarNode;
+import soot.options.SparkOptions;
+
+public class SCCCollapserTest {
+
+    @Test
+    public void testSeparateComponents() {
+        Scene.v().loadBasicClasses();
+        Type type = Scene.v().getObjectType();
+
+        SparkOptions sparkOptions = new SparkOptions(Collections.emptyMap());
+        PAG pag = new PAG(sparkOptions);
+
+        VarNode a = pag.makeGlobalVarNode("a", type);
+        VarNode b = pag.makeGlobalVarNode("b", type);
+        VarNode c = pag.makeGlobalVarNode("c", type);
+        pag.addEdge(a, b);
+        pag.addEdge(a, c);
+        pag.addEdge(b, c);
+
+        SCCCollapser sccCollapser = new SCCCollapser(pag, false);
+        sccCollapser.collapse();
+        pag.cleanUpMerges();
+
+        assertEquals(a, a.getReplacement());
+        assertEquals(b, b.getReplacement());
+        assertEquals(c, c.getReplacement());
+    }
+}