Compute topological sort using post-order traversal

* fixes #2080

Author: Michael Emmi

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

@@ -22,17 +22,16 @@
  * #L%
  */
 
-import java.util.ArrayList;
 import java.util.HashSet;
-import java.util.List;
+import java.util.Stack;
 
 import soot.jimple.spark.pag.Node;
 import soot.jimple.spark.pag.PAG;
 import soot.jimple.spark.pag.VarNode;
 
 /**
  * Performs a pseudo-topological sort on the VarNodes in a PAG.
- * 
+ *
  * @author Ondrej Lhotak
  */
 
@@ -64,23 +63,21 @@ protected void dfsVisit(VarNode n) {
     if (visited.contains(n)) {
       return;
     }
-    List<VarNode> stack = new ArrayList<>();
-    List<VarNode> all = new ArrayList<>();
+    Stack<VarNode> stack = new Stack<>();
     stack.add(n);
     while (!stack.isEmpty()) {
-      VarNode s = stack.remove(stack.size() - 1);
+      VarNode s = stack.peek();
       if (visited.add(s)) {
-        all.add(s);
         Node[] succs = pag.simpleLookup(s);
         for (Node element : succs) {
           if (ignoreTypes || pag.getTypeManager().castNeverFails(n.getType(), element.getType())) {
             stack.add((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 +1,40 @@
+package soot.jimple.spark.solver;
+
+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());
+    }
+}