buggy files form Closure #85

projects/Closure/85/com/google/javascript/jscomp/UnreachableCodeElimination.java

@@ -0,0 +1,236 @@
+/*
+ * Copyright 2008 The Closure Compiler Authors.
+ *
+ * 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.google.javascript.jscomp;
+
+import com.google.common.base.Preconditions;
+import com.google.javascript.jscomp.ControlFlowGraph.Branch;
+import com.google.javascript.jscomp.NodeTraversal.AbstractPostOrderCallback;
+import com.google.javascript.jscomp.NodeTraversal.ScopedCallback;
+import com.google.javascript.jscomp.graph.GraphReachability;
+import com.google.javascript.jscomp.graph.DiGraph.DiGraphEdge;
+import com.google.javascript.jscomp.graph.DiGraph.DiGraphNode;
+import com.google.javascript.rhino.Node;
+import com.google.javascript.rhino.Token;
+
+import java.util.Deque;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.logging.Level;
+import java.util.logging.Logger;
+
+/**
+ * Removes dead code from a parse tree. The kinds of dead code that this pass
+ * removes are:
+ *  - Any code following a return statement, such as the <code>alert</code>
+ *    call in: <code>if (x) { return; alert('unreachable'); }</code>.
+ *  - Statements that have no side effects, such as:
+ *    <code>a.b.MyClass.prototype.propertyName;</code> or <code>true;</code>.
+ *    That first kind of statement sometimes appears intentionally, so that
+ *    prototype properties can be annotated using JSDoc without actually
+ *    being initialized.
+ *
+ */
+class UnreachableCodeElimination extends AbstractPostOrderCallback
+    implements CompilerPass, ScopedCallback  {
+  private static final Logger logger =
+    Logger.getLogger(UnreachableCodeElimination.class.getName());
+
+  private final AbstractCompiler compiler;
+  private final boolean removeNoOpStatements;
+
+  Deque<ControlFlowGraph<Node>> cfgStack =
+      new LinkedList<ControlFlowGraph<Node>>();
+
+  ControlFlowGraph<Node> curCfg = null;
+
+  UnreachableCodeElimination(AbstractCompiler compiler,
+      boolean removeNoOpStatements) {
+    this.compiler = compiler;
+    this.removeNoOpStatements = removeNoOpStatements;
+  }
+
+  @Override
+  public void enterScope(NodeTraversal t) {
+    Scope scope = t.getScope();
+
+    // Computes the control flow graph.
+    ControlFlowAnalysis cfa = new ControlFlowAnalysis(compiler, false, false);
+    cfa.process(null, scope.getRootNode());
+    cfgStack.push(curCfg);
+    curCfg = cfa.getCfg();
+
+    new GraphReachability<Node, ControlFlowGraph.Branch>(curCfg)
+        .compute(curCfg.getEntry().getValue());
+  }
+
+  @Override
+  public void exitScope(NodeTraversal t) {
+    curCfg = cfgStack.pop();
+  }
+
+  @Override
+  public void process(Node externs, Node root) {
+    NodeTraversal.traverse(compiler, root, this);
+  }
+
+  @Override
+  public void visit(NodeTraversal t, Node n, Node parent) {
+    if (parent == null) {
+      return;
+    }
+    if (n.getType() == Token.FUNCTION || n.getType() == Token.SCRIPT) {
+      return;
+    }
+
+    DiGraphNode<Node, Branch> gNode = curCfg.getDirectedGraphNode(n);
+    if (gNode == null) { // Not in CFG.
+      return;
+    }
+    if (gNode.getAnnotation() != GraphReachability.REACHABLE ||
+        (removeNoOpStatements && !NodeUtil.mayHaveSideEffects(n))) {
+      removeDeadExprStatementSafely(n);
+      return;
+    }
+
+    tryRemoveUnconditionalBranching(n);
+  }
+
+  /**
+   * Tries to remove n if an unconditional branch node (break, continue or
+   * return) if the target of n is the same as the the follow of n. That is, if
+   * we remove n, the control flow remains the same. Also if n targets to
+   * another unconditional branch, this function will recursively try to remove
+   * the target branch as well. The reason why we want to cascade this removal
+   * is because we only run this pass once. If we have code such as
+   *
+   * break -> break -> break
+   *
+   * where all 3 break's are useless. The order of removal matters. When we
+   * first look at the first break, we see that it branches to the 2nd break.
+   * However, if we remove the last break, the 2nd break becomes useless and
+   * finally the first break becomes useless as well.
+   *
+   * @return The target of this jump. If the target is also useless jump,
+   *     the target of that useless jump recursively.
+   */
+  @SuppressWarnings("fallthrough")
+  private Node tryRemoveUnconditionalBranching(Node n) {
+    /*
+     * For each of the unconditional branching control flow node, check to see
+     * if the ControlFlowAnalysis.computeFollowNode of that node is same as
+     * the branching target. If it is, the branch node is safe to be removed.
+     *
+     * This is not as clever as MinimizeExitPoints because it doesn't do any
+     * if-else conversion but it handles more complicated switch statements
+     * much nicer.
+     */
+
+    // If n is null the target is the end of the function, nothing to do.
+    if (n == null) {
+       return n;
+    }
+
+    DiGraphNode<Node, Branch> gNode = curCfg.getDirectedGraphNode(n);
+
+    if (gNode == null) {
+      return n;
+    }
+
+    if (n.getParent() == null) {
+      List<DiGraphEdge<Node,Branch>> outEdges = gNode.getOutEdges();
+      if (outEdges.size() == 1) {
+        return tryRemoveUnconditionalBranching(outEdges.get(0).getDestination().getValue());
+      }
+    }
+    switch (n.getType()) {
+      case Token.BLOCK:
+        if (n.hasChildren()) {
+          Node first = n.getFirstChild();
+          return tryRemoveUnconditionalBranching(first);
+        } else {
+          return tryRemoveUnconditionalBranching(ControlFlowAnalysis.computeFollowNode(n));
+        }
+      case Token.RETURN:
+        if (n.hasChildren()) {
+          break;
+        }
+      case Token.BREAK:
+      case Token.CONTINUE:
+
+        // We are looking for a control flow changing statement that always
+        // branches to the same node. If removing it the control flow still
+        // branches to that same node. It is safe to remove it.
+        List<DiGraphEdge<Node,Branch>> outEdges = gNode.getOutEdges();
+        if (outEdges.size() == 1 &&
+            // If there is a next node, there is no chance this jump is useless.
+            (n.getNext() == null || n.getNext().getType() == Token.FUNCTION)) {
+
+          Preconditions.checkState(outEdges.get(0).getValue() == Branch.UNCOND);
+          Node fallThrough = tryRemoveUnconditionalBranching(computeFollowing(n));
+          Node nextCfgNode = outEdges.get(0).getDestination().getValue();
+          if (nextCfgNode == fallThrough) {
+            removeDeadExprStatementSafely(n);
+            return fallThrough;
+          }
+        }
+    }
+    return n;
+  }
+
+  private Node computeFollowing(Node n) {
+    Node next = ControlFlowAnalysis.computeFollowNode(n);
+    return next;
+  }
+
+  private void removeDeadExprStatementSafely(Node n) {
+    Node parent = n.getParent();
+    if (n.getType() == Token.EMPTY ||
+        (n.getType() == Token.BLOCK && !n.hasChildren())) {
+      // Not always trivial to remove, let FoldContants work its magic later.
+      return;
+    }
+
+    switch (n.getType()) {
+      // Removing an unreachable DO node is messy because it means we still have
+      // to execute one iteration. If the DO's body has breaks in the middle, it
+      // can get even more trickier and code size might actually increase.
+      case Token.DO:
+        return;
+
+      case Token.BLOCK:
+        // BLOCKs are used in several ways including wrapping CATCH blocks in TRYs
+        if (parent.getType() == Token.TRY) {
+          if (NodeUtil.isTryCatchNodeContainer(n)) {
+            return;
+          }
+        }
+        break;
+
+      case Token.CATCH:
+        Node tryNode = parent.getParent();
+        NodeUtil.maybeAddFinally(tryNode);
+        break;
+    }
+
+    NodeUtil.redeclareVarsInsideBranch(n);
+    compiler.reportCodeChange();
+    if (logger.isLoggable(Level.FINE)) {
+      logger.fine("Removing " + n.toString());
+    }
+    NodeUtil.removeChild(n.getParent(), n);
+  }
+}