reduces Java AST optimization time by bailing out when the rate of change slows to a crawl (only applies on optimization levels less than 9)

Change-Id: Iabf844d6a02f694e8d14103377f74f46e4e01915
Review-Link: https://gwt-review.googlesource.com/#/c/2580/

Review by: rluble@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11600 8db76d5a-ed1c-0410-87a9-c151d255dfc7

Author: stalcup@google.com

dev/core/src/com/google/gwt/dev/jjs/JavaToJavaScriptCompiler.java

@@ -251,16 +251,34 @@ public boolean visit(JNode x, Context ctx) {
 
   private static final String ENUM_NAME_OBFUSCATION_PROPERTY = "compiler.enum.obfuscate.names";
 
+  /**
+   * Continuing to apply optimizations till the rate of change reaches this value causes the AST to
+   * reach a fixed point.
+   */
+  private static final int FIXED_POINT_CHANGE_RATE = 0;
+
+  /**
+   * Ending optimization passes when the rate of change has reached this value results in
+   * gaining nearly all of the impact while avoiding the long tail of costly but low-impact passes.
+   */
+  private static final float EFFICIENT_CHANGE_RATE = 0.01f;
+
+  /**
+   * Limits the number of optimization passes against the possible danger of an AST that does not
+   * converge.
+   */
+  private static final int MAX_PASSES = 100;
+
   /**
    * Compiles a particular permutation, based on a precompiled unified AST.
    *
    * @param logger the logger to use
-   * @param unifiedAst the result of a
-   *          {@link #precompile(TreeLogger, ModuleDef, RebindPermutationOracle, String[], String[], JJSOptions, boolean, PrecompilationMetricsArtifact)}
+   * @param unifiedAst the result of a {@link #precompile(TreeLogger, ModuleDef,
+   *          RebindPermutationOracle, String[], String[], JJSOptions, boolean,
+   *          PrecompilationMetricsArtifact)}
    * @param permutation the permutation to compile
    * @return the output JavaScript
-   * @throws UnableToCompleteException if an error other than
-   *           {@link OutOfMemoryError} occurs
+   * @throws UnableToCompleteException if an error other than {@link OutOfMemoryError} occurs
    */
   public static PermutationResult compilePermutation(TreeLogger logger, UnifiedAst unifiedAst,
       Permutation permutation) throws UnableToCompleteException {
@@ -768,11 +786,16 @@ protected static void optimize(JJSOptions options, JProgram jprogram)
     Event optimizeEvent = SpeedTracerLogger.start(CompilerEventType.OPTIMIZE);
 
     List<OptimizerStats> allOptimizerStats = new ArrayList<OptimizerStats>();
-    int counter = 0;
-    int optimizationLevel = options.getOptimizationLevel();
+    int passCount = 0;
+    int nodeCount = getNodeCount(jprogram);
+    int lastNodeCount;
+
+    boolean atMaxLevel = options.getOptimizationLevel() == OptionOptimize.OPTIMIZE_LEVEL_MAX;
+    int passLimit = atMaxLevel ? MAX_PASSES : options.getOptimizationLevel();
+    float minChangeRate = atMaxLevel ? FIXED_POINT_CHANGE_RATE : EFFICIENT_CHANGE_RATE;
     while (true) {
-      counter++;
-      if (optimizationLevel < OptionOptimize.OPTIMIZE_LEVEL_MAX && counter > optimizationLevel) {
+      passCount++;
+      if (passCount > passLimit) {
         break;
       }
       if (Thread.interrupted()) {
@@ -781,9 +804,14 @@ protected static void optimize(JJSOptions options, JProgram jprogram)
       }
       AstDumper.maybeDumpAST(jprogram);
       OptimizerStats stats =
-          optimizeLoop("Pass " + counter, jprogram, options.isAggressivelyOptimize());
+          optimizeLoop("Pass " + passCount, jprogram, options.isAggressivelyOptimize(), nodeCount);
       allOptimizerStats.add(stats);
-      if (!stats.didChange()) {
+      lastNodeCount = nodeCount;
+      nodeCount = getNodeCount(jprogram);
+
+      float nodeChangeRate = stats.getNumMods() / (float) lastNodeCount;
+      float sizeChangeRate = (lastNodeCount - nodeCount) / (float) lastNodeCount;
+      if (nodeChangeRate <= minChangeRate && sizeChangeRate <= minChangeRate) {
         break;
       }
     }
@@ -849,15 +877,15 @@ protected static void optimizeJs(JJSOptions options, JsProgram jsProgram,
 
   protected static OptimizerStats optimizeLoop(String passName, JProgram jprogram,
       boolean isAggressivelyOptimize) {
-    Event optimizeEvent = SpeedTracerLogger.start(CompilerEventType.OPTIMIZE, "phase", "loop");
-
-    // Count the number of nodes in the AST so we can measure the efficiency of
-    // the optimizers.
-    Event countEvent = SpeedTracerLogger.start(CompilerEventType.OPTIMIZE, "phase", "countNodes");
     TreeStatistics treeStats = new TreeStatistics();
     treeStats.accept(jprogram);
     int numNodes = treeStats.getNodeCount();
-    countEvent.end();
+    return optimizeLoop("Early Optimization", jprogram, false, numNodes);
+  }
+
+  protected static OptimizerStats optimizeLoop(String passName, JProgram jprogram,
+      boolean isAggressivelyOptimize, int numNodes) {
+    Event optimizeEvent = SpeedTracerLogger.start(CompilerEventType.OPTIMIZE, "phase", "loop");
 
     // Recompute clinits each time, they can become empty.
     jprogram.typeOracle.recomputeAfterOptimizations();
@@ -911,6 +939,15 @@ protected static OptimizerStats optimizeLoop(String passName, JProgram jprogram,
     return stats;
   }
 
+  private static int getNodeCount(JProgram jProgram) {
+    Event countEvent = SpeedTracerLogger.start(CompilerEventType.OPTIMIZE, "phase", "countNodes");
+    TreeStatistics treeStats = new TreeStatistics();
+    treeStats.accept(jProgram);
+    int numNodes = treeStats.getNodeCount();
+    countEvent.end();
+    return numNodes;
+  }
+
   private static MultipleDependencyGraphRecorder chooseDependencyRecorder(boolean soycEnabled,
       OutputStream out) {
     MultipleDependencyGraphRecorder dependencyRecorder = CodeSplitter.NULL_RECORDER;