WeightedBranchTargets.java

/*
 *  This file is part of the Jikes RVM project (http://jikesrvm.org).
 *
 *  This file is licensed to You under the Eclipse Public License (EPL);
 *  You may not use this file except in compliance with the License. You
 *  may obtain a copy of the License at
 *
 *      http://www.opensource.org/licenses/eclipse-1.0.php
 *
 *  See the COPYRIGHT.txt file distributed with this work for information
 *  regarding copyright ownership.
 */
package org.jikesrvm.compilers.opt.ir;

import org.jikesrvm.VM;
import org.jikesrvm.compilers.opt.OptimizingCompilerException;
import org.jikesrvm.compilers.opt.ir.operand.BranchProfileOperand;

/**
 * Used to iterate over the branch targets (including the fall through edge)
 * and associated probabilites of a basic block.
 * Takes into account the ordering of branch instructions when
 * computing the edge weights such that the total target weight will always
 * be equal to 1.0 (flow in == flow out).
 */
public final class WeightedBranchTargets {
  private BasicBlock[] targets;
  private float[] weights;
  private int cur;
  private int max;

  public void reset() { cur = 0; }

  public boolean hasMoreElements() { return cur < max; }

  public void advance() { cur++; }

  public BasicBlock curBlock() { return targets[cur]; }

  public float curWeight() { return weights[cur]; }

  public WeightedBranchTargets(BasicBlock bb) {
    targets = new BasicBlock[3];
    weights = new float[3];
    cur = 0;
    max = 0;

    float prob = 1f;
    for (InstructionEnumeration ie = bb.enumerateBranchInstructions(); ie.hasMoreElements();) {
      Instruction s = ie.next();
      if (IfCmp.conforms(s)) {
        BasicBlock target = IfCmp.getTarget(s).target.getBasicBlock();
        BranchProfileOperand prof = IfCmp.getBranchProfile(s);
        float taken = prob * prof.takenProbability;
        prob = prob * (1f - prof.takenProbability);
        addEdge(target, taken);
      } else if (Goto.conforms(s)) {
        BasicBlock target = Goto.getTarget(s).target.getBasicBlock();
        addEdge(target, prob);
      } else if (InlineGuard.conforms(s)) {
        BasicBlock target = InlineGuard.getTarget(s).target.getBasicBlock();
        BranchProfileOperand prof = InlineGuard.getBranchProfile(s);
        float taken = prob * prof.takenProbability;
        prob = prob * (1f - prof.takenProbability);
        addEdge(target, taken);
      } else if (IfCmp2.conforms(s)) {
        BasicBlock target = IfCmp2.getTarget1(s).target.getBasicBlock();
        BranchProfileOperand prof = IfCmp2.getBranchProfile1(s);
        float taken = prob * prof.takenProbability;
        prob = prob * (1f - prof.takenProbability);
        addEdge(target, taken);
        target = IfCmp2.getTarget2(s).target.getBasicBlock();
        prof = IfCmp2.getBranchProfile2(s);
        taken = prob * prof.takenProbability;
        prob = prob * (1f - prof.takenProbability);
        addEdge(target, taken);
      } else if (TableSwitch.conforms(s)) {
        int lowLimit = TableSwitch.getLow(s).value;
        int highLimit = TableSwitch.getHigh(s).value;
        int number = highLimit - lowLimit + 1;
        float total = 0f;
        for (int i = 0; i < number; i++) {
          BasicBlock target = TableSwitch.getTarget(s, i).target.getBasicBlock();
          BranchProfileOperand prof = TableSwitch.getBranchProfile(s, i);
          float taken = prob * prof.takenProbability;
          total += prof.takenProbability;
          addEdge(target, taken);
        }
        BasicBlock target = TableSwitch.getDefault(s).target.getBasicBlock();
        BranchProfileOperand prof = TableSwitch.getDefaultBranchProfile(s);
        float taken = prob * prof.takenProbability;
        total += prof.takenProbability;
        if (VM.VerifyAssertions && !epsilon(total, 1f)) {
          VM.sysFail("Total outflow (" + total + ") does not sum to 1 for: " + s);
        }
        addEdge(target, taken);
      } else if (LowTableSwitch.conforms(s)) {
        int number = LowTableSwitch.getNumberOfTargets(s);
        float total = 0f;
        for (int i = 0; i < number; i++) {
          BasicBlock target = LowTableSwitch.getTarget(s, i).target.getBasicBlock();
          BranchProfileOperand prof = LowTableSwitch.getBranchProfile(s, i);
          float taken = prob * prof.takenProbability;
          total += prof.takenProbability;
          addEdge(target, taken);
        }
        if (VM.VerifyAssertions && !epsilon(total, 1f)) {
          VM.sysFail("Total outflow (" + total + ") does not sum to 1 for: " + s);
        }
      } else if (LookupSwitch.conforms(s)) {
        int number = LookupSwitch.getNumberOfTargets(s);
        float total = 0f;
        for (int i = 0; i < number; i++) {
          BasicBlock target = LookupSwitch.getTarget(s, i).target.getBasicBlock();
          BranchProfileOperand prof = LookupSwitch.getBranchProfile(s, i);
          float taken = prob * prof.takenProbability;
          total += prof.takenProbability;
          addEdge(target, taken);
        }
        BasicBlock target = LookupSwitch.getDefault(s).target.getBasicBlock();
        BranchProfileOperand prof = LookupSwitch.getDefaultBranchProfile(s);
        float taken = prob * prof.takenProbability;
        total += prof.takenProbability;
        if (VM.VerifyAssertions && !epsilon(total, 1f)) {
          VM.sysFail("Total outflow (" + total + ") does not sum to 1 for: " + s);
        }
        addEdge(target, taken);
      } else {
        throw new OptimizingCompilerException("TODO " + s + "\n");
      }
    }
    BasicBlock ft = bb.getFallThroughBlock();
    if (ft != null) addEdge(ft, prob);
  }

  private void addEdge(BasicBlock target, float weight) {
    if (max == targets.length) {
      BasicBlock[] tmp = new BasicBlock[targets.length << 1];
      for (int i = 0; i < targets.length; i++) {
        tmp[i] = targets[i];
      }
      targets = tmp;
      float[] tmp2 = new float[weights.length << 1];
      for (int i = 0; i < weights.length; i++) {
        tmp2[i] = weights[i];
      }
      weights = tmp2;
    }
    targets[max] = target;
    weights[max] = weight;
    max++;
  }

  private boolean epsilon(float a, float b) {
    return Math.abs(a - b) < 0.1;
  }
}