OPT_SplitBasicBlock.java

/*
 *  This file is part of the Jikes RVM project (http://jikesrvm.org).
 *
 *  This file is licensed to You under the Common Public License (CPL);
 *  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/cpl1.0.php
 *
 *  See the COPYRIGHT.txt file distributed with this work for information
 *  regarding copyright ownership.
 */
package org.jikesrvm.compilers.opt;

import org.jikesrvm.compilers.opt.ir.OPT_BasicBlock;
import org.jikesrvm.compilers.opt.ir.OPT_BasicBlockEnumeration;
import org.jikesrvm.compilers.opt.ir.OPT_IR;
import org.jikesrvm.compilers.opt.ir.OPT_Instruction;

/**
 * Splits a large basic block into smaller ones with size <= MAX_NUM_INSTRUCTIONS.
 */
public final class OPT_SplitBasicBlock extends OPT_CompilerPhase {

  private static final int MAX_NUM_INSTRUCTIONS = 300;

  public String getName() { return "SplitBasicBlock"; }

  public OPT_CompilerPhase newExecution(OPT_IR ir) { return this; }

  public void perform(OPT_IR ir) {
    for (OPT_BasicBlockEnumeration e = ir.getBasicBlocks(); e.hasMoreElements();) {
      OPT_BasicBlock bb = e.nextElement();

      if (!bb.isEmpty()) {
        while (bb != null) {
          bb = splitEachBlock(bb, ir);
        }
      }
    }
  }

  // Splits bb.
  // Returns null if no splitting is done.
  // returns the second block if splitting is done.
  OPT_BasicBlock splitEachBlock(OPT_BasicBlock bb, OPT_IR ir) {

    int instCount = MAX_NUM_INSTRUCTIONS;
    for (OPT_Instruction inst = bb.firstInstruction(); inst != bb.lastInstruction(); inst =
        inst.nextInstructionInCodeOrder()) {
      if ((--instCount) == 0) {
        if (inst.isBranch()) {
          return null; // no need to split because all the rests are just branches
        }
        if (inst.isMove()) {  // why do we need this?? --dave
          OPT_Instruction next = inst.nextInstructionInCodeOrder();
          if (next != bb.lastInstruction() && next.isImplicitLoad()) {
            inst = next;
          }
        }
        // Now, split!
        return bb.splitNodeWithLinksAt(inst, ir);
      }
    }

    return null; // no splitting happened
  }

}