Scala example source code file (DeadCodeElimination.scala)

This example Scala source code file (DeadCodeElimination.scala) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

Java - Scala tags/keywords

basicblock, basicblock, call_method, deadcode, imethod, imethod, int, int, list, list, load_exception, load_local, new, pair

The Scala DeadCodeElimination.scala source code

/* NSC -- new scala compiler
 * Copyright 2005-2011 LAMP/EPFL
 * @author  Iulian Dragos
 */


package scala.tools.nsc
package backend.opt

import scala.collection.{ mutable, immutable }
import symtab._

/**
 */
abstract class DeadCodeElimination extends SubComponent {
  import global._
  import icodes._
  import icodes.opcodes._

  val phaseName = "dce"

  /** Create a new phase */
  override def newPhase(p: Phase) = new DeadCodeEliminationPhase(p)

  /** Dead code elimination phase.
   */
  class DeadCodeEliminationPhase(prev: Phase) extends ICodePhase(prev) {

    def name = phaseName
    val dce = new DeadCode()

    override def apply(c: IClass) {
      if (settings.Xdce.value)
        dce.analyzeClass(c)
    }
  }

  /** closures that are instantiated at least once, after dead code elimination */
  val liveClosures: mutable.Set[Symbol] = new mutable.HashSet()

  /** Remove dead code.
   */
  class DeadCode {
    
    def analyzeClass(cls: IClass) {
      cls.methods.foreach { m =>
        this.method = m
        dieCodeDie(m)
        global.closureElimination.peephole(m)
      }
    }

    val rdef = new reachingDefinitions.ReachingDefinitionsAnalysis;

    /** Use-def chain: give the reaching definitions at the beginning of given instruction. */
    var defs: immutable.Map[(BasicBlock, Int), immutable.Set[rdef.lattice.Definition]] = immutable.HashMap.empty
    
    /** Useful instructions which have not been scanned yet. */
    val worklist: mutable.Set[(BasicBlock, Int)] = new mutable.LinkedHashSet
    
    /** what instructions have been marked as useful? */
    val useful: mutable.Map[BasicBlock, mutable.BitSet] = new mutable.HashMap
    
    /** what local variables have been accessed at least once? */
    var accessedLocals: List[Local] = Nil
    
    /** the current method. */
    var method: IMethod = _
    
    /** Map instructions who have a drop on some control path, to that DROP instruction. */
    val dropOf: mutable.Map[(BasicBlock, Int), (BasicBlock, Int)] = new mutable.HashMap()
    
    def dieCodeDie(m: IMethod) {
      if (m.code ne null) {
        log("dead code elimination on " + m);
        dropOf.clear
        m.code.blocks.clear
        accessedLocals = m.params.reverse
        m.code.blocks ++= linearizer.linearize(m)
        collectRDef(m)
        mark
        sweep(m)
        accessedLocals = accessedLocals.distinct
        if (m.locals diff accessedLocals nonEmpty) {
          log("Removed dead locals: " + (m.locals diff accessedLocals))
          m.locals = accessedLocals.reverse
        }
      }
    }

    /** collect reaching definitions and initial useful instructions for this method. */
    def collectRDef(m: IMethod): Unit = if (m.code ne null) {
      defs = immutable.HashMap.empty; worklist.clear; useful.clear;
      rdef.init(m);
      rdef.run;
     
      for (bb <- m.code.blocks.toList) {
        useful(bb) = new mutable.BitSet(bb.size)
        var rd = rdef.in(bb);
        for (Pair(i, idx) <- bb.toList.zipWithIndex) {
          i match {
            case LOAD_LOCAL(l) => 
              defs = defs + Pair(((bb, idx)), rd.vars)
//              Console.println(i + ": " + (bb, idx) + " rd: " + rd + " and having: " + defs)
            case RETURN(_) | JUMP(_) | CJUMP(_, _, _, _) | CZJUMP(_, _, _, _) | STORE_FIELD(_, _) |
                 THROW(_)   | STORE_ARRAY_ITEM(_) | SCOPE_ENTER(_) | SCOPE_EXIT(_) | STORE_THIS(_) |
                 LOAD_EXCEPTION(_) | SWITCH(_, _) | MONITOR_ENTER() | MONITOR_EXIT() => worklist += ((bb, idx))
            case CALL_METHOD(m1, _) if isSideEffecting(m1) => worklist += ((bb, idx)); log("marking " + m1)
            case CALL_METHOD(m1, SuperCall(_)) => 
              worklist += ((bb, idx)) // super calls to constructor
            case DROP(_) =>
              val necessary = rdef.findDefs(bb, idx, 1) exists { p =>
                val (bb1, idx1) = p
                bb1(idx1) match {
                  case CALL_METHOD(m1, _) if isSideEffecting(m1) => true
                  case LOAD_EXCEPTION(_) | DUP(_) | LOAD_MODULE(_) => true
                  case _ => 
                    dropOf((bb1, idx1)) = (bb, idx)
//                    println("DROP is innessential: " + i + " because of: " + bb1(idx1) + " at " + bb1 + ":" + idx1) 
                    false
                }
              }
              if (necessary) worklist += ((bb, idx))
            case _ => ()
          }
          rd = rdef.interpret(bb, idx, rd)
        }
      }
    }

    /** Mark useful instructions. Instructions in the worklist are each inspected and their
     *  dependencies are marked useful too, and added to the worklist.
     */
    def mark() {
//      log("Starting with worklist: " + worklist)
      while (!worklist.isEmpty) {
        val (bb, idx) = worklist.iterator.next
        worklist -= ((bb, idx))
        if (settings.debug.value)
          log("Marking instr: \tBB_" + bb + ": " + idx + " " + bb(idx))
          
        val instr = bb(idx)
        if (!useful(bb)(idx)) {
          useful(bb) += idx
          dropOf.get(bb, idx) match {
            case Some((bb1, idx1)) => useful(bb1) += idx1
            case None => ()
          }
          instr match {
            case LOAD_LOCAL(l1) =>
              for ((l2, bb1, idx1) <- defs((bb, idx)) if l1 == l2; if !useful(bb1)(idx1)) {
                log("\tAdding " + bb1(idx1))
                worklist += ((bb1, idx1))
              }
                
            case nw @ NEW(REFERENCE(sym)) =>
              assert(nw.init ne null, "null new.init at: " + bb + ": " + idx + "(" + instr + ")")
              worklist += findInstruction(bb, nw.init)
              if (inliner.isClosureClass(sym)) {
                liveClosures += sym
              }

            // it may be better to move static initializers from closures to
            // the enclosing class, to allow the optimizer to remove more closures.
            // right now, the only static fields in closures are created when caching
            // 'symbol literals.
            case LOAD_FIELD(sym, true) if inliner.isClosureClass(sym.owner) =>
              log("added closure class for field " + sym)
              liveClosures += sym.owner

            case LOAD_EXCEPTION(_) =>
              ()
              
            case _ =>
              for ((bb1, idx1) <- rdef.findDefs(bb, idx, instr.consumed) if !useful(bb1)(idx1)) {
                log("\tAdding " + bb1(idx1))
                worklist += ((bb1, idx1))
              }
          }
        }
      }
    }
    
    def sweep(m: IMethod) {
      val compensations = computeCompensations(m)
      
      for (bb <- m.code.blocks.toList) {
//        Console.println("** Sweeping block " + bb + " **")
        val oldInstr = bb.toList
        bb.open
        bb.clear
        for (Pair(i, idx) <- oldInstr.zipWithIndex) {
          if (useful(bb)(idx)) {
//            log(" " + i + " is useful")
            bb.emit(i, i.pos)
            compensations.get(bb, idx) match {
              case Some(is) => is foreach bb.emit
              case None => ()
            }
            // check for accessed locals
            i match {
              case LOAD_LOCAL(l) if !l.arg => 
                accessedLocals = l :: accessedLocals
              case STORE_LOCAL(l) if !l.arg =>
                accessedLocals = l :: accessedLocals
              case _ => ()
            }
          } else {
            i match {
              case NEW(REFERENCE(sym)) => 
                log("skipped object creation: " + sym + "inside " + m)
              case _ => ()
            }
            if (settings.debug.value) log("Skipped: bb_" + bb + ": " + idx + "( " + i + ")")
          }
        }

        if (bb.nonEmpty) bb.close
        else log("empty block encountered")
      }
    }
    
    private def computeCompensations(m: IMethod): mutable.Map[(BasicBlock, Int), List[Instruction]] = {
      val compensations: mutable.Map[(BasicBlock, Int), List[Instruction]] = new mutable.HashMap
      
      for (bb <- m.code.blocks) {
        assert(bb.closed, "Open block in computeCompensations")
        for ((i, idx) <- bb.toList.zipWithIndex) {
          if (!useful(bb)(idx)) {
            for ((consumedType, depth) <- i.consumedTypes.reverse.zipWithIndex) {
              log("Finding definitions of: " + i + "\n\t" + consumedType + " at depth: " + depth)
              val defs = rdef.findDefs(bb, idx, 1, depth)
              for (d <- defs) {
                val (bb, idx) = d
                bb(idx) match {
                  case DUP(_) if idx > 0 =>
                    bb(idx - 1) match {
                      case nw @ NEW(_) =>
                        val init = findInstruction(bb, nw.init)
                        log("Moving DROP to after <init> call: " + nw.init)
                        compensations(init) = List(DROP(consumedType))
                      case _ =>
                        compensations(d) = List(DROP(consumedType))
                    }
                  case _ =>
                    compensations(d) = List(DROP(consumedType))
                }
              }
            }
          }
        }
      }
      compensations
    }
    
    private def withClosed[a](bb: BasicBlock)(f: => a): a = {
      if (bb.nonEmpty) bb.close
      val res = f
      if (bb.nonEmpty) bb.open
      res
    }
    
    private def findInstruction(bb: BasicBlock, i: Instruction): (BasicBlock, Int) = {      
      for (b <- linearizer.linearizeAt(method, bb)) {
        val idx = b.toList indexWhere (_ eq i)
        if (idx != -1)
          return (b, idx)
      }
      abort("could not find init in: " + method)
    }

    /** Is 'sym' a side-effecting method? TODO: proper analysis.  */
    private def isSideEffecting(sym: Symbol): Boolean = {
      !((sym.isGetter && sym.isFinal && !sym.isLazy)
       || (sym.isConstructor 
           && !(sym.owner == method.symbol.owner && method.symbol.isConstructor) // a call to another constructor  
           && sym.owner.owner == definitions.RuntimePackage.moduleClass)
       || (sym.isConstructor && inliner.isClosureClass(sym.owner))
/*       || definitions.isBox(sym)
       || definitions.isUnbox(sym)*/)
    }
  } /* DeadCode */
}