Scala example source code file (LazyVals.scala)

This example Scala source code file (LazyVals.scala) is included in my "Source Code Warehouse" project. The intent of this project is to help you more easily find Scala source code examples by using tags.

All credit for the original source code belongs to scala-lang.org; I'm just trying to make examples easier to find. (For my Scala work, see my Scala examples and tutorials.)

Scala tags/keywords

block, collection, compiler, defdef, if, int, list, nil, nsc, symbol, tree, valdef, zero

The LazyVals.scala Scala example source code

package scala.tools.nsc
package transform

import scala.collection.{ mutable, immutable }

abstract class LazyVals extends Transform with TypingTransformers with ast.TreeDSL {
  // inherits abstract value `global` and class `Phase` from Transform

  import global._                  // the global environment
  import definitions._             // standard classes and methods
  import typer.{typed, atOwner}    // methods to type trees
  import CODE._

  val phaseName: String = "lazyvals"
  private val FLAGS_PER_BYTE: Int = 8 // Byte
  private def bitmapKind = ByteClass

  def newTransformer(unit: CompilationUnit): Transformer =
    new LazyValues(unit)

  private def lazyUnit(sym: Symbol) = sym.tpe.resultType.typeSymbol == UnitClass

  object LocalLazyValFinder extends Traverser {
    var result: Boolean  = _

    def find(t: Tree) = {result = false; traverse(t); result}
    def find(ts: List[Tree]) = {result = false; traverseTrees(ts); result}

    override def traverse(t: Tree) {
      if (!result)
        t match {
          case v@ValDef(_, _, _, _) if v.symbol.isLazy =>
            result = true

          case d@DefDef(_, _, _, _, _, _) if d.symbol.isLazy && lazyUnit(d.symbol) =>
            d.symbol.resetFlag(symtab.Flags.LAZY)
            result = true

          case ClassDef(_, _, _, _) | DefDef(_, _, _, _, _, _) | ModuleDef(_, _, _) =>

          case LabelDef(name, _, _) if nme.isLoopHeaderLabel(name) =>

          case _ =>
            super.traverse(t)
        }
    }
  }

  /**
   * Transform local lazy accessors to check for the initialized bit.
   */
  class LazyValues(unit: CompilationUnit) extends TypingTransformer(unit) {
    /** map from method symbols to the number of lazy values it defines. */
    private val lazyVals = perRunCaches.newMap[Symbol, Int]() withDefaultValue 0

    import symtab.Flags._

    /** Perform the following transformations:
     *  - for a lazy accessor inside a method, make it check the initialization bitmap
     *  - for all methods, add enough int vars to allow one flag per lazy local value
     *  - blocks in template bodies behave almost like methods. A single bitmaps section is
     *      added in the first block, for all lazy values defined in such blocks.
     *  - remove ACCESSOR flags: accessors in traits are not statically implemented,
     *    but moved to the host class. local lazy values should be statically implemented.
     */
    override def transform(tree: Tree): Tree = {
      val sym = tree.symbol
      curTree = tree

      tree match {

        case Block(_, _) =>
          val block1 = super.transform(tree)
          val Block(stats, expr) = block1
          val stats1 = stats.flatMap(_ match {
            case Block(List(d1@DefDef(_, n1, _, _, _, _)), d2@DefDef(_, n2, _, _, _, _)) if (nme.newLazyValSlowComputeName(n2) == n1) =>
              List(d1, d2)
            case stat =>
              List(stat)
          })
          treeCopy.Block(block1, stats1, expr)

        case DefDef(_, _, _, _, _, rhs) => atOwner(tree.symbol) {
          val (res, slowPathDef) = if (!sym.owner.isClass && sym.isLazy) {
            val enclosingClassOrDummyOrMethod = {
              val enclMethod = sym.enclMethod

              if (enclMethod != NoSymbol ) {
                val enclClass = sym.enclClass
                if (enclClass != NoSymbol && enclMethod == enclClass.enclMethod)
                  enclClass
                else
                  enclMethod
              } else
                sym.owner
            }
            debuglog(s"determined enclosing class/dummy/method for lazy val as $enclosingClassOrDummyOrMethod given symbol $sym")
            val idx = lazyVals(enclosingClassOrDummyOrMethod)
            lazyVals(enclosingClassOrDummyOrMethod) = idx + 1
            val (rhs1, sDef) = mkLazyDef(enclosingClassOrDummyOrMethod, transform(rhs), idx, sym)
            sym.resetFlag((if (lazyUnit(sym)) 0 else LAZY) | ACCESSOR)
            (rhs1, sDef)
          } else
            (transform(rhs), EmptyTree)

          val ddef1 = deriveDefDef(tree)(_ => if (LocalLazyValFinder.find(res)) typed(addBitmapDefs(sym, res)) else res)
          if (slowPathDef != EmptyTree) Block(slowPathDef, ddef1) else ddef1
        }

        case Template(_, _, body) => atOwner(currentOwner) {
          val body1 = super.transformTrees(body)
          var added = false
          val stats =
            for (stat <- body1) yield stat match {
              case Block(_, _) | Apply(_, _) | If(_, _, _) | Try(_, _, _) if !added =>
                // Avoid adding bitmaps when they are fully overshadowed by those
                // that are added inside loops
                if (LocalLazyValFinder.find(stat)) {
                  added = true
                  typed(addBitmapDefs(sym, stat))
                } else stat
              case ValDef(_, _, _, _) =>
                typed(deriveValDef(stat)(addBitmapDefs(stat.symbol, _)))
              case _ =>
                stat
            }
          val innerClassBitmaps = if (!added && currentOwner.isClass && bitmaps.contains(currentOwner)) {
              // add bitmap to inner class if necessary
                val toAdd0 = bitmaps(currentOwner).map(s => typed(ValDef(s, ZERO)))
                toAdd0.foreach(t => {
                    if (currentOwner.info.decl(t.symbol.name) == NoSymbol) {
                      t.symbol.setFlag(PROTECTED)
                      currentOwner.info.decls.enter(t.symbol)
                    }
                })
                toAdd0
            } else List()
          deriveTemplate(tree)(_ => innerClassBitmaps ++ stats)
        }

        case ValDef(_, _, _, _) if !sym.owner.isModule && !sym.owner.isClass =>
          deriveValDef(tree) { rhs0 =>
            val rhs = transform(rhs0)
            if (LocalLazyValFinder.find(rhs)) typed(addBitmapDefs(sym, rhs)) else rhs
          }

        case l@LabelDef(name0, params0, ifp0@If(_, _, _)) if name0.startsWith(nme.WHILE_PREFIX) =>
          val ifp1 = super.transform(ifp0)
          val If(cond0, thenp0, elsep0) = ifp1

          if (LocalLazyValFinder.find(thenp0))
            deriveLabelDef(l)(_ => treeCopy.If(ifp1, cond0, typed(addBitmapDefs(sym.owner, thenp0)), elsep0))
          else
            l

        case l@LabelDef(name0, params0, block@Block(stats0, expr))
          if name0.startsWith(nme.WHILE_PREFIX) || name0.startsWith(nme.DO_WHILE_PREFIX) =>
          val stats1 = super.transformTrees(stats0)
          if (LocalLazyValFinder.find(stats1))
            deriveLabelDef(l)(_ => treeCopy.Block(block, typed(addBitmapDefs(sym.owner, stats1.head))::stats1.tail, expr))
          else
            l

        case _ => super.transform(tree)
      }
    }

    /** Add the bitmap definitions to the rhs of a method definition.
     *  If the rhs has been tail-call transformed, insert the bitmap
     *  definitions inside the top-level label definition, so that each
     *  iteration has the lazy values un-initialized. Otherwise add them
     *  at the very beginning of the method.
     */
    private def addBitmapDefs(methSym: Symbol, rhs: Tree): Tree = {
      def prependStats(stats: List[Tree], tree: Tree): Block = tree match {
        case Block(stats1, res) => Block(stats ::: stats1, res)
        case _ => Block(stats, tree)
      }

      val bmps = bitmaps(methSym) map (ValDef(_, ZERO))

      def isMatch(params: List[Ident]) = (params.tail corresponds methSym.tpe.params)(_.tpe == _.tpe)

      if (bmps.isEmpty) rhs else rhs match {
        case Block(assign, l @ LabelDef(name, params, _))
          if (name string_== "_" + methSym.name) && isMatch(params) =>
            Block(assign, deriveLabelDef(l)(rhs => typed(prependStats(bmps, rhs))))

        case _ => prependStats(bmps, rhs)
      }
    }

    def mkSlowPathDef(clazz: Symbol, lzyVal: Symbol, cond: Tree, syncBody: List[Tree],
                      stats: List[Tree], retVal: Tree): Tree = {
      val defSym = clazz.newMethod(nme.newLazyValSlowComputeName(lzyVal.name.toTermName), lzyVal.pos, STABLE | PRIVATE)
      defSym setInfo MethodType(List(), lzyVal.tpe.resultType)
      defSym.owner = lzyVal.owner
      debuglog(s"crete slow compute path $defSym with owner ${defSym.owner} for lazy val $lzyVal")
      if (bitmaps.contains(lzyVal))
        bitmaps(lzyVal).map(_.owner = defSym)
      val rhs: Tree = (gen.mkSynchronizedCheck(clazz, cond, syncBody, stats)).changeOwner(currentOwner -> defSym)

      DefDef(defSym, addBitmapDefs(lzyVal, BLOCK(rhs, retVal)))
    }


    def mkFastPathBody(clazz: Symbol, lzyVal: Symbol, cond: Tree, syncBody: List[Tree],
                       stats: List[Tree], retVal: Tree): (Tree, Tree) = {
      val slowPathDef: Tree = mkSlowPathDef(clazz, lzyVal, cond, syncBody, stats, retVal)
      (If(cond, Apply(Ident(slowPathDef.symbol), Nil), retVal), slowPathDef)
    }

    /** return a 'lazified' version of rhs. Rhs should conform to the
     *  following schema:
     *  {
     *    l$ = <rhs>
     *    l$
     *  } or
     *  <rhs> when the lazy value has type Unit (for which there is no field
     *  to cache it's value.
     *
     *  Similarly as for normal lazy val members (see Mixin), the result will be a tree of the form
     *  { if ((bitmap&n & MASK) == 0) this.l$compute()
     *    else l$
     *
     *    def l$compute() = { synchronized(enclosing_class_or_dummy) {
     *      if ((bitmap$n & MASK) == 0) {
     *       l$ = <rhs>
     *       bitmap$n = bimap$n | MASK
     *      }}
     *      l$
     *    }
     *  }
     *  where bitmap$n is a byte value acting as a bitmap of initialized values. It is
     *  the 'n' is (offset / 8), the MASK is (1 << (offset % 8)). If the value has type
     *  unit, no field is used to cache the value, so the l$compute will now look as following:
     *  {
     *    def l$compute() = { synchronized(enclosing_class_or_dummy) {
     *      if ((bitmap$n & MASK) == 0) {
     *       <rhs>;
     *       bitmap$n = bimap$n | MASK
     *      }}
     *    ()
     *    }
     *  }
     */
    private def mkLazyDef(methOrClass: Symbol, tree: Tree, offset: Int, lazyVal: Symbol): (Tree, Tree) = {
      val bitmapSym           = getBitmapFor(methOrClass, offset)
      val mask                = LIT(1 << (offset % FLAGS_PER_BYTE))
      val bitmapRef = if (methOrClass.isClass) Select(This(methOrClass), bitmapSym) else Ident(bitmapSym)

      def mkBlock(stmt: Tree) = BLOCK(stmt, mkSetFlag(bitmapSym, mask, bitmapRef), UNIT)

      debuglog(s"create complete lazy def in $methOrClass for $lazyVal")
      val (block, res) = tree match {
        case Block(List(assignment), res) if !lazyUnit(lazyVal) =>
          (mkBlock(assignment),  res)
        case rhs                          =>
          (mkBlock(rhs),         UNIT)
      }

      val cond = (bitmapRef GEN_& (mask, bitmapKind)) GEN_== (ZERO, bitmapKind)
      val lazyDefs = mkFastPathBody(methOrClass.enclClass, lazyVal, cond, List(block), Nil, res)
      (atPos(tree.pos)(localTyper.typed {lazyDefs._1 }), atPos(tree.pos)(localTyper.typed {lazyDefs._2 }))
    }

    private def mkSetFlag(bmp: Symbol, mask: Tree, bmpRef: Tree): Tree =
      bmpRef === (bmpRef GEN_| (mask, bitmapKind))

    val bitmaps = mutable.Map[Symbol, List[Symbol]]() withDefaultValue Nil

    /** Return the symbol corresponding of the right bitmap int inside meth,
     *  given offset.
     */
    private def getBitmapFor(meth: Symbol, offset: Int): Symbol = {
      val n = offset / FLAGS_PER_BYTE
      val bmps = bitmaps(meth)
      if (bmps.length > n)
        bmps(n)
      else {
        val sym = meth.newVariable(nme.newBitmapName(nme.BITMAP_NORMAL, n), meth.pos).setInfo(ByteTpe)
        enteringTyper {
          sym addAnnotation VolatileAttr
        }

        bitmaps(meth) = (sym :: bmps).reverse
        sym
      }
    }
  }
}