Revised implementation

Author: odersky

tests/run/lazy-impl.scala

@@ -15,55 +15,47 @@
  *                        for the result
  *      otherwise         Variable is initialized
  *
- *  Note 1: This assumes that fields cannot have `null` as normal value. Once we have
- *  nullability checking, this should be the standard case. We can still accommodate
- *  fields that can be null by representing `null` with a special value (say `NULL`)
- *  and storing `NULL` instead of `null` in the field. The necessary tweaks are added
- *  as comment lines to the code below.
  *
  *  A lazy val `x: A = rhs` is compiled to the following code scheme:
- *
- *  private var _x: AnyRef = null
- *  def x: A =
- *      _x match
- *      case current: A =>
- *          current
- *      case null =>
- *          if CAS(_x, null, Evaluating) then
- *              var result = rhs
- *  //          if result == null then result == NULL
- *              if !CAS(_x, Evaluating, result) then
- *                  val lock = _x.asInstanceOf[Waiting]
- *                  _x = result
- *                  lock.release(result)
- *          x
- *      case Evaluating =>
- *          CAS(x, Evaluating, new Waiting)
- *          x
- *      case current: Waiting =>
- *          _x = current.awaitRelease()
- *          x
- *  //  case NULL =>
- *  //      null
- *
+
+    private @volatile var _x: AnyRef = null
+
+    @tailrec def x: A =
+        _x match
+        case current: A =>
+            current
+        case null =>
+            if CAS(_x, null, Evaluating) then
+                var result: A = null
+                try
+                    result = rhs
+                    if result == null then result = NULL // drop if A is non-nullable
+                finally
+                    if !CAS(_x, Evaluating, result) then
+                        val lock = _x.asInstanceOf[Waiting]
+                        CAS(_x, lock, result)
+                        lock.release()
+            x
+        case Evaluating =>
+            CAS(x, Evaluating, new Waiting)
+            x
+        case current: Waiting =>
+            _x = current.awaitRelease()
+            x
+        case NULL => null                                // drop if A is non-nullable
 
  *  The code makes use of the following runtime class:
- *
- *  class Waiting:
- *
- *      private var done = false
- *      private var result: AnyRef = _
- *
- *      def release(result: AnyRef): Unit = synchronized:
- *          this.result = result
- *          done = true
- *          notifyAll()
- *
- *      def awaitRelease(): AnyRef = synchronized:
- *          while !done do wait()
- *          result
- *
- *  Note 2: The code assumes that the getter result type `A` is disjoint from the type
+
+    class Waiting:
+        private var done = false
+        def release(): Unit = synchronized:
+            done = true
+            notifyAll()
+
+        def awaitRelease(): Unit = synchronized:
+            while !done do wait()
+
+ *  Note: The code assumes that the getter result type `A` is disjoint from the type
  *  of `Evaluating` and the `Waiting` class. If this is not the case (e.g. `A` is AnyRef),
  *  then the conditions in the match have to be re-ordered so that case `_x: A` becomes
  *  the final default case.
@@ -75,14 +67,14 @@
  *      whether cache has updated
  *    - no synchronization operations on reads after the first one
  *    - If there is contention, we see in addition
- *       - for the initializing thread: a synchronized notifyAll
+ *       - for the initializing thread: another CAS and a synchronized notifyAll
  *       - for a reading thread: 0 or 1 CAS and a synchronized wait
  *
  *  Code sizes for getter:
  *
- *   this scheme, if nulls are excluded in type: 72 bytes
- *   current Dotty scheme: 131 bytes
- *   Scala 2 scheme: 39 bytes + 1 exception handler
+ *   this scheme, if nulls are excluded in type: 86 bytes
+ *   current Dotty scheme: 125 bytes
+ *   Scala 2 scheme: 39 bytes
  *
  *  Advantages of the scheme:
  *
@@ -95,7 +87,6 @@
  *
  *  Disadvantages:
  *
- *   - does not work for local lazy vals (but maybe these could be unsynchronized anyway?)
  *   - lazy vals of primitive types are boxed
  */
 import sun.misc.Unsafe._
@@ -106,31 +97,37 @@ class C {
     println(s"initialize $name"); "result"
   }
 
-  private[this] var _x: AnyRef = null
+  @volatile private[this] var _x: AnyRef = _
 
-  // Expansion of:  lazy val x: String = init
+  // Expansion of:  lazy val x: String = init("x")
 
   def x: String = {
     val current = _x
     if (current.isInstanceOf[String])
       current.asInstanceOf[String]
     else
-      x$lzy_compute
+      x$lzy
   }
 
-  def x$lzy_compute: String = {
+  def x$lzy: String = {
     val current = _x
     if (current.isInstanceOf[String])
       current.asInstanceOf[String]
     else {
       val offset = C.x_offset
       if (current == null) {
-        if (LazyRuntime.isUnitialized(this, offset))
-          LazyRuntime.initialize(this, offset, init("x"))
+        if (LazyRuntime.isUnitialized(this, offset)) {
+          try LazyRuntime.initialize(this, offset, init("x"))
+          catch {
+            case ex: Throwable =>
+              LazyRuntime.initialize(this, offset, null)
+              throw ex
+          }
+        }
       }
       else
         LazyRuntime.awaitInitialized(this, offset, current)
-      x$lzy_compute
+      x$lzy
     }
   }
 
@@ -164,13 +161,13 @@ object LazyRuntime {
   def initialize(base: Object, offset: Long, result: Object): Unit =
     if (!unsafe.compareAndSwapObject(base, offset, Evaluating, result)) {
       val lock = unsafe.getObject(base, offset).asInstanceOf[Waiting]
-      unsafe.putObject(base, offset, result)
-      lock.release(result)
+      unsafe.compareAndSwapObject(base, offset, lock, result)
+      lock.release()
     }
 
   def awaitInitialized(base: Object, offset: Long, current: Object): Unit =
     if (current.isInstanceOf[Waiting])
-      unsafe.putObject(base, offset, current.asInstanceOf[Waiting].awaitRelease())
+      current.asInstanceOf[Waiting].awaitRelease()
     else
       unsafe.compareAndSwapObject(base, offset, Evaluating, new Waiting)
 }
@@ -180,17 +177,14 @@ class LazyControl
 class Waiting extends LazyControl {
 
   private var done = false
-  private var result: AnyRef = _
 
-  def release(result: AnyRef) = synchronized {
-    this.result = result
+  def release(): Unit = synchronized {
     done = true
     notifyAll()
   }
 
-  def awaitRelease(): AnyRef = synchronized {
+  def awaitRelease(): Unit = synchronized {
     while (!done) wait()
-    result
   }
 }