Merge pull request #6979 from dotty-staging/test-lazy-scheme

Test a new scheme to implement lazy vals

Author: odersky

tests/run/lazy-impl.check

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+initialize x
+result
+result
+initialize y
+result
+initialize x

tests/run/lazy-impl.scala

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+/** A demonstrator for a new algorithm to handle lazy vals. The idea is that
+ *  we use the field slot itself for all synchronization; there are no separate bitmaps
+ *  or locks. The type of a field is always Object. The field goes through the following
+ *  state changes:
+ *
+ *      null -> Evaluating -+--------------+-> Initialized
+ *                          |              |
+ *                          +--> Waiting --+
+ *
+ *  The states of a field are characterized as follows:
+ *
+ *      x == null         Nobody has evaluated the variable yet
+ *      x == Evaluating   A thread has started evaluating
+ *      x: Waiting        A thread has started evaluating and other threads are waiting
+ *                        for the result
+ *      otherwise         Variable is initialized
+ *
+ *
+ *  A lazy val `x: A = rhs` is compiled to the following code scheme:
+
+    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
+        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.
+ *
+ *  Cost analysis:
+ *
+ *    - 2 CAS on contention-free initialization
+ *    - 0 or 1 CAS on first read in thread other than initializer thread, depending on
+ *      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: 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: 86 bytes
+ *   current Dotty scheme: 125 bytes
+ *   Scala 2 scheme: 39 bytes
+ *
+ *  Advantages of the scheme:
+ *
+ *   - no slot other than the field itself is needed
+ *   - no locks are shared among lazy val initializations and between lazy val initializations
+ *     and normal code
+ *   - no deadlocks (other than those inherent in user code)
+ *   - synchronized code is executed only if there is contention
+ *   - simpler than current Dotty scheme
+ *
+ *  Disadvantages:
+ *
+ *   - lazy vals of primitive types are boxed
+ */
+import sun.misc.Unsafe._
+
+class C {
+  def init(name: String) = {
+    Thread.sleep(10)
+    println(s"initialize $name"); "result"
+  }
+
+  @volatile private[this] var _x: AnyRef = _
+
+  // Expansion of:  lazy val x: String = init("x")
+
+  def x: String = {
+    val current = _x
+    if (current.isInstanceOf[String])
+      current.asInstanceOf[String]
+    else
+      x$lzy
+  }
+
+  def x$lzy: String = {
+    val current = _x
+    if (current.isInstanceOf[String])
+      current.asInstanceOf[String]
+    else {
+      val offset = C.x_offset
+      if (current == null) {
+        if (LazyRuntime.isUnitialized(this, offset)) {
+          try LazyRuntime.initialize(this, offset, init("x"))
+          catch {
+            case ex: Throwable =>
+              LazyRuntime.initialize(this, offset, null)
+              throw ex
+          }
+        }
+      }
+      else
+        LazyRuntime.awaitInitialized(this, offset, current)
+      x$lzy
+    }
+  }
+
+  // Compare with bytecodes for regular lazy val:
+  lazy val y = init("y")
+}
+
+object C {
+  import LazyRuntime.fieldOffset
+  val x_offset = fieldOffset(classOf[C], "_x")
+}
+
+object LazyRuntime {
+  val Evaluating = new LazyControl()
+
+  private val unsafe: sun.misc.Unsafe = {
+    val f: java.lang.reflect.Field = classOf[sun.misc.Unsafe].getDeclaredField("theUnsafe");
+    f.setAccessible(true)
+    f.get(null).asInstanceOf[sun.misc.Unsafe]
+  }
+
+  def fieldOffset(cls: Class[_], name: String): Long = {
+    val fld = cls.getDeclaredField(name)
+    fld.setAccessible(true)
+    unsafe.objectFieldOffset(fld)
+  }
+
+  def isUnitialized(base: Object, offset: Long): Boolean =
+    unsafe.compareAndSwapObject(base, offset, null, Evaluating)
+
+  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.compareAndSwapObject(base, offset, lock, result)
+      lock.release()
+    }
+
+  def awaitInitialized(base: Object, offset: Long, current: Object): Unit =
+    if (current.isInstanceOf[Waiting])
+      current.asInstanceOf[Waiting].awaitRelease()
+    else
+      unsafe.compareAndSwapObject(base, offset, Evaluating, new Waiting)
+}
+
+class LazyControl
+
+class Waiting extends LazyControl {
+
+  private var done = false
+
+  def release(): Unit = synchronized {
+    done = true
+    notifyAll()
+  }
+
+  def awaitRelease(): Unit = synchronized {
+    while (!done) wait()
+  }
+}
+
+object Test {
+  def main(args: Array[String]) = {
+    val c = new C()
+    println(c.x)
+    println(c.x)
+    println(c.y)
+    multi()
+  }
+
+  def multi() = {
+    val rand = java.util.Random()
+    val c = new C()
+    val readers =
+      for i <- 0 until 1000 yield
+        new Thread {
+          override def run() = {
+            Thread.sleep(rand.nextInt(50))
+            assert(c.x == "result")
+          }
+        }
+    for (t <- readers) t.start()
+    for (t <- readers) t.join()
+  }
+}