Is this a bug in Scala 2.9.1 lazy implementation or just an artifact of decompilation

Question

I am considering using Scala on a pretty computationally intensive program. Profiling the C++ version of our code reveals that we could benefit significantly from Lazy evaluation. I have tried it out in Scala 2.9.1 and really like it. However, when I ran the class through a decompiler the implemenation didn't look quite right. I'm assuming that it's an artifact of the decompiler, but I wanted to get a more conclusive answer...

consider the following trivial example:

class TrivialAngle(radians : Double) 
{
    lazy val sin = math.sin(radians)
}

when I decompile it, I get this:

import scala.ScalaObject;
import scala.math.package.;
import scala.reflect.ScalaSignature;

@ScalaSignature(bytes="omitted")
public class TrivialAngle
  implements ScalaObject
{
  private final double radians;
  private double sin;
  public volatile int bitmap$0;

  public double sin()
  {
    if ((this.bitmap$0 & 0x1) == 0);
    synchronized (this)
    {
      if (
        (this.bitmap$0 & 0x1) == 0)
      {
        this.sin = package..MODULE$.sin(this.radians);
        this.bitmap$0 |= 1; 
      } 
      return this.sin;
    }
  }

  public TrivialAngle(double radians)
  {
  }
}

To me, the return block is in the wrong spot, and you will always acquire the lock. This can't be what the real code is doing, but I am unable to confirm this. Can anyone confirm or deny that I have a bogus decompilation, and that the lazy implementation is somewhat reasonable (ie, only locks when it is computing the value, and doesn't acquire the lock for subsequent calls?)

Thanks!

For reference, this is the decompiler I used: http://java.decompiler.free.fr/?q=jdgui

Answer 1

scala -Xprint:jvm reveals the true story:

[[syntax trees at end of jvm]]// Scala source: lazy.scala
package <empty> {
  class TrivialAngle extends java.lang.Object with ScalaObject {
    @volatile protected var bitmap$0: Int = 0;
    <paramaccessor> private[this] val radians: Double = _;
    lazy private[this] var sin: Double = _;
    <stable> <accessor> lazy def sin(): Double = {
      if (TrivialAngle.this.bitmap$0.&(1).==(0))
        {
          TrivialAngle.this.synchronized({
            if (TrivialAngle.this.bitmap$0.&(1).==(0))
              {
                TrivialAngle.this.sin = scala.math.`package`.sin(TrivialAngle.this.radians);
                TrivialAngle.this.bitmap$0 = TrivialAngle.this.bitmap$0.|(1);
                ()
              };
            scala.runtime.BoxedUnit.UNIT
          });
          ()
        };
      TrivialAngle.this.sin
    };
    def this(radians: Double): TrivialAngle = {
      TrivialAngle.this.radians = radians;
      TrivialAngle.super.this();
      ()
    }
  }
}

It's a (since JVM 1.5) safe, and very fast, double checked lock.

More details:

What's the (hidden) cost of Scala's lazy val?

Be aware that if you have multiple lazy val members in a class, only one of them can be initialized at once, as they are guarded by synchronized(this) { ... }.

Answer 2

What I get with javap -c does not correspond to your decompile. In particular, there is no monitor enter when the field is found to be initialized. Version 2.9.1 too. There is still the memory barrier implied by the volatile access of course, so it does not come completely free. Comments starting with /// are mine

public double sin();
  Code:
   0:   aload_0
   1:   getfield        #14; //Field bitmap$0:I
   4:   iconst_1
   5:   iand
   6:   iconst_0
   7:   if_icmpne       54 /// if getField & 1 == O goto 54, skip lock
   10:  aload_0
   11:  dup
   12:  astore_1
   13:  monitorenter
            /// 14 to 52 reasonably equivalent to synchronized block 
            /// in your decompiled code, without the return
   53:  monitorexit
   54:  aload_0
   55:  getfield        #27; //Field sin:D
   58:  dreturn        /// return outside lock
   59:  aload_1        /// (this would be the finally implied by the lock)
   60:  monitorexit
   61:  athrow
  Exception table:
   from   to  target type
    14    54    59   any