SD-225 Use a "lzycompute" method for module initialization

The monitors and module instantation were inliuned into the module accessor
method in b2e0911. However, this seems to have had a detrimental impact on
performance. This might be because the module accessors are now above the
"always inline" HotSpot threshold of 35 bytes, or perhaps because they
contain monitor-entry/exit and exception handlers.

This commit returns to the the 2.11.8 appraoch of factoring
the the second check of the doublecheck locking into a method.
I've done this by declaring a nested method within the accessor;
this will be lifted out to the class level by lambdalift.

This represents a slight deviation from the implementation strategy used
for lazy accessors, which create a symbol for the slowpath method in
the info transform and generate the corresponding DefDef as a class
member. I don't believe this deviation is particular worrisome, though.

I have bootstrapped the compiler through this commit and found that
the drastic regression in compiling the shapeless test suite is solved.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/Fields.scala b/src/compiler/scala/tools/nsc/transform/Fields.scala
index a383b65192..45741eb391 100644
--- a/src/compiler/scala/tools/nsc/transform/Fields.scala
+++ b/src/compiler/scala/tools/nsc/transform/Fields.scala
@@ -22,7 +22,7 @@ import symtab.Flags._
   * in the first (closest in the subclassing lattice) subclass (not a trait) of a trait.
   *
   * For lazy vals and modules, we emit accessors that using double-checked locking (DCL) to balance thread safety
-  * and performance. A lazy val gets a compute method for the DCL's slow path, for a module it's all done in the accessor.
+  * and performance. For both lazy vals and modules, the a compute method contains the DCL's slow path.
   *
   * Local lazy vals do not receive bitmaps, but use a Lazy*Holder that has the volatile init bit and the computed value.
   * See `mkLazyLocalDef`.
@@ -236,7 +236,9 @@ abstract class Fields extends InfoTransform with ast.TreeDSL with TypingTransfor
       *
       * TODO: optimize using local variable?
       */
-    Block(If(needsInit, gen.mkSynchronized(monitorHolder)(If(needsInit, init, EmptyTree)), EmptyTree) :: Nil, moduleVarRef)
+    val computeName = nme.newLazyValSlowComputeName(module.name)
+    val computeMethod = DefDef(NoMods, computeName, Nil, ListOfNil, TypeTree(UnitTpe), gen.mkSynchronized(monitorHolder)(If(needsInit, init, EmptyTree)))
+    Block(computeMethod :: If(needsInit, Apply(Ident(computeName), Nil), EmptyTree) :: Nil, moduleVarRef)
   }
 
   // NoSymbol for lazy accessor sym with unit result type