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, 3 insertions, 2 deletions

diff --git a/test/files/run/t6028.check b/test/files/run/t6028.check
index d6cc452bbf..80f8698ecf 100644
--- a/test/files/run/t6028.check
+++ b/test/files/run/t6028.check
@@ -54,10 +54,11 @@ package <empty> {
       <synthetic> <stable> <artifact> def $outer(): T = MethodLocalObject$2.this.$outer;
       <synthetic> <stable> <artifact> def $outer(): T = MethodLocalObject$2.this.$outer
     };
+    final private[this] def MethodLocalObject$lzycompute$1(barParam$1: Int, MethodLocalObject$module$1: runtime.VolatileObjectRef): Unit = T.this.synchronized[Unit](if (MethodLocalObject$module$1.elem.$asInstanceOf[T#MethodLocalObject$2.type]().eq(null))
+      MethodLocalObject$module$1.elem = new T#MethodLocalObject$2.type(T.this, barParam$1));
     final <stable> private[this] def MethodLocalObject$1(barParam$1: Int, MethodLocalObject$module$1: runtime.VolatileObjectRef): T#MethodLocalObject$2.type = {
       if (MethodLocalObject$module$1.elem.$asInstanceOf[T#MethodLocalObject$2.type]().eq(null))
-        T.this.synchronized[Unit](if (MethodLocalObject$module$1.elem.$asInstanceOf[T#MethodLocalObject$2.type]().eq(null))
-          MethodLocalObject$module$1.elem = new T#MethodLocalObject$2.type(T.this, barParam$1));
+        T.this.MethodLocalObject$lzycompute$1(barParam$1, MethodLocalObject$module$1);
       MethodLocalObject$module$1.elem.$asInstanceOf[T#MethodLocalObject$2.type]()
     };
     @SerialVersionUID(value = 0) final <synthetic> class $anonfun$tryy$1 extends scala.runtime.AbstractFunction0$mcV$sp with Serializable {