Desugar module var and accessor in refchecks/lazyvals

Rather than leaving it until mixin.

The broader motivation is to simplify the mixin phase of the
compiler before we get rid of implementatation classes in
favour of using JDK8 default interface methods.

The current code in mixin is used for both lazy val and modules,
and puts the "slow path" code that uses the monitor into a
dedicated method (`moduleName$lzyCompute`). I tracked this
back to a3d4d17b77. I can't tell from that commit whether the
performance sensititivity was related to modules or lazy vals,
from the commit message I'd say the latter.

As the initialization code for a module is just a constructor call,
rather than an arbitraryly large chunk of code for a lazy initializer,
this commit opts to inline the `lzycompute` method.

During refchecks, mixin module accessors are added to classes, so
that mixed in and defined modules are translated uniformly. Trait
owned modules get an accessor method with an empty body (that shares
the module symbol), but no module var.

Defer synthesis of the double checked locking idiom to the lazyvals
phase, which gets us a step closer to a unified translation of
modules and lazy vals.

I had to change the `atOwner` methods to to avoid using the
non-existent module class of a module accessor method as the
current owner. This fixes a latent bug. Without this change,
retypechecking of the module accessor method during erasure crashes
with an accessibility error selecting the module var.

In the process, I've tweaked a tree generation utility method
to wvoid synthesizing redundant blocks in module desugaring.

1 files changed, 41 insertions, 11 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/LazyVals.scala b/src/compiler/scala/tools/nsc/transform/LazyVals.scala
index b6695efb0b..73c15c1f53 100644
--- a/src/compiler/scala/tools/nsc/transform/LazyVals.scala
+++ b/src/compiler/scala/tools/nsc/transform/LazyVals.scala
@@ -54,14 +54,35 @@ abstract class LazyVals extends Transform with TypingTransformers with ast.TreeD
     private val lazyVals = perRunCaches.newMap[Symbol, Int]() withDefaultValue 0
 
     import symtab.Flags._
+    private def flattenThickets(stats: List[Tree]): List[Tree] = stats.flatMap(_ match {
+      case b @ Block(List(d1@DefDef(_, n1, _, _, _, _)), d2@DefDef(_, n2, _, _, _, _)) if b.tpe == null && n1.endsWith(nme.LAZY_SLOW_SUFFIX) =>
+        List(d1, d2)
+      case stat =>
+        List(stat)
+    })
 
     /** Perform the following transformations:
      *  - for a lazy accessor inside a method, make it check the initialization bitmap
+     *  - implement double checked locking of member modules for non-trait owners (trait just have the abstract accessor)
+     *     ```
+     *     // typer
+     *     class C { object x }
+     *     // refchecks
+     *     class C { var x$module; def x() = { x$module = new x; x$module }
+     *     // lazyvals
+     *     class C {
+     *       var x$module                                                                               // module var
+     *       def x() = { if (x$module == null) x$lzycompute() else x$module                             // fast path
+     *       def x$lzycompute() = { synchronized { if (x$module == null) x$module = new x }; x$module } // slow path
+     *     }
+     *     ```
      *  - for all methods, add enough int vars to allow one flag per lazy local value
      *  - blocks in template bodies behave almost like methods. A single bitmaps section is
      *      added in the first block, for all lazy values defined in such blocks.
      *  - remove ACCESSOR flags: accessors in traits are not statically implemented,
      *    but moved to the host class. local lazy values should be statically implemented.
+     *
+     * The general pattern is
      */
     override def transform(tree: Tree): Tree = {
       val sym = tree.symbol
@@ -72,20 +93,14 @@ abstract class LazyVals extends Transform with TypingTransformers with ast.TreeD
         case Block(_, _) =>
           val block1 = super.transform(tree)
           val Block(stats, expr) = block1
-          val stats1 = stats.flatMap(_ match {
-            case Block(List(d1@DefDef(_, n1, _, _, _, _)), d2@DefDef(_, n2, _, _, _, _)) if (nme.newLazyValSlowComputeName(n2) == n1) =>
-              List(d1, d2)
-            case stat =>
-              List(stat)
-          })
-          treeCopy.Block(block1, stats1, expr)
+          treeCopy.Block(block1, flattenThickets(stats), expr)
 
         case DefDef(_, _, _, _, _, rhs) => atOwner(tree.symbol) {
           val (res, slowPathDef) = if (!sym.owner.isClass && sym.isLazy) {
             val enclosingClassOrDummyOrMethod = {
               val enclMethod = sym.enclMethod
 
-              if (enclMethod != NoSymbol ) {
+              if (enclMethod != NoSymbol) {
                 val enclClass = sym.enclClass
                 if (enclClass != NoSymbol && enclMethod == enclClass.enclMethod)
                   enclClass
@@ -100,11 +115,26 @@ abstract class LazyVals extends Transform with TypingTransformers with ast.TreeD
             val (rhs1, sDef) = mkLazyDef(enclosingClassOrDummyOrMethod, transform(rhs), idx, sym)
             sym.resetFlag((if (lazyUnit(sym)) 0 else LAZY) | ACCESSOR)
             (rhs1, sDef)
-          } else
+          } else if (sym.hasAllFlags(MODULE | METHOD) && !sym.owner.isTrait) {
+            rhs match {
+              case b @ Block((assign @ Assign(moduleRef, _)) :: Nil, expr) =>
+                val cond = Apply(Select(moduleRef, Object_eq), List(Literal(Constant(null))))
+                val (fastPath, slowPath) = mkFastPathBody(sym.owner.enclClass, moduleRef.symbol, cond, transform(assign) :: Nil, Nil, transform(expr))
+                (localTyper.typedPos(tree.pos)(fastPath), localTyper.typedPos(tree.pos)(slowPath))
+              case rhs =>
+                global.reporter.error(tree.pos, "Unexpected tree on the RHS of a module accessor: " + rhs)
+                (rhs, EmptyTree)
+            }
+          } else {
             (transform(rhs), EmptyTree)
+          }
 
           val ddef1 = deriveDefDef(tree)(_ => if (LocalLazyValFinder.find(res)) typed(addBitmapDefs(sym, res)) else res)
-          if (slowPathDef != EmptyTree) Block(slowPathDef, ddef1) else ddef1
+          if (slowPathDef != EmptyTree) {
+            // The contents of this block are flattened into the enclosing statement sequence, see flattenThickets
+            // This is a poor man's version of dotty's Thicket: https://github.com/lampepfl/dotty/blob/d5280358d1/src/dotty/tools/dotc/ast/Trees.scala#L707
+            Block(slowPathDef, ddef1)
+          } else ddef1
         }
 
         case Template(_, _, body) => atOwner(currentOwner) {
@@ -135,7 +165,7 @@ abstract class LazyVals extends Transform with TypingTransformers with ast.TreeD
                 })
                 toAdd0
             } else List()
-          deriveTemplate(tree)(_ => innerClassBitmaps ++ stats)
+          deriveTemplate(tree)(_ => innerClassBitmaps ++ flattenThickets(stats))
         }
 
         case ValDef(_, _, _, _) if !sym.owner.isModule && !sym.owner.isClass =>