Further clean-up of new lazy vals

Author: szymon-rd

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -2,33 +2,20 @@ package dotty.tools
 package dotc
 package core
 
-import scala.annotation.threadUnsafe as tu
-import Types.*
-import Contexts.*
-import Symbols.*
-import SymDenotations.*
-import StdNames.*
-import Names.*
-import Phases.*
-import Flags.*
-import Scopes.*
-import Decorators.*
-import NameOps.*
-import Periods.*
-import NullOpsDecorator.*
+import scala.annotation.{threadUnsafe => tu}
+import Types._, Contexts._, Symbols._, SymDenotations._, StdNames._, Names._, Phases._
+import Flags._, Scopes._, Decorators._, NameOps._, Periods._, NullOpsDecorator._
 import unpickleScala2.Scala2Unpickler.ensureConstructor
-
 import scala.collection.mutable
 import collection.mutable
 import Denotations.{SingleDenotation, staticRef}
-import util.{NoSource, SimpleIdentityMap, SourceFile}
+import util.{SimpleIdentityMap, SourceFile, NoSource}
 import typer.ImportInfo.RootRef
 import Comments.CommentsContext
 import Comments.Comment
 import util.Spans.NoSpan
 import Symbols.requiredModuleRef
-import cc.{CaptureSet, CapturingType, EventuallyCapturingType}
-import dotty.tools.dotc.transform.LazyVals.lazyNme
+import cc.{CapturingType, CaptureSet, EventuallyCapturingType}
 
 import scala.annotation.tailrec
 

compiler/src/dotty/tools/dotc/transform/LazyVals.scala

@@ -104,8 +104,8 @@ class LazyVals extends MiniPhase with IdentityDenotTransformer {
     }
   }
 
-  /** 
-   * Append offset fields to companion objects.
+
+  /** Append offset fields to companion objects
    */
   override def transformTemplate(template: Template)(using Context): Tree = {
     val cls = ctx.owner.asClass
@@ -122,9 +122,9 @@ class LazyVals extends MiniPhase with IdentityDenotTransformer {
     case _ => prefix ::: stats
   }
 
-  /** 
-   * Make an eager val that would implement synthetic module.
+  /** Make an eager val that would implement synthetic module.
    * Eager val ensures thread safety and has less code generated.
+   *
    */
   def transformSyntheticModule(tree: ValOrDefDef)(using Context): Thicket = {
     val sym = tree.symbol
@@ -135,8 +135,7 @@ class LazyVals extends MiniPhase with IdentityDenotTransformer {
     Thicket(field, getter)
   }
 
-  /** 
-   * Desugar a local `lazy val x: Int = <RHS>` into:
+  /** Desugar a local `lazy val x: Int = <RHS>` into:
    *
    *  ```
    *  val x$lzy = new scala.runtime.LazyInt()
@@ -276,75 +275,47 @@ class LazyVals extends MiniPhase with IdentityDenotTransformer {
   }
 
   /**
-   * Create a threadsafe lazy accessor equivalent to the following code:
+   * Create a threadsafe lazy accessor and function that computes the field's value. `Evaluating` and
+   * `NullValue` are represented by `object`s and `Waiting` by a class that allows awaiting the completion
+   * of the evaluation. Note that since tail-recursive functions are transformed *before* lazy-vals,
+   * this implementation does involve explicit while loop. `PatternMatcher` is coming before `LazyVals`,
+   * therefore the pattern matching is implemented using if-s.
+   *
    * ```
    * private @volatile var _x: AnyRef = null
-   * @tailrec def x: A =
-   *   _x match
-   *   case current: A =>
-   *     current
-   *   case NullValue => null
-   *   case null =>
-   *     if CAS(_x, null, Evaluating) then
-   *       var result: AnyRef = null // here, we need `AnyRef` to possibly assign `NullValue`
-   *       try
-   *         result = rhs
-   *         nullable = null // if the field is nullable; see `CollectNullableFields`
-   *       finally
-   *         if result == null then result = NullValue // drop if A is non-nullable
-   *         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 =>
-   *     current.awaitRelease()
-   *     x
-   * ```
-   * Where `Evaluating` and `NullValue` are represented by `object`s and `Waiting` by a class that
-   * allows awaiting the completion of the evaluation. Note that since tail-recursive
-   * functions are transformed *before* lazy-vals, this implementation directly implements
-   * the resulting loop. `PatternMatcher` coming before `LazyVals`, the pattern matching block
-   * is implemented using if-s. Additionally, the code can be optimized to be better inlined by
-   * introducing separate function for computing the lazy value and awaiting it. That is:
-   * 
-   * ```
-   * private var _x: AnyRef = null
    * 
    * def x: A =
-   *   if !(_x == null || _x.isInstanceOf[LazyValControlState]) then
-   *     return _x.asInstanceOf[A]
-   *   else if _x == NullValue then
-   *     return null
+   *   val result = _x
+   *   if result.isInstanceOf[A] then
+   *     result // possible unboxing applied here
+   *   else if result.eq(NullValue) then
+   *     null // possible unboxing applied here
    *   else
-   *     return x_compute()
+   *     return x_compute() // possible unboxing applied here
    * 
    * private def x_compute() =
    *   while <EmptyTree> do
    *     val current: AnyRef = _x
-   *     if current == null then
+   *     if current.eq(null) then
    *       if CAS(_x, null, Evaluating) then
    *         var resultNullable: AnyRef = null
    *         var result: AnyRef = null
    *         try
    *           resultNullable = rhs
-   *           nullable = null
-   *           if resultNullable == null then
+   *           nullable = null // nulls out the nullable fields used only in initialization
+   *           if resultNullable.eq(null) then
    *             result = NullValue
    *           else
    *             result = resultNullable
-   *           return result
    *         finally
    *            if !CAS(_x, Evaluating, result) then
    *              val lock = _x.asInstanceOf[Waiting]
    *              CAS(_x, lock, result)
    *              lock.release()
+   *         return resultNullable
    *     else
    *       if current.isInstanceOf[LazyValControlState] then
-   *         if current.isInstanceOf[Evaluating] then // To avoid creating Waiting instance
+   *         if current.eq(Evaluating) then // To avoid creating Waiting instance
    *           CAS(current, Evaluating, new Waiting)
    *         else if current.isInstanceOf[Waiting] then
    *           current.asInstanceOf[Waiting].await()
@@ -353,7 +324,7 @@ class LazyVals extends MiniPhase with IdentityDenotTransformer {
    *         return current.asInstanceOf[A]
    *   end while
    *  * ```
-   * 
+   *
    * @param memberDef     the transformed lazy field member definition
    * @param claz          the class containing this lazy val field
    * @param target        the target synthetic field
@@ -469,7 +440,7 @@ class LazyVals extends MiniPhase with IdentityDenotTransformer {
         Return(ref(current), lazyInitMethodSymbol)
       )
 
-    val initBody = Block(ValDef(current, ref(target)) :: Nil, If(ref(current).equal(nullLiteral), initialize, ifNotUninitialized).withType(defn.UnitType))
+    val initBody = Block(ValDef(current, ref(target)) :: Nil, If(ref(current).select(defn.Object_eq).appliedTo(nullLiteral), initialize, ifNotUninitialized).withType(defn.UnitType))
     val initMainLoop = WhileDo(EmptyTree, initBody) // becomes: while (true) do { body }
     val initMethodDef = DefDef(lazyInitMethodSymbol, initMainLoop)
     (accessorDef, initMethodDef)

compiler/test/dotc/pos-lazy-vals-tests.allowlist

@@ -34,4 +34,4 @@ t6278-synth-def.scala
 t6925b.scala
 t7011.scala
 t8306.scala
-zipped.scala
+zipped.scala

compiler/test/dotc/run-lazy-vals-tests.allowlist

@@ -63,4 +63,4 @@ t7406.scala
 t8245.scala
 unapply.scala
 unit-lazy-val.scala
-view-iterator-stream.scala
+view-iterator-stream.scala

library/src/scala/runtime/LazyVals.scala

@@ -169,6 +169,5 @@ object LazyVals {
     final val wait4Notification = "wait4Notification"
     final val get = "get"
     final val getOffset = "getOffset"
-    final val getOffsetStatic = "getOffsetStatic"
   }
 }

project/MiMaFilters.scala

@@ -3,20 +3,9 @@ import com.typesafe.tools.mima.core._
 
 object MiMaFilters {
   val Library: Seq[ProblemFilter] = Seq(
-    ProblemFilters.exclude[DirectMissingMethodProblem]("scala.runtime.LazyVals.getStaticOffset"),
-    ProblemFilters.exclude[DirectMissingMethodProblem]("scala.runtime.LazyVals#Names.getOffsetStatic"),
     ProblemFilters.exclude[DirectMissingMethodProblem]("scala.runtime.LazyVals.getStaticFieldOffset"),
-    ProblemFilters.exclude[DirectMissingMethodProblem]("scala.runtime.LazyVals#Names.getStaticFieldOffset"),
     ProblemFilters.exclude[DirectMissingMethodProblem]("scala.runtime.LazyVals.objCAS"),
-    ProblemFilters.exclude[DirectMissingMethodProblem]("scala.runtime.LazyVals#Names.evaluating"),
-    ProblemFilters.exclude[DirectMissingMethodProblem]("scala.runtime.LazyVals#Names.getStaticOffset"),
-    ProblemFilters.exclude[DirectMissingMethodProblem]("scala.runtime.LazyVals#Names.nullValue"),
-    ProblemFilters.exclude[DirectMissingMethodProblem]("scala.runtime.LazyVals#Names.objCas"),
-    ProblemFilters.exclude[DirectMissingMethodProblem]("scala.runtime.LazyVals#Names.waiting"),
-    ProblemFilters.exclude[DirectMissingMethodProblem]("scala.runtime.LazyVals#Names.waitingAwaitRelease"),
-    ProblemFilters.exclude[DirectMissingMethodProblem]("scala.runtime.LazyVals#Names.waitingRelease"),
     ProblemFilters.exclude[MissingClassProblem]("scala.runtime.LazyVals$LazyValControlState"),
-    ProblemFilters.exclude[DirectMissingMethodProblem]("scala.runtime.LazyVals#Names.controlState"),
     ProblemFilters.exclude[MissingClassProblem]("scala.runtime.LazyVals$Evaluating$"),
     ProblemFilters.exclude[MissingClassProblem]("scala.runtime.LazyVals$NullValue$"),
     ProblemFilters.exclude[MissingClassProblem]("scala.runtime.LazyVals$Waiting"),

tests/printing/transformed/lazy-vals-legacy.scala

@@ -1,3 +1,3 @@
 object A {
     lazy val x: Int = 2
-}
+}

tests/printing/transformed/lazy-vals-new.check

@@ -35,7 +35,7 @@ package <empty> {
       while <empty> do 
         {
           val current: Object = A#x$lzy1
-          if current.==(null) then 
+          if current.eq(null) then 
             if 
               scala.runtime.LazyVals.objCAS(classOf[A], A.OFFSET$_m_0, null, 
                 scala.runtime.LazyVals.Evaluating

tests/printing/transformed/lazy-vals-new.scala

@@ -1,3 +1,3 @@
 object A {
     lazy val x: Int = 2
-}
+}

tests/run/lazyVals_c3.1.0.scala

@@ -10,4 +10,4 @@ class Foo:
 @main def Test =
   val foo = new Foo
   println(foo.x)
-  println(foo.y)
+  println(foo.y)