Rewrite closure invocations to the lambda body method

When an indylambda closure is allocated and invoked within the same
method, rewrite the invocation to the implementation method.

This works for any indylambda / SAM type, not only Scala functions.
However, the Scala compiler (under -Xexperimental) currently desugars
function literals for non-FunctionN types to an anonymous class during
typer.

No testing yet, waiting for FunctionN to become SAMs first.

The feature requires scala-java8-compat to be on the classpath and a
number of compiler flags:
-Ydelambdafy:method -Ybackend:GenBCode -Yopt:closure-elimination -target:jvm-1.8

➜  scala git:(opt/closureInlining) ant -Dscala-java8-compat.package=1 -Dlocker.skip=1
➜  scala git:(opt/closureInlining) cd sandbox

➜  sandbox git:(opt/closureInlining) cat Fun.java
public interface Fun<T> {
  T apply(T x);
}
➜  sandbox git:(opt/closureInlining) javac Fun.java

➜  sandbox git:(opt/closureInlining) cat Test.scala
class C {
  val z = "too"
  def f = {
    val kap = "me! me!"
    val f: Tuple2[String, String] => String = (o => z + kap + o.toString)
    f(("a", "b"))
  }

  def g = {
    val f: Int => String = x => x.toString
    f(10)
  }

  def h = {
    val f: Fun[Int] = x => x + 100 // Java SAM, requires -Xexperimental, will create an anonymous class in typer
    f(10)
  }

  def i = {
    val l = 10l
    val f: (Long, String) => String = (x, s) => s + l + z + x
    f(20l, "n")
  }

  def j = {
    val f: Int => Int = x => x + 101 // specialized
    f(33)
  }
}
➜  sandbox git:(opt/closureInlining) ../build/quick/bin/scalac -target:jvm-1.8 -Yopt:closure-elimination -Ydelambdafy:method -Ybackend:GenBCode -Xexperimental -cp ../build/quick/scala-java8-compat:. Test.scala

➜  sandbox git:(opt/closureInlining) asm -a C.class
➜  sandbox git:(opt/closureInlining) cat C.asm

[...]

  public g()Ljava/lang/String;
   L0
    INVOKEDYNAMIC apply()Lscala/compat/java8/JFunction1; [
      // handle kind 0x6 : INVOKESTATIC
      java/lang/invoke/LambdaMetafactory.altMetafactory(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite;
      // arguments:
      (Ljava/lang/Object;)Ljava/lang/Object;,
      // handle kind 0x6 : INVOKESTATIC
      C.C$$$anonfun$2$adapted(Ljava/lang/Object;)Ljava/lang/String;,
      (Ljava/lang/Object;)Ljava/lang/String;,
      3,
      1,
      Lscala/Serializable;.class,
      0
    ]
    CHECKCAST scala/Function1
   L1
    ASTORE 1
   L2
    ALOAD 1
    BIPUSH 10
    INVOKESTATIC scala/runtime/BoxesRunTime.boxToInteger (I)Ljava/lang/Integer;
    ASTORE 2
    POP
    ALOAD 2
    INVOKESTATIC C.C$$$anonfun$2$adapted (Ljava/lang/Object;)Ljava/lang/String;
    CHECKCAST java/lang/String
   L3
    ARETURN

[...]

1 files changed, 6 insertions, 1 deletions

diff --git a/src/compiler/scala/tools/nsc/settings/ScalaSettings.scala b/src/compiler/scala/tools/nsc/settings/ScalaSettings.scala
index d3cdf69d30..0cdece59e1 100644
--- a/src/compiler/scala/tools/nsc/settings/ScalaSettings.scala
+++ b/src/compiler/scala/tools/nsc/settings/ScalaSettings.scala
@@ -235,6 +235,7 @@ trait ScalaSettings extends AbsScalaSettings
     val emptyLabels             = Choice("empty-labels",              "Eliminate and collapse redundant labels in the bytecode.")
     val compactLocals           = Choice("compact-locals",            "Eliminate empty slots in the sequence of local variables.")
     val nullnessTracking        = Choice("nullness-tracking",         "Track nullness / non-nullness of local variables and apply optimizations.")
+    val closureElimination      = Choice("closure-elimination" ,      "Rewrite closure invocations to the implementation method and eliminate closures.")
     val inlineProject           = Choice("inline-project",            "Inline only methods defined in the files being compiled.")
     val inlineGlobal            = Choice("inline-global",             "Inline methods from any source, including classfiles on the compile classpath.")
 
@@ -243,7 +244,7 @@ trait ScalaSettings extends AbsScalaSettings
     private val defaultChoices = List(unreachableCode)
     val lDefault        = Choice("l:default",   "Enable default optimizations: "+ defaultChoices.mkString(","),                                    expandsTo = defaultChoices)
 
-    private val methodChoices = List(unreachableCode, simplifyJumps, emptyLineNumbers, emptyLabels, compactLocals, nullnessTracking)
+    private val methodChoices = List(unreachableCode, simplifyJumps, emptyLineNumbers, emptyLabels, compactLocals, nullnessTracking, closureElimination)
     val lMethod         = Choice("l:method",    "Enable intra-method optimizations: "+ methodChoices.mkString(","),                                expandsTo = methodChoices)
 
     private val projectChoices = List(lMethod, inlineProject)
@@ -266,11 +267,15 @@ trait ScalaSettings extends AbsScalaSettings
   def YoptEmptyLabels             = Yopt.contains(YoptChoices.emptyLabels)
   def YoptCompactLocals           = Yopt.contains(YoptChoices.compactLocals)
   def YoptNullnessTracking        = Yopt.contains(YoptChoices.nullnessTracking)
+  def YoptClosureElimination      = Yopt.contains(YoptChoices.closureElimination)
 
   def YoptInlineProject           = Yopt.contains(YoptChoices.inlineProject)
   def YoptInlineGlobal            = Yopt.contains(YoptChoices.inlineGlobal)
   def YoptInlinerEnabled          = YoptInlineProject || YoptInlineGlobal
 
+  def YoptBuildCallGraph          = YoptInlinerEnabled || YoptClosureElimination
+  def YoptAddToBytecodeRepository = YoptInlinerEnabled || YoptClosureElimination
+
   val YoptInlineHeuristics = ChoiceSetting(
     name = "-Yopt-inline-heuristics",
     helpArg = "strategy",