[indylambda] Enable caching for lambda deserialization

We add a static field to each class that defines lambdas that
will hold a `ju.Map[String, MethodHandle]` to cache references to
the constructors of the classes originally created by
`LambdaMetafactory`.

The cache is initially null, and created on the first deserialization.

In case of a race between two threads deserializing the first
lambda hosted by a class, the last one to finish will clobber
the one-element cache of the first.

This lack of strong guarantees mirrors the current policy in
`LambdaDeserializer`.

We should consider whether to strengthen the combinaed guarantee here.
A useful benchmark would be those of the invokedynamic instruction,
which allows multiple threads to call the boostrap method in parallel,
but guarantees that if that happens, the results of all but one will
be discarded:

> If several threads simultaneously execute the bootstrap method for
> the same dynamic call site, the Java Virtual Machine must choose
> one returned call site object and install it visibly to all threads.

We could meet this guarantee easily, albeit excessively, by
synchronizing `$deserializeLambda$`. But a more fine grained
approach is possible and desirable.

A test is included that shows we are able to garbage collect
classloaders of classes that have hosted lambda deserialization.

3 files changed, 38 insertions, 8 deletions

diff --git a/src/compiler/scala/tools/nsc/backend/jvm/BCodeHelpers.scala b/src/compiler/scala/tools/nsc/backend/jvm/BCodeHelpers.scala
index 783c89584e..6aa3a62295 100644
--- a/src/compiler/scala/tools/nsc/backend/jvm/BCodeHelpers.scala
+++ b/src/compiler/scala/tools/nsc/backend/jvm/BCodeHelpers.scala
@@ -685,24 +685,50 @@ abstract class BCodeHelpers extends BCodeIdiomatic with BytecodeWriters {
 
     /**
      * Add:
-     *
+     * private static java.util.Map $deserializeLambdaCache$ = null
      * private static Object $deserializeLambda$(SerializedLambda l) {
-     *   return scala.compat.java8.runtime.LambdaDeserializer.deserializeLambda(MethodHandles.lookup(), null, l);
+     *   var cache = $deserializeLambdaCache$
+     *   if (cache eq null) {
+     *     cache = new java.util.HashMap()
+     *     $deserializeLambdaCache$ = cache
+     *   }
+     *   return scala.compat.java8.runtime.LambdaDeserializer.deserializeLambda(MethodHandles.lookup(), cache, l);
      * }
-     * @param jclass
      */
-    // TODO add a static cache field to the class, and pass that as the second argument to `deserializeLambda`.
-    // This will make the test at run/lambda-serialization.scala:15 work
-    def addLambdaDeserialize(jclass: asm.ClassVisitor): Unit = {
+    def addLambdaDeserialize(clazz: Symbol, jclass: asm.ClassVisitor): Unit = {
       val cw = jclass
       import scala.tools.asm.Opcodes._
+
+      // Need to force creation of BTypes for these as `getCommonSuperClass` is called on
+      // automatically computing the max stack size (`visitMaxs`) during method writing.
+      javaUtilHashMapReference
+      javaUtilMapReference
+
       cw.visitInnerClass("java/lang/invoke/MethodHandles$Lookup", "java/lang/invoke/MethodHandles", "Lookup", ACC_PUBLIC + ACC_FINAL + ACC_STATIC)
 
       {
+        val fv = cw.visitField(ACC_PRIVATE + ACC_STATIC + ACC_SYNTHETIC, "$deserializeLambdaCache$", "Ljava/util/Map;", null, null)
+        fv.visitEnd()
+      }
+
+      {
         val mv = cw.visitMethod(ACC_PRIVATE + ACC_STATIC + ACC_SYNTHETIC, "$deserializeLambda$", "(Ljava/lang/invoke/SerializedLambda;)Ljava/lang/Object;", null, null)
         mv.visitCode()
+        mv.visitFieldInsn(GETSTATIC, clazz.javaBinaryName.encoded, "$deserializeLambdaCache$", "Ljava/util/Map;")
+        mv.visitVarInsn(ASTORE, 1)
+        mv.visitVarInsn(ALOAD, 1)
+        val l0 = new asm.Label()
+        mv.visitJumpInsn(IFNONNULL, l0)
+        mv.visitTypeInsn(NEW, "java/util/HashMap")
+        mv.visitInsn(DUP)
+        mv.visitMethodInsn(INVOKESPECIAL, "java/util/HashMap", "<init>", "()V", false)
+        mv.visitVarInsn(ASTORE, 1)
+        mv.visitVarInsn(ALOAD, 1)
+        mv.visitFieldInsn(PUTSTATIC, clazz.javaBinaryName.encoded, "$deserializeLambdaCache$", "Ljava/util/Map;")
+        mv.visitLabel(l0)
+        mv.visitFrame(asm.Opcodes.F_APPEND,1, Array("java/util/Map"), 0, null)
         mv.visitMethodInsn(INVOKESTATIC, "java/lang/invoke/MethodHandles", "lookup", "()Ljava/lang/invoke/MethodHandles$Lookup;", false)
-        mv.visitInsn(asm.Opcodes.ACONST_NULL)
+        mv.visitVarInsn(ALOAD, 1)
         mv.visitVarInsn(ALOAD, 0)
         mv.visitMethodInsn(INVOKESTATIC, "scala/compat/java8/runtime/LambdaDeserializer", "deserializeLambda", "(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/util/Map;Ljava/lang/invoke/SerializedLambda;)Ljava/lang/Object;", false)
         mv.visitInsn(ARETURN)
diff --git a/src/compiler/scala/tools/nsc/backend/jvm/BCodeSkelBuilder.scala b/src/compiler/scala/tools/nsc/backend/jvm/BCodeSkelBuilder.scala
index b2011f8e0c..a2fd22d24c 100644
--- a/src/compiler/scala/tools/nsc/backend/jvm/BCodeSkelBuilder.scala
+++ b/src/compiler/scala/tools/nsc/backend/jvm/BCodeSkelBuilder.scala
@@ -131,7 +131,7 @@ abstract class BCodeSkelBuilder extends BCodeHelpers {
           && indyLambdaHosts.contains(claszSymbol))
 
       if (shouldAddLambdaDeserialize)
-        addLambdaDeserialize(cnode)
+        addLambdaDeserialize(claszSymbol, cnode)
 
       addInnerClassesASM(cnode, innerClassBufferASM.toList)
 
diff --git a/src/compiler/scala/tools/nsc/backend/jvm/CoreBTypes.scala b/src/compiler/scala/tools/nsc/backend/jvm/CoreBTypes.scala
index 492fe3ae79..00ca096e59 100644
--- a/src/compiler/scala/tools/nsc/backend/jvm/CoreBTypes.scala
+++ b/src/compiler/scala/tools/nsc/backend/jvm/CoreBTypes.scala
@@ -114,6 +114,8 @@ class CoreBTypes[BTFS <: BTypesFromSymbols[_ <: Global]](val bTypes: BTFS) {
   lazy val jioSerializableReference    : ClassBType = classBTypeFromSymbol(JavaSerializableClass)     // java/io/Serializable
   lazy val scalaSerializableReference  : ClassBType = classBTypeFromSymbol(SerializableClass)         // scala/Serializable
   lazy val classCastExceptionReference : ClassBType = classBTypeFromSymbol(ClassCastExceptionClass)   // java/lang/ClassCastException
+  lazy val javaUtilMapReference        : ClassBType = classBTypeFromSymbol(JavaUtilMap)               // java/util/Map
+  lazy val javaUtilHashMapReference    : ClassBType = classBTypeFromSymbol(JavaUtilHashMap)           // java/util/HashMap
 
   lazy val srBooleanRef : ClassBType = classBTypeFromSymbol(requiredClass[scala.runtime.BooleanRef])
   lazy val srByteRef    : ClassBType = classBTypeFromSymbol(requiredClass[scala.runtime.ByteRef])
@@ -258,6 +260,8 @@ final class CoreBTypesProxy[BTFS <: BTypesFromSymbols[_ <: Global]](val bTypes:
   def jioSerializableReference    : ClassBType = _coreBTypes.jioSerializableReference
   def scalaSerializableReference  : ClassBType = _coreBTypes.scalaSerializableReference
   def classCastExceptionReference : ClassBType = _coreBTypes.classCastExceptionReference
+  def javaUtilMapReference        : ClassBType = _coreBTypes.javaUtilMapReference
+  def javaUtilHashMapReference    : ClassBType = _coreBTypes.javaUtilHashMapReference
 
   def srBooleanRef : ClassBType = _coreBTypes.srBooleanRef
   def srByteRef    : ClassBType = _coreBTypes.srByteRef