Fix #505: Remove unnecessary boxing of synchronized block

`synchronized` is now treated as a magic method by the compiler and is no
longer erased (similarly to `asInstanceOf` and `isInstanceOf`). Backend
is updated to handle this new magic polymorphic method.

compiler/src/dotty/tools/backend/jvm/DottyBackendInterface.scala

@@ -116,6 +116,7 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
   val Throwable_Type: Type = defn.ThrowableType
   val Object_isInstanceOf: Symbol = defn.Any_isInstanceOf
   val Object_asInstanceOf: Symbol = defn.Any_asInstanceOf
+  val Object_synchronized: Symbol = defn.Object_synchronized
   val Object_equals: Symbol = defn.Any_equals
   val ArrayClass: Symbol = defn.ArrayClass
   val UnitClass: Symbol = defn.UnitClass

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

@@ -1074,7 +1074,7 @@ class Definitions {
   lazy val NoInitClasses: Set[Symbol] = NotRuntimeClasses + FunctionXXLClass
 
   def isPolymorphicAfterErasure(sym: Symbol): Boolean =
-     (sym eq Any_isInstanceOf) || (sym eq Any_asInstanceOf)
+     (sym eq Any_isInstanceOf) || (sym eq Any_asInstanceOf) || (sym eq Object_synchronized)
 
   def isTupleType(tp: Type)(implicit ctx: Context): Boolean = {
     val arity = tp.dealias.argInfos.length

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

@@ -167,11 +167,10 @@ class LazyVals extends MiniPhase with IdentityDenotTransformer {
     val initSymbol = ctx.newSymbol(x.symbol.owner, initName, initFlags, MethodType(Nil, tpe), coord = x.pos)
     val rhs = x.rhs.changeOwnerAfter(x.symbol, initSymbol, this)
     val initialize = holderRef.select(lazyNme.initialize).appliedTo(rhs)
-    val initBody =
-      adaptToType(
-        holderRef.select(defn.Object_synchronized).appliedTo(
-          adaptToType(If(initialized, getValue, initialize), defn.ObjectType)),
-        tpe)
+    val initBody = holderRef
+      .select(defn.Object_synchronized)
+      .appliedToType(tpe)
+      .appliedTo(If(initialized, getValue, initialize).ensureConforms(tpe))
     val initTree = DefDef(initSymbol, initBody)
 
     // def x(): Int = if (x$lzy.initialized()) x$lzy.value() else x$lzycompute()

compiler/test/dotty/tools/backend/jvm/DottyBytecodeTests.scala

@@ -504,6 +504,7 @@ class TestBCode extends DottyBytecodeTest {
   /** Test that the size of the lazy val initialiazer is under a certain threshold
    *
    *  - Fix to #5340 reduced the size from 39 instructions to 34
+   *  - Fix to #505  reduced the size from 34 instructions to 32
    */
   @Test def i5340 = {
     val source =
@@ -519,7 +520,30 @@ class TestBCode extends DottyBytecodeTest {
       val clsIn   = dir.lookupName("Test.class", directory = false).input
       val clsNode = loadClassNode(clsIn)
       val method  = getMethod(clsNode, "x$lzyINIT1$1")
-      assertEquals(34, instructionsFromMethod(method).size)
+      assertEquals(32, instructionsFromMethod(method).size)
+    }
+  }
+
+  /** Test that synchronize blocks don't box */
+  @Test def i505 = {
+    val source =
+      """class Test {
+        |  def test: Int = synchronized(1)
+        |}
+      """.stripMargin
+
+    checkBCode(source) { dir =>
+      val clsIn   = dir.lookupName("Test.class", directory = false).input
+      val clsNode = loadClassNode(clsIn)
+      val method  = getMethod(clsNode, "test")
+
+      val doBox = instructionsFromMethod(method).exists {
+        case Invoke(_, _, name, _, _) =>
+          name == "boxToInteger" || name == "unboxToInt"
+        case _ =>
+          false
+      }
+      assertFalse(doBox)
     }
   }
 }

tests/run/i505.scala

@@ -0,0 +1,15 @@
+object Test {
+  def main(args: Array[String]): Unit = {
+    val a: Int = synchronized(1)
+    val b: Long = synchronized(1L)
+    val c: Boolean = synchronized(true)
+    val d: Float = synchronized(1f)
+    val e: Double = synchronized(1.0)
+    val f: Byte = synchronized(1.toByte)
+    val g: Char = synchronized('1')
+    val h: Short = synchronized(1.toShort)
+    val i: String = synchronized("Hello")
+    val j: List[Int] = synchronized(List(1))
+    synchronized(())
+  }
+}