Clear temp var for captured var expr to permit GC

Author: som-snytt

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

@@ -10,6 +10,7 @@ import core.Decorators._
 import core.StdNames.nme
 import core.Names._
 import core.NameKinds.TempResultName
+import core.Constants._
 import ast.Trees._
 import util.Store
 import collection.mutable
@@ -45,6 +46,9 @@ class CapturedVars extends MiniPhase with IdentityDenotTransformer { thisPhase =
 
     val boxedRefClasses: collection.Set[Symbol] =
       refClassKeys.flatMap(k => Set(refClass(k), volatileRefClass(k)))
+
+    val objectRefClasses: collection.Set[Symbol] =
+      Set(refClass(defn.ObjectClass), volatileRefClass(defn.ObjectClass))
   }
 
   private var myRefInfo: RefInfo = null
@@ -138,17 +142,21 @@ class CapturedVars extends MiniPhase with IdentityDenotTransformer { thisPhase =
     *  Also: If the ref type lhs is followed by a cast (can be an artifact of nested translation),
     *  drop the cast.
     */
-  override def transformAssign(tree: Assign)(using Context): Tree = {
+  override def transformAssign(tree: Assign)(using Context): Tree =
     def recur(lhs: Tree): Tree = lhs match {
       case TypeApply(Select(qual, nme.asInstanceOf_), _) =>
         val Select(_, nme.elem) = qual
         recur(qual)
       case Select(_, nme.elem) if refInfo.boxedRefClasses.contains(lhs.symbol.maybeOwner) =>
-        val tempDef = transformFollowing(SyntheticValDef(TempResultName.fresh(), tree.rhs))
-        transformFollowing(Block(tempDef :: Nil, cpy.Assign(tree)(lhs, ref(tempDef.symbol))))
+        val tempDef = transformFollowing {
+          ValDef(newSymbol(ctx.owner, TempResultName.fresh(), Mutable | Synthetic, tree.rhs.tpe.widen, coord = tree.rhs.span), tree.rhs)
+        }
+        val update  = cpy.Assign(tree)(lhs, ref(tempDef.symbol))
+        def reset   = cpy.Assign(tree)(ref(tempDef.symbol), nullLiteral.cast(tempDef.symbol.info))
+        val res     = if (refInfo.objectRefClasses(lhs.symbol.maybeOwner)) reset else Literal(Constant(()))
+        transformFollowing(Block(tempDef :: update :: Nil, res))
       case _ =>
         tree
     }
     recur(tree.lhs)
-  }
 }

tests/run/i14198.scala

@@ -0,0 +1,34 @@
+
+import java.lang.ref.WeakReference
+import java.util.concurrent.atomic.AtomicReference
+
+final class Mark
+
+object Test {
+
+  def main(args: Array[String]): Unit = {
+    myTest()
+  }
+
+  final def myAssert(cond: => Boolean): Unit = {
+    assert(cond)
+  }
+
+  def myTest(): Unit = {
+    val ref = new AtomicReference[WeakReference[AnyRef]]
+    var mark: AnyRef = null
+    assert(ref.compareAndSet(null, new WeakReference(new Mark)))
+    mark = ref.get().get()
+    myAssert(mark ne null) // in theory this could fail, but it isn't
+    mark = null
+    System.gc()
+    var n = 10
+    while (n > 0 && ref.get().get() != null) {
+      System.gc()
+      Thread.`yield`()
+      //print(".")
+      n -= 1
+    }
+    assert(ref.get().get() == null)
+  }
+}

tests/run/liftedTry.scala

@@ -16,15 +16,34 @@ object Test {
     assert(x == 1)
     assert(foo(2) == 2)
     assert(foo(try raise(3) catch handle) == 3)
-    Tr.foo
+    assert(Tr.foo == 3)
   }
 }
 
 object Tr {
   def fun(a: Int => Unit) = a(2)
   def foo: Int = {
     var s = 1
-    s = try {fun(s = _); 3} catch{ case ex: Throwable => val x = 4; s = x; 5 }
+    s = try {fun(s = _); 3} catch { case ex: Throwable => val x = 4; s = x; 5 }
     s
   }
 }
+
+/* was:
+Caused by: java.lang.VerifyError: Inconsistent stackmap frames at branch target 33
+Exception Details:
+  Location:
+    Tr$.foo()I @30: goto
+  Reason:
+    Current frame's stack size doesn't match stackmap.
+  Current Frame:
+    bci: @30
+    flags: { }
+    locals: { 'Tr$', 'scala/runtime/IntRef', 'java/lang/Throwable', integer }
+    stack: { integer }
+  Stackmap Frame:
+    bci: @33
+    flags: { }
+    locals: { 'Tr$', 'scala/runtime/IntRef' }
+    stack: { top, integer }
+ */