WIP Avoid ObjectRef overhead for effectively final case vars

[retronym]

Given

class C {
    lazy val s: reflect.internal.SymbolTable = ???
    import s._
    def test(t: Tree) = {
        t match {
            case ap @ Apply(sel @ Select(_, _), _) if (ap, sel).hashCode > 0 =>
                1
            case ap @ Apply(sel @ Select(_, _), _) if (ap, sel).hashCode == 0 =>
                () => ap
        }
    }
}

$ scalac  -Xprint:jvm sandbox/test.scala  2>&1 | tee /tmp/old && qscalac  -Xprint:jvm sandbox/test.scala  2>&1 | tee /tmp/new && diff -U1000 /tmp/{old,new}

--- /tmp/old    2019-08-27 10:47:15.000000000 +1000
+++ /tmp/new    2019-08-27 10:47:18.000000000 +1000
@@ -1,80 +1,80 @@
 [[syntax trees at end of                       jvm]] // test.scala
 package <empty> {
   class C extends Object {
     final <synthetic> lazy private[this] var s: scala.reflect.internal.SymbolTable = _;
     @volatile private[this] var bitmap$0: Boolean = _;
     private def s$lzycompute(): scala.reflect.internal.SymbolTable = {
       C.this.synchronized(if (C.this.bitmap$0.unary_!())
         {
           C.this.s = (scala.Predef.???(): scala.reflect.internal.SymbolTable);
           C.this.bitmap$0 = true
         });
       C.this.s
     };
-    <stable> <accessor> lazy def s(): scala.reflect.internal.SymbolTable = (if (C.this.bitmap$0.unary_!())
+    <stable> <accessor> lazy def s(): scala.reflect.internal.SymbolTable = if (C.this.bitmap$0.unary_!())
       C.this.s$lzycompute()
     else
-      C.this.s: scala.reflect.internal.SymbolTable);
+      C.this.s;
     def test(t: reflect.internal.Trees$Tree): Object = {
       <synthetic> var rc13: Boolean = false;
-      <synthetic> var x2: runtime.ObjectRef = scala.runtime.ObjectRef.create((null: reflect.internal.Trees$Apply));
+      <synthetic> <stable> var x2: reflect.internal.Trees$Apply = (null: reflect.internal.Trees$Apply);
       {
         case <synthetic> val x1: reflect.internal.Trees$Tree = t;
         case15(){
           if (x1.$isInstanceOf[reflect.internal.Trees$Apply]())
             {
               rc13 = true;
-              x2.elem = (x1.$asInstanceOf[reflect.internal.Trees$Apply](): reflect.internal.Trees$Apply);
+              x2 = (x1.$asInstanceOf[reflect.internal.Trees$Apply](): reflect.internal.Trees$Apply);
               {
-                val sel: reflect.internal.Trees$Tree = x2.elem.$asInstanceOf[reflect.internal.Trees$Apply]().fun();
+                val sel: reflect.internal.Trees$Tree = x2.fun();
                 if (sel.$isInstanceOf[reflect.internal.Trees$Select]())
                   {
                     <synthetic> val x4: reflect.internal.Trees$Select = (sel.$asInstanceOf[reflect.internal.Trees$Select](): reflect.internal.Trees$Select);
-                    if (new Tuple2(x2.elem.$asInstanceOf[reflect.internal.Trees$Apply](), x4).hashCode().>(0))
+                    if (new Tuple2(x2, x4).hashCode().>(0))
                       matchEnd14(scala.Int.box(1))
                     else
                       case16()
                   }
                 else
                   case16()
               }
             }
           else
             case16()
         };
         case16(){
           if (rc13)
             {
-              val sel: reflect.internal.Trees$Tree = x2.elem.$asInstanceOf[reflect.internal.Trees$Apply]().fun();
+              val sel: reflect.internal.Trees$Tree = x2.fun();
               if (sel.$isInstanceOf[reflect.internal.Trees$Select]())
                 {
                   <synthetic> val x9: reflect.internal.Trees$Select = (sel.$asInstanceOf[reflect.internal.Trees$Select](): reflect.internal.Trees$Select);
-                  if (new Tuple2(x2.elem.$asInstanceOf[reflect.internal.Trees$Apply](), x9).hashCode().==(0))
+                  if (new Tuple2(x2, x9).hashCode().==(0))
                     matchEnd14({
                       $anonfun(x2)
                     })
                   else
                     case17()
                 }
               else
                 case17()
             }
           else
             case17()
         };
         case17(){
           matchEnd14(throw new MatchError(x1))
         };
         matchEnd14(x: Object){
           x
         }
       }
     };
-    final <static> <artifact> def $anonfun$test$1(x2$1: runtime.ObjectRef): reflect.internal.Trees$Apply = x2$1.elem.$asInstanceOf[reflect.internal.Trees$Apply]();
+    final <static> <artifact> def $anonfun$test$1(x2$1: reflect.internal.Trees$Apply): reflect.internal.Trees$Apply = x2$1;
     def <init>(): C = {
       C.super.<init>();
       ()
     }
   }
 }

[retronym]

Upstream: scala#8375