SI-8601 Avoid over-eager optimization of LOAD_FIELD

It can NPE or trigger static class initilization, we can't elimiate
it without changing semantics.

Author: retronym

src/compiler/scala/tools/nsc/backend/opt/ClosureElimination.scala

@@ -56,11 +56,10 @@ abstract class ClosureElimination extends SubComponent {
       case (BOX(t1), UNBOX(t2)) if (t1 == t2) =>
         Some(Nil)
 
-      case (LOAD_FIELD(sym, isStatic), DROP(_)) if !sym.hasAnnotation(definitions.VolatileAttr) =>
-        if (isStatic)
-          Some(Nil)
-        else
-          Some(DROP(REFERENCE(definitions.ObjectClass)) :: Nil)
+      // Can't eliminate (LOAD_FIELD, DROP) without eliding side effects:
+      //  - static class initialization
+      //  - NPE if the receiver is null
+      // We could replace the LOAD/DROP with a null check to preserve semantics.
 
       case _ => None
     }

src/compiler/scala/tools/nsc/backend/opt/DeadCodeElimination.scala

@@ -167,7 +167,7 @@ abstract class DeadCodeElimination extends SubComponent {
               set += idx
               localStores(key) = set
 
-            case RETURN(_) | JUMP(_) | CJUMP(_, _, _, _) | CZJUMP(_, _, _, _) | STORE_FIELD(_, _) |
+            case RETURN(_) | JUMP(_) | CJUMP(_, _, _, _) | CZJUMP(_, _, _, _) | STORE_FIELD(_, _) | LOAD_FIELD(_, _) | // Why LOAD_FIELD? It can NPE!
                  THROW(_)   | LOAD_ARRAY_ITEM(_) | STORE_ARRAY_ITEM(_) | SCOPE_ENTER(_) | SCOPE_EXIT(_) | STORE_THIS(_) |
                  LOAD_EXCEPTION(_) | SWITCH(_, _) | MONITOR_ENTER() | MONITOR_EXIT() | CHECK_CAST(_) =>
               moveToWorkList()

test/files/run/t8601c.flags

@@ -0,0 +1 @@
+-optimize

test/files/run/t8601c.scala

@@ -0,0 +1,12 @@
+object Test {
+  def loadField(x: scala.runtime.IntRef): Unit = x.elem
+  def storeField(x: scala.runtime.IntRef): Unit = x.elem = 42
+
+  def check(x: => Any) = try { x; sys.error("failed to throw NPE!") } catch { case _: NullPointerException => }
+
+  def main(args: Array[String]) {
+    check(loadField(null)) // bug: did not NPE under -Ydead-code
+    check(storeField(null))
+
+  }
+}