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.

To make sure we don't thwart closure elimination, I've allowed DCE
to eliminate a non-static LOAD_FIELD of a member of a closure class.
It would be more general to track nullity of the reciever (e.g, `this`
or `new Foo` cannot be null), but that would require more
infrastructure in this phase.

I've added a test for closure inlining based on a a suggestion by
@dragos. This actually passes if we remove the (LOAD_FIELD, DROP)
peephole optimization for `closelim` altogether. But I chose to
adapt that optimization (only allow it for non-static, closure
fields), rather then remove it alogether, in the interests of
treading lightly.

1 files changed, 2 insertions, 5 deletions

diff --git a/src/compiler/scala/tools/nsc/backend/opt/ClosureElimination.scala b/src/compiler/scala/tools/nsc/backend/opt/ClosureElimination.scala
index c49f23852f..a866173a88 100644
--- a/src/compiler/scala/tools/nsc/backend/opt/ClosureElimination.scala
+++ b/src/compiler/scala/tools/nsc/backend/opt/ClosureElimination.scala
@@ -56,11 +56,8 @@ 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)
+      case (LOAD_FIELD(sym, /* isStatic */false), DROP(_)) if !sym.hasAnnotation(definitions.VolatileAttr) && inliner.isClosureClass(sym.owner) =>
+        Some(DROP(REFERENCE(definitions.ObjectClass)) :: Nil)
 
       case _ => None
     }