Fix incorrect assertion failures in noopt regarding removing classes that still have constants.

runtime/vm/precompiler.cc

Index: runtime/vm/precompiler.cc
diff --git a/runtime/vm/precompiler.cc b/runtime/vm/precompiler.cc
index b8bfc1898fab8195d08277bfe8bb93f404c7b67d..2bda728bafe5a3ebad5de30ae0dfae785a8a3563 100644
--- a/runtime/vm/precompiler.cc
+++ b/runtime/vm/precompiler.cc
@@ -203,6 +203,12 @@ void Precompiler::DoCompileAll(
     TraceTypesFromRetainedClasses();
     DropTypes();
     DropTypeArguments();
+
+    // Clear these before dropping classes as they may hold onto otherwise
+    // dead instances of classes we will remove.
+    I->object_store()->set_compile_time_constants(Array::null_array());
+    I->object_store()->set_unique_dynamic_targets(Array::null_array());
+
     DropClasses();
     DropLibraries();
 
@@ -216,9 +222,6 @@ void Precompiler::DoCompileAll(
       DedupInstructions();
     }
 
-    I->object_store()->set_compile_time_constants(Array::null_array());
-    I->object_store()->set_unique_dynamic_targets(Array::null_array());
-
     zone_ = NULL;
   }
 
@@ -1473,6 +1476,7 @@ void Precompiler::TraceTypesFromRetainedClasses() {
         constants = Array::MakeArray(retained_constants);
         cls.set_constants(constants);
       } else {
+        constants = Object::empty_array().raw();
         cls.set_constants(Object::empty_array());
       }
 
@@ -1492,14 +1496,17 @@ void Precompiler::TraceTypesFromRetainedClasses() {
 void Precompiler::DropClasses() {
   Library& lib = Library::Handle(Z);
   Class& cls = Class::Handle(Z);
-  Array& members = Array::Handle(Z);
+  Array& constants = Array::Handle(Z);
   String& name = String::Handle(Z);
 
 #if defined(DEBUG)
-  {
-    // Force GC for allocation stats.
-    I->heap()->CollectAllGarbage();
-  }
+  // We are about to remove classes from the class table. For this to be safe,
+  // there must be no instances of these classes on the heap, not even
+  // corpses because the class table entry may be used to find the size of
+  // corpses. Request a GC twice so we are sure the sweep tasks of the first
+  // have completed before we continue.
+  I->heap()->CollectAllGarbage();
+  I->heap()->CollectAllGarbage();

[rmacnak, 2016/03/08 23:57:27]

Is there a better way to wait?

[siva, 2016/03/09 07:03:33]

You could add a method in heap.h called WaitForSweeperTasks which does something
like
    {
      MonitorLocker ml(old_space_.tasks_lock());
      while (old_space_.tasks() > 0) {
        ml.WaitWithSafepointCheck(thread);
      }
    }

[rmacnak, 2016/03/09 20:09:41]

Added as debug-only.

 #endif
 
   ClassTable* class_table = I->class_table();
@@ -1518,22 +1525,16 @@ void Precompiler::DropClasses() {
       // removed.
       continue;
     }
-    if (cls.is_enum_class()) {
-      // Enum classes have live instances, so we cannot unregister
-      // them.
-      continue;
-    }
-    members = cls.constants();
-    if (members.Length() > 0) {
-      // --compile_all?
-      continue;
-    }
 
     bool retain = classes_to_retain_.Lookup(&cls) != NULL;
     if (retain) {
       continue;
     }
 
+    ASSERT(!cls.is_allocated());
+    constants = cls.constants();
+    ASSERT(constants.Length() == 0);
+
 #if defined(DEBUG)
     intptr_t instances =
         class_table->StatsWithUpdatedSize(cid)->post_gc.new_count +