Deletion barrier preparation: validate overwritten references.

runtime/vm/raw_object.cc

Index: runtime/vm/raw_object.cc
===================================================================
--- runtime/vm/raw_object.cc	(revision 42583)
+++ runtime/vm/raw_object.cc	(working copy)
@@ -14,6 +14,9 @@
 
 namespace dart {
 
+#if defined(DEBUG)
+DEFINE_FLAG(bool, validate_overwrite, true, "Verify overwritten fields.");
+#endif  // DEBUG
 
 const intptr_t RawPcDescriptors::kFullRecSize =
     sizeof(RawPcDescriptors::PcDescriptorRec);
@@ -35,6 +38,10 @@
   if (!IsHeapObject()) {
     return;
   }
+  // Slightly more readable than a segfault.
+  if (this == reinterpret_cast<RawObject*>(kHeapObjectTag)) {
+    FATAL("RAW_NULL encountered");
+  }
   // Validate that the tags_ field is sensible.
   uword tags = ptr()->tags_;
   intptr_t reserved = ReservedBits::decode(tags);
@@ -45,6 +52,11 @@
   if (!isolate->class_table()->IsValidIndex(class_id)) {
     FATAL1("Invalid class id encountered %" Pd "\n", class_id);
   }
+  if ((class_id == kNullCid) &&
+      (isolate->class_table()->At(class_id) == NULL)) {
+    // Null class not yet initialized; skip.
+    return;
+  }
   intptr_t size = SizeTag::decode(tags);
   if (size != 0 && size != SizeFromClass()) {
     FATAL1("Inconsistent class size encountered %" Pd "\n", size);
@@ -200,6 +212,22 @@
 }
 
 
+#if defined(DEBUG)
+void RawObject::ValidateOverwrittenPointer(RawObject* raw) {
+  if (FLAG_validate_overwrite) {
+    raw->Validate(Isolate::Current());
+  }
+}
+
+
+void RawObject::ValidateOverwrittenSmi(RawSmi* raw) {
+  if (FLAG_validate_overwrite && raw->IsHeapObject() && raw != Object::null()) {
+    FATAL1("Expected smi/null, found: %" Px "\n", reinterpret_cast<uword>(raw));
+  }
+}
+#endif  // DEBUG
+
+
 intptr_t RawObject::VisitPointers(ObjectPointerVisitor* visitor) {
   intptr_t size = 0;
   NoHandleScope no_handles(visitor->isolate());