Always use NULL instead of root_hcls

Refcounting added a second way to represent "no attrs" -- NULL as a hcls.
This was causing guards to fail since a check would be written for one
but then it would fail when objects came in with the other.

So canonicalize to using NULL as the "no attributes" hcls.

src/core/types.h

@@ -630,7 +630,7 @@ public:
     HiddenClass* hcls;
     AttrList* attr_list;
 
-    HCAttrs(HiddenClass* hcls = root_hcls) : hcls(hcls), attr_list(nullptr) {}
+    HCAttrs(HiddenClass* hcls = NULL) : hcls(hcls), attr_list(nullptr) {}
 
     int traverse(visitproc visit, void* arg) noexcept;
     void clear() noexcept;

src/runtime/hiddenclass.cpp

@@ -136,6 +136,8 @@ HiddenClass* HiddenClass::delAttrToMakeHC(BoxedString* attr) {
             cur = cur->getOrMakeChild(attr);
         curidx++;
     }
+    if (cur == root_hcls)
+        return NULL;
     return cur;
 }
 

src/runtime/objmodel.cpp

@@ -1118,6 +1118,8 @@ void Box::appendNewHCAttr(BORROWED(Box*) new_attr, SetattrRewriteArgs* rewrite_a
     HCAttrs* attrs = getHCAttrsPtr();
     HiddenClass* hcls = attrs->hcls;
 
+    if (hcls == NULL)
+        hcls = root_hcls;
     assert(hcls->type == HiddenClass::NORMAL || hcls->type == HiddenClass::SINGLETON);
 
     int numattrs = hcls->attributeArraySize();
@@ -1188,7 +1190,7 @@ void Box::setattr(BoxedString* attr, BORROWED(Box*) val, SetattrRewriteArgs* rew
             // We could update PyObject_Init and PyObject_INIT to do this, but that has a small compatibility
             // issue (what if people don't call either of those) and I'm not sure that this check will be that
             // harmful.  But if it is we might want to try pushing this assignment to allocation time.
-            hcls = attrs->hcls = root_hcls;
+            hcls = root_hcls;
         }
 
         if (hcls->type == HiddenClass::DICT_BACKED) {
@@ -1212,7 +1214,7 @@ void Box::setattr(BoxedString* attr, BORROWED(Box*) val, SetattrRewriteArgs* rew
                 REWRITE_ABORTED("");
                 rewrite_args = NULL;
             } else {
-                rewrite_args->obj->addAttrGuard(cls->attrs_offset + offsetof(HCAttrs, hcls), (intptr_t)hcls);
+                rewrite_args->obj->addAttrGuard(cls->attrs_offset + offsetof(HCAttrs, hcls), (intptr_t)attrs->hcls);
                 if (hcls->type == HiddenClass::SINGLETON)
                     hcls->addDependence(rewrite_args->rewriter);
             }

src/runtime/types.cpp

@@ -2063,7 +2063,7 @@ Box* moduleInit(BoxedModule* self, Box* name, Box* doc) {
 
     HCAttrs* attrs = self->getHCAttrsPtr();
 
-    if (attrs->hcls->attributeArraySize() == 0) {
+    if (!attrs->hcls || attrs->hcls->attributeArraySize() == 0) {
         attrs->hcls = HiddenClass::makeSingleton();
 
         self->giveAttrBorrowed("__name__", name);
@@ -2167,7 +2167,7 @@ public:
         // This check doesn't cover all cases, since an attrwrapper could be created around
         // a normal object which then becomes dict-backed, so we RELEASE_ASSERT later
         // that that doesn't happen.
-        assert(b->getHCAttrsPtr()->hcls->type == HiddenClass::NORMAL
+        assert(!b->getHCAttrsPtr()->hcls || b->getHCAttrsPtr()->hcls->type == HiddenClass::NORMAL
                || b->getHCAttrsPtr()->hcls->type == HiddenClass::SINGLETON);
     }
 
@@ -2579,6 +2579,8 @@ public:
 
         // Add the existing attrwrapper object (ie self) back as the attrwrapper:
         self->b->appendNewHCAttr(self, NULL);
+        if (!attrs->hcls)
+            attrs->hcls = root_hcls;
         attrs->hcls = attrs->hcls->getAttrwrapperChild();
     }
 
@@ -2742,7 +2744,7 @@ Box* Box::getAttrWrapper() {
     HiddenClass* hcls = attrs->hcls;
 
     if (!hcls)
-        hcls = attrs->hcls = root_hcls;
+        hcls = root_hcls;
 
     if (hcls->type == HiddenClass::DICT_BACKED) {
         return incref(attrs->attr_list->attrs[0]);
@@ -3727,7 +3729,7 @@ void HCAttrs::clear() noexcept {
     auto old_attr_list = this->attr_list;
     auto old_attr_list_size = hcls->attributeArraySize();
 
-    new ((void*)this) HCAttrs(root_hcls);
+    new ((void*)this) HCAttrs(NULL);
 
     if (old_attr_list) {
         for (int i = 0; i < old_attr_list_size; i++) {