Remove the HAS_MOVED_GIVTBL bit

We no longer use the HAS_MOVED_GIVTBL bit in the hidden header.  Now
when we try to get the givtbl of a potentially moved object, we first
look up the global general_iv_tbl_ and, if not found, look up the
RubyBinding::moved_givtbl table in the Rust part of the binding.

internal/mmtk.h

@@ -29,8 +29,6 @@ typedef uint32_t MMTk_AllocationSemantics;
 
 #define MMTK_GC_THREAD_KIND_WORKER 1
 
-#define MMTK_HAS_MOVED_GIVTBL 9223372036854775808ull
-
 #define MMTK_HIDDEN_SIZE_MASK 281474976710655
 
 typedef struct st_table st_table;
@@ -271,4 +269,6 @@ void mmtk_object_reference_write_post(MMTk_Mutator *mutator, MMTk_ObjectReferenc
  */
 void mmtk_enumerate_objects(void (*callback)(MMTk_ObjectReference, void*), void *data);
 
+bool mmtk_hidden_header_is_sane(const struct MMTk_HiddenHeader *hidden_header);
+
 #endif /* MMTK_H */

mmtk_support.c

@@ -404,6 +404,10 @@ rb_mmtk_init_hidden_header(VALUE object, size_t payload_size)
 
     struct MMTk_HiddenHeader *hidden_header = (struct MMTk_HiddenHeader*)(object - MMTK_OBJREF_OFFSET);
     hidden_header->prefix = payload_size;
+
+    RUBY_ASSERT(mmtk_hidden_header_is_sane(hidden_header),
+                "Hidden header is not sane on construction.  Object: %p, prefix: %zx",
+                (void*) object, hidden_header->prefix);
 }
 
 size_t
@@ -412,8 +416,9 @@ rb_mmtk_get_payload_size(VALUE object)
     struct MMTk_HiddenHeader *hidden_header = (struct MMTk_HiddenHeader*)(object - MMTK_OBJREF_OFFSET);
     size_t prefix = hidden_header->prefix;
 
-    RUBY_ASSERT((prefix & ~(MMTK_HIDDEN_SIZE_MASK | MMTK_HAS_MOVED_GIVTBL)) == 0,
-                "Hidden field is corrupted.  Object: %p, prefix: %zx", (void*) object, prefix);
+    RUBY_ASSERT(mmtk_hidden_header_is_sane(hidden_header),
+                "Hidden header is corrupted.  Object: %p, prefix: %zx",
+                (void*) object, prefix);
 
     return prefix & MMTK_HIDDEN_SIZE_MASK;
 }