Fix givtbl handling

mmtk/src/abi.rs

@@ -13,6 +13,9 @@ pub const GC_THREAD_KIND_WORKER: libc::c_int = 1;
 const HAS_MOVED_GIVTBL: usize = 1 << 63;
 const HIDDEN_SIZE_MASK: usize = 0x0000FFFFFFFFFFFF;
 
+// Should keep in sync with C code.
+const RUBY_FL_EXIVAR: usize = 1 << 10;
+
 /// Provide convenient methods for accessing Ruby objects.
 /// TODO: Wrap C functions in `RubyUpcalls` as Rust-friendly methods.
 pub struct RubyObjectAccess {
@@ -68,6 +71,18 @@ impl RubyObjectAccess {
         self.update_hidden_field(|old| old & !HIDDEN_SIZE_MASK | size & HIDDEN_SIZE_MASK);
     }
 
+    fn flags_field(&self) -> Address {
+        self.objref.to_raw_address()
+    }
+
+    pub fn load_flags(&self) -> usize {
+        unsafe { self.flags_field().load::<usize>() }
+    }
+
+    pub fn has_exivar_flag(&self) -> bool {
+        (self.load_flags() & RUBY_FL_EXIVAR) != 0
+    }
+
     pub fn has_moved_givtbl(&self) -> bool {
         (self.load_hidden_field() & HAS_MOVED_GIVTBL) != 0
     }

mmtk/src/api.rs

@@ -267,7 +267,7 @@ pub extern "C" fn mmtk_register_ppps(objects: *const ObjectReference, len: usize
 }
 
 #[no_mangle]
-pub extern "C" fn mmtk_get_givtbl(object: ObjectReference) -> *mut libc::c_void {
+pub extern "C" fn mmtk_get_givtbl_during_gc(object: ObjectReference) -> *mut libc::c_void {
     let acc = RubyObjectAccess::from_objref(object);
     acc.get_givtbl()
 }

mmtk/src/object_model.rs

@@ -43,7 +43,11 @@ impl ObjectModel<Ruby> for VMObjectModel {
         copy_context: &mut GCWorkerCopyContext<Ruby>,
     ) -> ObjectReference {
         let from_acc = RubyObjectAccess::from_objref(from);
-        let maybe_givtbl = from_acc.get_original_givtbl();
+        let maybe_givtbl = from_acc.has_exivar_flag().then(|| {
+            from_acc
+                .get_original_givtbl()
+                .unwrap_or_else(|| panic!("Object {} has FL_EXIVAR but no givtbl.", from))
+        });
         let from_start = from_acc.obj_start();
         let object_size = from_acc.object_size();
         let to_start = copy_context.alloc_copy(from, object_size, MIN_OBJ_ALIGN, 0, semantics);