Remove NULL ObjectReference (#62)

Parent PR: mmtk/mmtk-core#1064

---------

Co-authored-by: mmtkgc-bot <mmtkgc.bot@gmail.com>

mmtk/Cargo.toml

@@ -12,7 +12,7 @@ edition = "2021"
 # Metadata for the Ruby repository
 [package.metadata.ci-repos.ruby]
 repo = "mmtk/ruby" # This is used by actions/checkout, so the format is "owner/repo", not URL.
-rev = "f63ea9a7ab846f8c1e0fab3ea61cdd1b364a661c"
+rev = "2a566599cdb83134df4c64a4991b26e2a96dc24a"
 
 [lib]
 name = "mmtk_ruby"
@@ -37,10 +37,10 @@ features = ["is_mmtk_object", "object_pinning"]
 
 # Uncomment the following lines to use mmtk-core from the official repository.
 git = "https://github.com/mmtk/mmtk-core.git"
-rev = "5a01555b5aee984b634f81e9d137f1ae0410fd39"
+rev = "a02803b4104519ff2289234101a2dd8ceedd1bc7"
 
 # Uncomment the following line to use mmtk-core from a local repository.
-#path = "../../mmtk-core"
+# path = "../../mmtk-core"
 
 [features]
 default = []

mmtk/cbindgen.toml

@@ -20,6 +20,7 @@ typedef rb_thread_t* MMTk_VMMutatorThread;
 typedef struct MMTk_GCThreadTLS* MMTk_VMWorkerThread;
 typedef void* MMTk_Address;
 typedef void* MMTk_ObjectReference;
+typedef void* MMTk_NullableObjectReference;
 typedef uint32_t MMTk_AllocationSemantics;
 """
 
@@ -34,6 +35,7 @@ typedef uint32_t MMTk_AllocationSemantics;
 "VMWorkerThread" = "MMTk_VMWorkerThread"
 "Address" = "MMTk_Address"
 "ObjectReference" = "MMTk_ObjectReference"
+"NullableObjectReference" = "MMTk_NullableObjectReference"
 "RawVecOfObjRef" = "MMTk_RawVecOfObjRef"
 "AllocationSemantics" = "MMTk_AllocationSemantics"
 "GC_THREAD_KIND_CONTROLLER" = "MMTK_GC_THREAD_KIND_CONTROLLER"

mmtk/src/api.rs

@@ -17,6 +17,7 @@ use mmtk::memory_manager;
 use mmtk::memory_manager::mmtk_init;
 use mmtk::util::alloc::AllocatorInfo;
 use mmtk::util::alloc::AllocatorSelector;
+use mmtk::util::api_util::NullableObjectReference;
 use mmtk::util::constants::MIN_OBJECT_SIZE;
 use mmtk::util::options::GCTriggerSelector;
 use mmtk::util::options::PlanSelector;
@@ -208,10 +209,8 @@ pub extern "C" fn mmtk_is_live_object(object: ObjectReference) -> bool {
 }
 
 #[no_mangle]
-pub extern "C" fn mmtk_get_forwarded_object(object: ObjectReference) -> ObjectReference {
-    object
-        .get_forwarded_object::<Ruby>()
-        .unwrap_or(ObjectReference::NULL)
+pub extern "C" fn mmtk_get_forwarded_object(object: ObjectReference) -> NullableObjectReference {
+    object.get_forwarded_object::<Ruby>().into()
 }
 
 #[no_mangle]

mmtk/src/object_model.rs

@@ -51,11 +51,13 @@ impl ObjectModel<Ruby> for VMObjectModel {
         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);
+        debug_assert!(!to_start.is_zero());
         let to_payload = to_start.add(OBJREF_OFFSET);
         unsafe {
             copy_nonoverlapping::<u8>(from_start.to_ptr(), to_start.to_mut_ptr(), object_size);
         }
-        let to_obj = ObjectReference::from_raw_address(to_payload);
+        // unsafe: `to_payload`` cannot be zero because `alloc_copy`` never returns zero.
+        let to_obj = unsafe { ObjectReference::from_raw_address_unchecked(to_payload) };
         copy_context.post_copy(to_obj, object_size, semantics);
         trace!("Copied object from {} to {}", from, to_obj);
 
@@ -123,7 +125,8 @@ impl ObjectModel<Ruby> for VMObjectModel {
     }
 
     fn address_to_ref(addr: Address) -> ObjectReference {
-        ObjectReference::from_raw_address(addr)
+        debug_assert!(!addr.is_zero());
+        unsafe { ObjectReference::from_raw_address_unchecked(addr) }
     }
 
     fn get_size_when_copied(object: ObjectReference) -> usize {

mmtk/src/reference_glue.rs

@@ -8,7 +8,7 @@ pub struct VMReferenceGlue {}
 impl ReferenceGlue<Ruby> for VMReferenceGlue {
     type FinalizableType = ObjectReference;
 
-    fn get_referent(_object: ObjectReference) -> ObjectReference {
+    fn get_referent(_object: ObjectReference) -> Option<ObjectReference> {
         unimplemented!()
     }
 
@@ -19,4 +19,8 @@ impl ReferenceGlue<Ruby> for VMReferenceGlue {
     fn enqueue_references(_references: &[ObjectReference], _tls: VMWorkerThread) {
         unimplemented!()
     }
+
+    fn clear_referent(_new_reference: ObjectReference) {
+        unimplemented!()
+    }
 }