Remove NULL ObjectReference (#169)

Upstream PR: mmtk/mmtk-core#1064

---------

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

mmtk/Cargo.toml

@@ -28,7 +28,7 @@ log = {version = "0.4", features = ["max_level_trace", "release_max_level_off"]
 # - change branch/rev
 # - change repo name
 # But other changes including adding/removing whitespaces in commented lines may break the CI.
-mmtk = { git = "https://github.com/mmtk/mmtk-core.git", rev = "e79e94e744660c486d5471f252ff05c4248bcea9" }
+mmtk = { git = "https://github.com/mmtk/mmtk-core.git", rev = "a02803b4104519ff2289234101a2dd8ceedd1bc7" }
 # Uncomment the following to build locally - if you change the path locally, do not commit the change in a PR
 # mmtk = { path = "../repos/mmtk-core" }
 

mmtk/src/api.rs

@@ -5,6 +5,7 @@ use libc::c_char;
 use libc::c_void;
 use mmtk::memory_manager;
 use mmtk::scheduler::*;
+use mmtk::util::api_util::NullableObjectReference;
 use mmtk::util::opaque_pointer::*;
 use mmtk::util::{Address, ObjectReference};
 
@@ -210,11 +211,8 @@ pub extern "C" fn add_phantom_candidate(reff: ObjectReference, referent: ObjectR
 }
 
 #[no_mangle]
-pub extern "C" fn get_forwarded_object(object: ObjectReference) -> ObjectReference {
-    match object.get_forwarded_object::<JikesRVM>() {
-        Some(ref_obj) => ref_obj,
-        None => ObjectReference::NULL,
-    }
+pub extern "C" fn get_forwarded_object(object: ObjectReference) -> NullableObjectReference {
+    object.get_forwarded_object::<JikesRVM>().into()
 }
 
 #[no_mangle]
@@ -285,11 +283,8 @@ pub extern "C" fn add_finalizer(object: ObjectReference) {
 }
 
 #[no_mangle]
-pub extern "C" fn get_finalized_object() -> ObjectReference {
-    match memory_manager::get_finalized_object(&SINGLETON) {
-        Some(obj) => obj,
-        None => ObjectReference::NULL,
-    }
+pub extern "C" fn get_finalized_object() -> NullableObjectReference {
+    memory_manager::get_finalized_object(&SINGLETON).into()
 }
 
 // Allocation slow path

mmtk/src/object_model.rs

@@ -21,6 +21,15 @@ use memory_manager_constants::*;
 use tib_layout_constants::*;
 use JikesRVM;
 
+/// Used as a parameter of `move_object` to specify where to move an object to.
+enum MoveTarget {
+    /// Move an object to the address returned from `alloc_copy`.
+    ToAddress(Address),
+    /// Move an object to an `ObjectReference` pointing to an object previously computed from
+    /// `get_reference_when_copied_to`.
+    ToObject(ObjectReference),
+}
+
 /** Should we gather stats on hash code state transitions for address-based hashing? */
 const HASH_STATS: bool = false;
 /** count number of Object.hashCode() operations */
@@ -131,7 +140,7 @@ impl ObjectModel<JikesRVM> for VMObjectModel {
 
         let bytes = if copy {
             let bytes = Self::bytes_required_when_copied(from, rvm_type);
-            Self::move_object(unsafe { Address::zero() }, from, to, bytes, rvm_type);
+            Self::move_object(from, MoveTarget::ToObject(to), bytes, rvm_type);
             bytes
         } else {
             Self::bytes_used(from, rvm_type)
@@ -156,7 +165,8 @@ impl ObjectModel<JikesRVM> for VMObjectModel {
             }
         }
 
-        ObjectReference::from_raw_address(res + OBJECT_REF_OFFSET)
+        debug_assert!(!res.is_zero());
+        unsafe { ObjectReference::from_raw_address_unchecked(res + OBJECT_REF_OFFSET) }
     }
 
     fn get_current_size(object: ObjectReference) -> usize {
@@ -229,7 +239,8 @@ impl ObjectModel<JikesRVM> for VMObjectModel {
 
     #[inline(always)]
     fn address_to_ref(addr: Address) -> ObjectReference {
-        ObjectReference::from_raw_address(addr + (-TIB_OFFSET))
+        debug_assert!(!addr.is_zero());
+        unsafe { ObjectReference::from_raw_address_unchecked(addr + (-TIB_OFFSET)) }
     }
 
     fn dump_object(_object: ObjectReference) {
@@ -256,7 +267,7 @@ impl VMObjectModel {
         let offset = Self::get_offset_for_alignment_class(from, rvm_type);
         let region = copy_context.alloc_copy(from, bytes, align, offset, copy);
 
-        let to_obj = Self::move_object(region, from, ObjectReference::NULL, bytes, rvm_type);
+        let to_obj = Self::move_object(from, MoveTarget::ToAddress(region), bytes, rvm_type);
         copy_context.post_copy(to_obj, bytes, copy);
         to_obj
     }
@@ -275,7 +286,7 @@ impl VMObjectModel {
         let offset = Self::get_offset_for_alignment_array(from, rvm_type);
         let region = copy_context.alloc_copy(from, bytes, align, offset, copy);
 
-        let to_obj = Self::move_object(region, from, ObjectReference::NULL, bytes, rvm_type);
+        let to_obj = Self::move_object(from, MoveTarget::ToAddress(region), bytes, rvm_type);
         copy_context.post_copy(to_obj, bytes, copy);
         // XXX: Do not sync icache/dcache because we do not support PowerPC
         to_obj
@@ -371,16 +382,12 @@ impl VMObjectModel {
 
     #[inline(always)]
     fn move_object(
-        immut_to_address: Address,
         from_obj: ObjectReference,
-        immut_to_obj: ObjectReference,
+        mut to: MoveTarget,
         num_bytes: usize,
         _rvm_type: Address,
     ) -> ObjectReference {
         trace!("VMObjectModel.move_object");
-        let mut to_address = immut_to_address;
-        let mut to_obj = immut_to_obj;
-        debug_assert!(to_address.is_zero() || to_obj.to_raw_address().is_zero());
 
         // Default values
         let mut copy_bytes = num_bytes;
@@ -399,8 +406,8 @@ impl VMObjectModel {
 
                     if !DYNAMIC_HASH_OFFSET {
                         // The hashcode is the first word, so we copy to object one word higher
-                        if to_obj.to_raw_address().is_zero() {
-                            to_address += HASHCODE_BYTES;
+                        if let MoveTarget::ToAddress(ref mut addr) = to {
+                            *addr += HASHCODE_BYTES;
                         }
                     }
                 } else if !DYNAMIC_HASH_OFFSET && hash_state == HASH_STATE_HASHED_AND_MOVED {
@@ -410,9 +417,18 @@ impl VMObjectModel {
             }
         }
 
-        if !to_obj.to_raw_address().is_zero() {
-            to_address = to_obj.to_raw_address() + (-obj_ref_offset);
-        }
+        let (to_address, to_obj) = match to {
+            MoveTarget::ToAddress(addr) => {
+                let obj =
+                    unsafe { ObjectReference::from_raw_address_unchecked(addr + obj_ref_offset) };
+                (addr, obj)
+            }
+            MoveTarget::ToObject(obj) => {
+                let addr = obj.to_raw_address() + (-obj_ref_offset);
+                debug_assert!(obj.to_raw_address() == addr + obj_ref_offset);
+                (addr, obj)
+            }
+        };
 
         // Low memory word of source object
         let from_address = from_obj.to_raw_address() + (-obj_ref_offset);
@@ -422,12 +438,6 @@ impl VMObjectModel {
             Self::aligned_32_copy(to_address, from_address, copy_bytes);
         }
 
-        if to_obj.is_null() {
-            to_obj = ObjectReference::from_raw_address(to_address + obj_ref_offset);
-        } else {
-            debug_assert!(to_obj.to_raw_address() == to_address + obj_ref_offset);
-        }
-
         // Do we need to copy the hash code?
         if hash_state == HASH_STATE_HASHED {
             unsafe {

mmtk/src/reference_glue.rs

@@ -1,4 +1,5 @@
 use mmtk::util::opaque_pointer::*;
+use mmtk::util::Address;
 use mmtk::util::ObjectReference;
 use mmtk::vm::ReferenceGlue;
 
@@ -25,15 +26,14 @@ impl ReferenceGlue<JikesRVM> for VMReferenceGlue {
         }
     }
 
-    fn get_referent(object: ObjectReference) -> ObjectReference {
+    fn get_referent(object: ObjectReference) -> Option<ObjectReference> {
         if cfg!(feature = "binding_side_ref_proc") {
             panic!();
         } else {
-            debug_assert!(!object.is_null());
-            unsafe {
-                (object.to_raw_address() + REFERENCE_REFERENT_FIELD_OFFSET)
-                    .load::<ObjectReference>()
-            }
+            let addr = unsafe {
+                (object.to_raw_address() + REFERENCE_REFERENT_FIELD_OFFSET).load::<Address>()
+            };
+            ObjectReference::from_raw_address(addr)
         }
     }
 
@@ -52,4 +52,15 @@ impl ReferenceGlue<JikesRVM> for VMReferenceGlue {
             }
         }
     }
+
+    fn clear_referent(new_reference: ObjectReference) {
+        if cfg!(feature = "binding_side_ref_proc") {
+            panic!();
+        } else {
+            unsafe {
+                (new_reference.to_raw_address() + REFERENCE_REFERENT_FIELD_OFFSET)
+                    .store(Address::ZERO);
+            }
+        }
+    }
 }