Remove NULL ObjectReference (#146)

Parent PR: mmtk/mmtk-core#1064

---------

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

mmtk/Cargo.toml

@@ -31,7 +31,7 @@ lazy_static = "1.1"
 # - change branch
 # - 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
 # mmtk = { path = "../repos/mmtk-core" }
 log = {version = "0.4", features = ["max_level_trace", "release_max_level_off"] }

mmtk/src/edges.rs

@@ -17,7 +17,7 @@ pub enum JuliaVMEdge {
 unsafe impl Send for JuliaVMEdge {}
 
 impl Edge for JuliaVMEdge {
-    fn load(&self) -> ObjectReference {
+    fn load(&self) -> Option<ObjectReference> {
         match self {
             JuliaVMEdge::Simple(e) => e.load(),
             JuliaVMEdge::Offset(e) => e.load(),
@@ -74,9 +74,10 @@ impl OffsetEdge {
 }
 
 impl Edge for OffsetEdge {
-    fn load(&self) -> ObjectReference {
+    fn load(&self) -> Option<ObjectReference> {
         let middle = unsafe { (*self.slot_addr).load(atomic::Ordering::Relaxed) };
         let begin = middle - self.offset;
+        debug_assert!(!begin.is_zero());
         ObjectReference::from_raw_address(begin)
     }
 

mmtk/src/julia_finalizer.rs

@@ -111,7 +111,9 @@ impl ArrayListT {
 }
 
 fn gc_ptr_clear_tag(addr: Address, tag: usize) -> ObjectReference {
-    ObjectReference::from_raw_address(unsafe { Address::from_usize(addr & !tag) })
+    let addr = unsafe { Address::from_usize(addr & !tag) };
+    debug_assert!(!addr.is_zero());
+    unsafe { ObjectReference::from_raw_address_unchecked(addr) }
 }
 
 fn gc_ptr_tag(addr: Address, tag: usize) -> bool {
@@ -201,7 +203,8 @@ fn mark_finlist<T: ObjectTracer>(list: &mut ArrayListT, start: usize, tracer: &m
             cur_tag = 1;
             gc_ptr_clear_tag(cur, 1)
         } else {
-            ObjectReference::from_raw_address(cur)
+            // unsafe: We checked `cur.is_zero()` before.
+            unsafe { ObjectReference::from_raw_address_unchecked(cur) }
         };
         if gc_ptr_tag(cur, 2) {
             i += 1;

mmtk/src/julia_scanning.rs

@@ -4,7 +4,6 @@ use crate::edges::JuliaVMEdge;
 use crate::edges::OffsetEdge;
 use crate::julia_types::*;
 use crate::object_model::mmtk_jl_array_ndims;
-use crate::JuliaVM;
 use crate::JULIA_BUFF_TAG;
 use crate::UPCALLS;
 use memoffset::offset_of;
@@ -416,27 +415,34 @@ fn get_stack_addr(addr: Address, offset: isize, lb: u64, ub: u64) -> Address {
     }
 }
 
-use mmtk::vm::edge_shape::Edge;
-
 #[inline(always)]
 pub fn process_edge<EV: EdgeVisitor<JuliaVMEdge>>(closure: &mut EV, slot: Address) {
     let simple_edge = SimpleEdge::from_address(slot);
-    debug_assert!(
-        simple_edge.load().is_null()
-            || mmtk::memory_manager::is_in_mmtk_spaces::<JuliaVM>(simple_edge.load()),
-        "Object {:?} in slot {:?} is not mapped address",
-        simple_edge.load(),
-        simple_edge
-    );
 
-    // captures wrong edges before creating the work
-    debug_assert!(
-        simple_edge.load().to_raw_address().as_usize() % 16 == 0
-            || simple_edge.load().to_raw_address().as_usize() % 8 == 0,
-        "Object {:?} in slot {:?} is not aligned to 8 or 16",
-        simple_edge.load(),
-        simple_edge
-    );
+    #[cfg(debug_assertions)]
+    {
+        use crate::JuliaVM;
+        use mmtk::vm::edge_shape::Edge;
+
+        if let Some(objref) = simple_edge.load() {
+            debug_assert!(
+                mmtk::memory_manager::is_in_mmtk_spaces::<JuliaVM>(objref),
+                "Object {:?} in slot {:?} is not mapped address",
+                objref,
+                simple_edge
+            );
+
+            let raw_addr_usize = objref.to_raw_address().as_usize();
+
+            // captures wrong edges before creating the work
+            debug_assert!(
+                raw_addr_usize % 16 == 0 || raw_addr_usize % 8 == 0,
+                "Object {:?} in slot {:?} is not aligned to 8 or 16",
+                objref,
+                simple_edge
+            );
+        }
+    }
 
     closure.visit_edge(JuliaVMEdge::Simple(simple_edge));
 }
@@ -485,13 +491,20 @@ pub fn process_offset_edge<EV: EdgeVisitor<JuliaVMEdge>>(
     offset: usize,
 ) {
     let offset_edge = OffsetEdge::new_with_offset(slot, offset);
-    debug_assert!(
-        offset_edge.load().is_null()
-            || mmtk::memory_manager::is_in_mmtk_spaces::<JuliaVM>(offset_edge.load()),
-        "Object {:?} in slot {:?} is not mapped address",
-        offset_edge.load(),
-        offset_edge
-    );
+    #[cfg(debug_assertions)]
+    {
+        use crate::JuliaVM;
+        use mmtk::vm::edge_shape::Edge;
+
+        if let Some(objref) = offset_edge.load() {
+            debug_assert!(
+                mmtk::memory_manager::is_in_mmtk_spaces::<JuliaVM>(objref),
+                "Object {:?} in slot {:?} is not mapped address",
+                objref,
+                offset_edge
+            );
+        }
+    }
 
     closure.visit_edge(JuliaVMEdge::Offset(offset_edge));
 }
@@ -603,5 +616,6 @@ pub fn mmtk_jl_bt_num_uintvals(bt_entry: *mut mmtk_jl_bt_element_t) -> usize {
 
 pub fn mmtk_jl_bt_entry_jlvalue(bt_entry: *mut mmtk_jl_bt_element_t, i: usize) -> ObjectReference {
     let entry = unsafe { (*bt_entry.add(2 + i)).__bindgen_anon_1.jlvalue };
-    ObjectReference::from_raw_address(Address::from_mut_ptr(entry))
+    debug_assert!(!entry.is_null());
+    unsafe { ObjectReference::from_raw_address_unchecked(Address::from_mut_ptr(entry)) }
 }

mmtk/src/object_model.rs

@@ -62,25 +62,28 @@ impl ObjectModel<JuliaVM> for VMObjectModel {
         copy_context: &mut GCWorkerCopyContext<JuliaVM>,
     ) -> ObjectReference {
         let bytes = Self::get_current_size(from);
-        let from_start_ref = ObjectReference::from_raw_address(Self::ref_to_object_start(from));
-        let header_offset =
-            from.to_raw_address().as_usize() - from_start_ref.to_raw_address().as_usize();
+        let from_addr = from.to_raw_address();
+        let from_start = Self::ref_to_object_start(from);
+        let header_offset = from_addr - from_start;
 
         let dst = if header_offset == 8 {
             // regular object
-            copy_context.alloc_copy(from_start_ref, bytes, 16, 8, semantics)
+            // Note: The `from` reference is not used by any allocator currently in MMTk core.
+            copy_context.alloc_copy(from, bytes, 16, 8, semantics)
         } else if header_offset == 16 {
             // buffer should not be copied
             unimplemented!();
         } else {
             unimplemented!()
         };
+        // `alloc_copy` should never return zero.
+        debug_assert!(!dst.is_zero());
 
-        let src = Self::ref_to_object_start(from);
+        let src = from_start;
         unsafe {
             std::ptr::copy_nonoverlapping::<u8>(src.to_ptr(), dst.to_mut_ptr(), bytes);
         }
-        let to_obj = ObjectReference::from_raw_address(dst + header_offset);
+        let to_obj = unsafe { ObjectReference::from_raw_address_unchecked(dst + header_offset) };
 
         trace!("Copying object from {} to {}", from, to_obj);
 
@@ -99,7 +102,7 @@ impl ObjectModel<JuliaVM> for VMObjectModel {
         {
             use atomic::Ordering;
             unsafe {
-                libc::memset(from_start_ref.to_raw_address().to_mut_ptr(), 0, bytes);
+                libc::memset(from_start.to_mut_ptr(), 0, bytes);
             }
 
             Self::LOCAL_FORWARDING_BITS_SPEC.store_atomic::<JuliaVM, u8>(
@@ -160,7 +163,9 @@ impl ObjectModel<JuliaVM> for VMObjectModel {
 
     #[inline(always)]
     fn address_to_ref(address: Address) -> ObjectReference {
-        ObjectReference::from_raw_address(address)
+        // `address` is a result of `ref_to_address(object)`, where `object` cannot be NULL.
+        debug_assert!(!address.is_zero());
+        unsafe { ObjectReference::from_raw_address_unchecked(address) }
     }
 
     #[inline(always)]

mmtk/src/reference_glue.rs

@@ -26,40 +26,54 @@ impl Finalizable for JuliaFinalizableObject {
         self.set_reference(trace.trace_object(self.get_reference()));
         if !self.2 {
             // not a void pointer
-            trace.trace_object(ObjectReference::from_raw_address(self.1));
+            debug_assert!(!self.1.is_zero());
+            let objref = unsafe { ObjectReference::from_raw_address_unchecked(self.1) };
+            trace.trace_object(objref);
         }
     }
 }
 
 pub struct VMReferenceGlue {}
 
-impl ReferenceGlue<JuliaVM> for VMReferenceGlue {
-    type FinalizableType = JuliaFinalizableObject;
-    fn set_referent(reference: ObjectReference, referent: ObjectReference) {
+impl VMReferenceGlue {
+    fn load_referent_raw(reference: ObjectReference) -> *mut mmtk_jl_value_t {
+        let reff = reference.to_raw_address().to_ptr::<mmtk_jl_weakref_t>();
+        unsafe { (*reff).value }
+    }
+
+    fn set_referent_raw(reference: ObjectReference, referent_raw: *mut mmtk_jl_value_t) {
+        let reff = reference.to_raw_address().to_mut_ptr::<mmtk_jl_weakref_t>();
         unsafe {
-            let reff = reference.to_raw_address().to_mut_ptr::<mmtk_jl_weakref_t>();
-            let referent_raw = referent.to_raw_address().to_mut_ptr::<mmtk_jl_value_t>();
             (*reff).value = referent_raw;
         }
     }
+}
 
-    fn clear_referent(new_reference: ObjectReference) {
-        Self::set_referent(new_reference, unsafe {
-            ObjectReference::from_raw_address(Address::from_mut_ptr(jl_nothing))
-        });
+impl ReferenceGlue<JuliaVM> for VMReferenceGlue {
+    type FinalizableType = JuliaFinalizableObject;
+
+    fn set_referent(reference: ObjectReference, referent: ObjectReference) {
+        Self::set_referent_raw(reference, referent.to_raw_address().to_mut_ptr());
     }
 
-    fn get_referent(object: ObjectReference) -> ObjectReference {
-        let referent;
-        unsafe {
-            let reff = object.to_raw_address().to_mut_ptr::<mmtk_jl_weakref_t>();
-            referent = ObjectReference::from_raw_address(Address::from_mut_ptr((*reff).value));
-        }
-        referent
+    fn clear_referent(new_reference: ObjectReference) {
+        Self::set_referent_raw(new_reference, unsafe { jl_nothing });
     }
 
-    fn is_referent_cleared(referent: ObjectReference) -> bool {
-        unsafe { referent.to_raw_address().to_mut_ptr() == jl_nothing }
+    fn get_referent(object: ObjectReference) -> Option<ObjectReference> {
+        let value = Self::load_referent_raw(object);
+        if value == unsafe { jl_nothing } {
+            return None;
+        } else {
+            debug_assert!(
+                !value.is_null(),
+                "A weak reference {} contains null referent pointer",
+                object
+            );
+            let objref =
+                unsafe { ObjectReference::from_raw_address_unchecked(Address::from_ptr(value)) };
+            Some(objref)
+        }
     }
 
     fn enqueue_references(_references: &[ObjectReference], _tls: VMWorkerThread) {}

mmtk/src/scanning.rs

@@ -4,6 +4,7 @@ use mmtk::memory_manager;
 use mmtk::scheduler::*;
 use mmtk::util::opaque_pointer::*;
 use mmtk::util::ObjectReference;
+use mmtk::vm::edge_shape::Edge;
 use mmtk::vm::EdgeVisitor;
 use mmtk::vm::ObjectTracerContext;
 use mmtk::vm::RootsWorkFactory;
@@ -31,9 +32,7 @@ impl Scanning<JuliaVM> for VMScanning {
             fn visit_edge(&mut self, edge: JuliaVMEdge) {
                 match edge {
                     JuliaVMEdge::Simple(se) => {
-                        let slot = se.as_address();
-                        let object = unsafe { slot.load::<ObjectReference>() };
-                        if !object.is_null() {
+                        if let Some(object) = se.load() {
                             self.buffer.push(object);
                         }
                     }
@@ -67,7 +66,11 @@ impl Scanning<JuliaVM> for VMScanning {
                         Address::from_ptr(task)
                     );
 
-                    node_buffer.push(ObjectReference::from_raw_address(Address::from_ptr(task)));
+                    // unsafe: We checked `!task.is_null()` before.
+                    let objref = unsafe {
+                        ObjectReference::from_raw_address_unchecked(Address::from_ptr(task))
+                    };
+                    node_buffer.push(objref);
                 }
             }
         };
@@ -88,9 +91,12 @@ impl Scanning<JuliaVM> for VMScanning {
         root_scan_task(ptls.next_task, true);
         root_scan_task(ptls.previous_task, true);
         if !ptls.previous_exception.is_null() {
-            node_buffer.push(ObjectReference::from_raw_address(Address::from_mut_ptr(
-                ptls.previous_exception,
-            )));
+            node_buffer.push(unsafe {
+                // unsafe: We have just checked `ptls.previous_exception` is not null.
+                ObjectReference::from_raw_address_unchecked(Address::from_mut_ptr(
+                    ptls.previous_exception,
+                ))
+            });
         }
 
         // Scan backtrace buffer: See gc_queue_bt_buf in gc.c