Make ObjectReferenct non-nullable

src/memory_manager.rs

@@ -708,9 +708,6 @@ pub fn is_mmtk_object(addr: Address) -> bool {
 /// * `object`: The object reference to query.
 pub fn is_in_mmtk_spaces<VM: VMBinding>(object: ObjectReference) -> bool {
     use crate::mmtk::SFT_MAP;
-    if object.is_null() {
-        return false;
-    }
     SFT_MAP
         .get_checked(object.to_address::<VM>())
         .is_in_space(object)

src/plan/generational/gc_work.rs

@@ -36,18 +36,16 @@ impl<VM: VMBinding, P: GenerationalPlanExt<VM> + PlanTraceObject<VM>> ProcessEdg
         Self { plan, base }
     }
     fn trace_object(&mut self, object: ObjectReference) -> ObjectReference {
-        debug_assert!(!object.is_null());
-
         // We cannot borrow `self` twice in a call, so we extract `worker` as a local variable.
         let worker = self.worker();
         self.plan
             .trace_object_nursery(&mut self.base.nodes, object, worker)
     }
     fn process_edge(&mut self, slot: EdgeOf<Self>) {
-        let object = slot.load();
-        if object.is_null() {
+        let Some(object) = slot.load() else {
+            // Skip slots that are not holding an object reference.
             return;
-        }
+        };
         let new_object = self.trace_object(object);
         debug_assert!(!self.plan.is_object_in_nursery(new_object));
         // Note: If `object` is a mature object, `trace_object` will not call `space.trace_object`,

src/plan/tracing.rs

@@ -117,7 +117,7 @@ impl<'a, E: ProcessEdgesWork> EdgeVisitor<EdgeOf<E>> for ObjectsClosure<'a, E> {
         {
             use crate::vm::edge_shape::Edge;
             trace!(
-                "(ObjectsClosure) Visit edge {:?} (pointing to {})",
+                "(ObjectsClosure) Visit edge {:?} (pointing to {:?})",
                 slot,
                 slot.load()
             );

src/policy/copyspace.rs

@@ -217,7 +217,6 @@ impl<VM: VMBinding> CopySpace<VM> {
         worker: &mut GCWorker<VM>,
     ) -> ObjectReference {
         trace!("copyspace.trace_object(, {:?}, {:?})", object, semantics,);
-        debug_assert!(!object.is_null());
 
         // If this is not from space, we do not need to trace it (the object has been copied to the tosapce)
         if !self.is_from_space() {

src/policy/immix/immixspace.rs

@@ -184,7 +184,6 @@ impl<VM: VMBinding> crate::policy::gc_work::PolicyTraceObject<VM> for ImmixSpace
         copy: Option<CopySemantics>,
         worker: &mut GCWorker<VM>,
     ) -> ObjectReference {
-        debug_assert!(!object.is_null());
         if KIND == TRACE_KIND_TRANSITIVE_PIN {
             self.trace_object_without_moving(queue, object)
         } else if KIND == TRACE_KIND_DEFRAG {

src/policy/immortalspace.rs

@@ -187,7 +187,6 @@ impl<VM: VMBinding> ImmortalSpace<VM> {
         queue: &mut Q,
         object: ObjectReference,
     ) -> ObjectReference {
-        debug_assert!(!object.is_null());
         #[cfg(feature = "vo_bit")]
         debug_assert!(
             crate::util::metadata::vo_bit::is_vo_bit_set::<VM>(object),

src/policy/largeobjectspace.rs

@@ -189,7 +189,6 @@ impl<VM: VMBinding> LargeObjectSpace<VM> {
         queue: &mut Q,
         object: ObjectReference,
     ) -> ObjectReference {
-        debug_assert!(!object.is_null());
         #[cfg(feature = "vo_bit")]
         debug_assert!(
             crate::util::metadata::vo_bit::is_vo_bit_set::<VM>(object),

src/policy/markcompactspace.rs

@@ -37,12 +37,7 @@ impl<VM: VMBinding> SFT for MarkCompactSpace<VM> {
     }
 
     fn get_forwarded_object(&self, object: ObjectReference) -> Option<ObjectReference> {
-        let forwarding_pointer = Self::get_header_forwarding_pointer(object);
-        if forwarding_pointer.is_null() {
-            None
-        } else {
-            Some(forwarding_pointer)
-        }
+        Self::get_header_forwarding_pointer(object)
     }
 
     fn is_live(&self, object: ObjectReference) -> bool {
@@ -130,7 +125,6 @@ impl<VM: VMBinding> crate::policy::gc_work::PolicyTraceObject<VM> for MarkCompac
         _copy: Option<CopySemantics>,
         _worker: &mut GCWorker<VM>,
     ) -> ObjectReference {
-        debug_assert!(!object.is_null());
         debug_assert!(
             KIND != TRACE_KIND_TRANSITIVE_PIN,
             "MarkCompact does not support transitive pin trace."
@@ -177,8 +171,9 @@ impl<VM: VMBinding> MarkCompactSpace<VM> {
     }
 
     /// Get header forwarding pointer for an object
-    fn get_header_forwarding_pointer(object: ObjectReference) -> ObjectReference {
-        unsafe { Self::header_forwarding_pointer_address(object).load::<ObjectReference>() }
+    fn get_header_forwarding_pointer(object: ObjectReference) -> Option<ObjectReference> {
+        let addr = unsafe { Self::header_forwarding_pointer_address(object).load::<Address>() };
+        ObjectReference::from_raw_address(addr)
     }
 
     /// Store header forwarding pointer for an object
@@ -251,7 +246,9 @@ impl<VM: VMBinding> MarkCompactSpace<VM> {
             queue.enqueue(object);
         }
 
-        Self::get_header_forwarding_pointer(object)
+        Self::get_header_forwarding_pointer(object).unwrap_or_else(|| {
+            panic!("trace_forward_object called when an object is not forwarded, yet. object: {object}")
+        })
     }
 
     pub fn test_and_mark(object: ObjectReference) -> bool {
@@ -388,10 +385,9 @@ impl<VM: VMBinding> MarkCompactSpace<VM> {
                 // clear the VO bit
                 vo_bit::unset_vo_bit::<VM>(obj);
 
-                let forwarding_pointer = Self::get_header_forwarding_pointer(obj);
-
-                trace!("Compact {} to {}", obj, forwarding_pointer);
-                if !forwarding_pointer.is_null() {
+                let maybe_forwarding_pointer = Self::get_header_forwarding_pointer(obj);
+                if let Some(forwarding_pointer) = maybe_forwarding_pointer {
+                    trace!("Compact {} to {}", obj, forwarding_pointer);
                     let new_object = forwarding_pointer;
                     Self::clear_header_forwarding_pointer(new_object);
 
@@ -403,6 +399,8 @@ impl<VM: VMBinding> MarkCompactSpace<VM> {
                     vo_bit::set_vo_bit::<VM>(new_object);
                     to = new_object.to_object_start::<VM>() + copied_size;
                     debug_assert_eq!(end_of_new_object, to);
+                } else {
+                    trace!("Skipping dead object {}", obj);
                 }
             }
         }

src/policy/marksweepspace/malloc_ms/global.rs

@@ -400,8 +400,6 @@ impl<VM: VMBinding> MallocSpace<VM> {
         queue: &mut Q,
         object: ObjectReference,
     ) -> ObjectReference {
-        debug_assert!(!object.is_null());
-
         assert!(
             self.in_space(object),
             "Cannot mark an object {} that was not alloced by malloc.",

src/policy/marksweepspace/native_ms/block.rs

@@ -287,7 +287,8 @@ impl Block {
         while cell + cell_size <= self.start() + Block::BYTES {
             // The invariants we checked earlier ensures that we can use cell and object reference interchangably
             // We may not really have an object in this cell, but if we do, this object reference is correct.
-            let potential_object = ObjectReference::from_raw_address(cell);
+            // About unsafe: We know `cell` is non-zero here.
+            let potential_object = unsafe { ObjectReference::from_raw_address_unchecked(cell) };
 
             if !VM::VMObjectModel::LOCAL_MARK_BIT_SPEC
                 .is_marked::<VM>(potential_object, Ordering::SeqCst)
@@ -327,9 +328,12 @@ impl Block {
 
         while cell + cell_size <= self.end() {
             // possible object ref
-            let potential_object_ref = ObjectReference::from_raw_address(
-                cursor + VM::VMObjectModel::OBJECT_REF_OFFSET_LOWER_BOUND,
-            );
+            let potential_object_ref = unsafe {
+                // We know cursor plus an offset cannot be 0.
+                ObjectReference::from_raw_address_unchecked(
+                    cursor + VM::VMObjectModel::OBJECT_REF_OFFSET_LOWER_BOUND,
+                )
+            };
             trace!(
                 "{:?}: cell = {}, last cell in free list = {}, cursor = {}, potential object = {}",
                 self,

src/policy/marksweepspace/native_ms/global.rs

@@ -241,7 +241,6 @@ impl<VM: VMBinding> MarkSweepSpace<VM> {
         queue: &mut Q,
         object: ObjectReference,
     ) -> ObjectReference {
-        debug_assert!(!object.is_null());
         debug_assert!(
             self.in_space(object),
             "Cannot mark an object {} that was not alloced by free list allocator.",

src/scheduler/gc_work.rs

@@ -274,7 +274,6 @@ impl<E: ProcessEdgesWork> ObjectTracer for ProcessEdgesWorkTracer<E> {
     /// Forward the `trace_object` call to the underlying `ProcessEdgesWork`,
     /// and flush as soon as the underlying buffer of `process_edges_work` is full.
     fn trace_object(&mut self, object: ObjectReference) -> ObjectReference {
-        debug_assert!(!object.is_null());
         let result = self.process_edges_work.trace_object(object);
         self.flush_if_full();
         result
@@ -659,12 +658,11 @@ pub trait ProcessEdgesWork:
     /// Process an edge, including loading the object reference from the memory slot,
     /// trace the object and store back the new object reference if necessary.
     fn process_edge(&mut self, slot: EdgeOf<Self>) {
-        let object = slot.load();
-        if object.is_null() {
+        let Some(object) = slot.load() else {
+            // Skip slots that are not holding an object reference.
             return;
-        }
+        };
         let new_object = self.trace_object(object);
-        debug_assert!(!new_object.is_null());
         if Self::OVERWRITE_REFERENCE && new_object != object {
             slot.store(new_object);
         }
@@ -722,8 +720,6 @@ impl<VM: VMBinding> ProcessEdgesWork for SFTProcessEdges<VM> {
     fn trace_object(&mut self, object: ObjectReference) -> ObjectReference {
         use crate::policy::sft::GCWorkerMutRef;
 
-        debug_assert!(!object.is_null());
-
         // Erase <VM> type parameter
         let worker = GCWorkerMutRef::new(self.worker());
 
@@ -996,20 +992,18 @@ impl<VM: VMBinding, P: PlanTraceObject<VM> + Plan<VM = VM>, const KIND: TraceKin
     }
 
     fn trace_object(&mut self, object: ObjectReference) -> ObjectReference {
-        debug_assert!(!object.is_null());
         // We cannot borrow `self` twice in a call, so we extract `worker` as a local variable.
         let worker = self.worker();
         self.plan
             .trace_object::<VectorObjectQueue, KIND>(&mut self.base.nodes, object, worker)
     }
 
     fn process_edge(&mut self, slot: EdgeOf<Self>) {
-        let object = slot.load();
-        if object.is_null() {
+        let Some(object) = slot.load() else {
+            // Skip slots that are not holding an object reference.
             return;
-        }
+        };
         let new_object = self.trace_object(object);
-        debug_assert!(!new_object.is_null());
         if P::may_move_objects::<KIND>() && new_object != object {
             slot.store(new_object);
         }

src/util/address.rs

@@ -3,6 +3,7 @@ use bytemuck::NoUninit;
 
 use std::fmt;
 use std::mem;
+use std::num::NonZeroUsize;
 use std::ops::*;
 use std::sync::atomic::Ordering;
 
@@ -479,28 +480,33 @@ use crate::vm::VMBinding;
 /// the opaque `ObjectReference` type, and we haven't seen a use case for now.
 #[repr(transparent)]
 #[derive(Copy, Clone, Eq, Hash, PartialOrd, Ord, PartialEq, NoUninit)]
-pub struct ObjectReference(usize);
+pub struct ObjectReference(NonZeroUsize);
 
 impl ObjectReference {
-    /// The null object reference, represented as zero.
-    pub const NULL: ObjectReference = ObjectReference(0);
-
     /// Cast the object reference to its raw address. This method is mostly for the convinience of a binding.
     ///
     /// MMTk should not make any assumption on the actual location of the address with the object reference.
     /// MMTk should not assume the address returned by this method is in our allocation. For the purposes of
     /// setting object metadata, MMTk should use [`crate::vm::ObjectModel::ref_to_address()`] or [`crate::vm::ObjectModel::ref_to_header()`].
     pub fn to_raw_address(self) -> Address {
-        Address(self.0)
+        Address(self.0.get())
     }
 
     /// Cast a raw address to an object reference. This method is mostly for the convinience of a binding.
     /// This is how a binding creates `ObjectReference` instances.
     ///
+    /// If `addr` is 0, the result is `None`.
+    ///
     /// MMTk should not assume an arbitrary address can be turned into an object reference. MMTk can use [`crate::vm::ObjectModel::address_to_ref()`]
     /// to turn addresses that are from [`crate::vm::ObjectModel::ref_to_address()`] back to object.
-    pub fn from_raw_address(addr: Address) -> ObjectReference {
-        ObjectReference(addr.0)
+    pub fn from_raw_address(addr: Address) -> Option<ObjectReference> {
+        NonZeroUsize::new(addr.0).map(ObjectReference)
+    }
+
+    /// Like `from_raw_address`, but assume `addr` is not zero.
+    pub unsafe fn from_raw_address_unchecked(addr: Address) -> ObjectReference {
+        debug_assert!(!addr.is_zero());
+        ObjectReference(NonZeroUsize::new_unchecked(addr.0))
     }
 
     /// Get the in-heap address from an object reference. This method is used by MMTk to get an in-heap address
@@ -541,54 +547,41 @@ impl ObjectReference {
         obj
     }
 
-    /// is this object reference null reference?
-    pub fn is_null(self) -> bool {
-        self.0 == 0
-    }
-
     /// returns the ObjectReference
     pub fn value(self) -> usize {
-        self.0
+        self.0.get()
     }
 
     /// Is the object reachable, determined by the policy?
     /// Note: Objects in ImmortalSpace may have `is_live = true` but are actually unreachable.
     pub fn is_reachable(self) -> bool {
-        if self.is_null() {
-            false
-        } else {
-            unsafe { SFT_MAP.get_unchecked(Address(self.0)) }.is_reachable(self)
-        }
+        unsafe { SFT_MAP.get_unchecked(self.to_raw_address()) }.is_reachable(self)
     }
 
     /// Is the object live, determined by the policy?
     pub fn is_live(self) -> bool {
-        if self.0 == 0 {
-            false
-        } else {
-            unsafe { SFT_MAP.get_unchecked(Address(self.0)) }.is_live(self)
-        }
+        unsafe { SFT_MAP.get_unchecked(self.to_raw_address()) }.is_live(self)
     }
 
     /// Can the object be moved?
     pub fn is_movable(self) -> bool {
-        unsafe { SFT_MAP.get_unchecked(Address(self.0)) }.is_movable()
+        unsafe { SFT_MAP.get_unchecked(self.to_raw_address()) }.is_movable()
     }
 
     /// Get forwarding pointer if the object is forwarded.
     pub fn get_forwarded_object(self) -> Option<Self> {
-        unsafe { SFT_MAP.get_unchecked(Address(self.0)) }.get_forwarded_object(self)
+        unsafe { SFT_MAP.get_unchecked(self.to_raw_address()) }.get_forwarded_object(self)
     }
 
     /// Is the object in any MMTk spaces?
     pub fn is_in_any_space(self) -> bool {
-        unsafe { SFT_MAP.get_unchecked(Address(self.0)) }.is_in_space(self)
+        unsafe { SFT_MAP.get_unchecked(self.to_raw_address()) }.is_in_space(self)
     }
 
     /// Is the object sane?
     #[cfg(feature = "sanity")]
     pub fn is_sane(self) -> bool {
-        unsafe { SFT_MAP.get_unchecked(Address(self.0)) }.is_sane()
+        unsafe { SFT_MAP.get_unchecked(self.to_raw_address()) }.is_sane()
     }
 }
 

src/util/metadata/vo_bit/mod.rs

@@ -100,7 +100,10 @@ pub fn is_vo_bit_set<VM: VMBinding>(object: ObjectReference) -> bool {
 /// Check if an address can be turned directly into an object reference using the VO bit.
 /// If so, return `Some(object)`. Otherwise return `None`.
 pub fn is_vo_bit_set_for_addr<VM: VMBinding>(address: Address) -> Option<ObjectReference> {
-    let potential_object = ObjectReference::from_raw_address(address);
+    let Some(potential_object) = ObjectReference::from_raw_address(address) else {
+        return None;
+    };
+
     let addr = potential_object.to_address::<VM>();
 
     // If we haven't mapped VO bit for the address, it cannot be an object
@@ -123,7 +126,10 @@ pub fn is_vo_bit_set_for_addr<VM: VMBinding>(address: Address) -> Option<ObjectR
 ///
 /// This is unsafe: check the comment on `side_metadata::load`
 pub unsafe fn is_vo_bit_set_unsafe<VM: VMBinding>(address: Address) -> Option<ObjectReference> {
-    let potential_object = ObjectReference::from_raw_address(address);
+    let Some(potential_object) = ObjectReference::from_raw_address(address) else {
+        return None;
+    };
+
     let addr = potential_object.to_address::<VM>();
 
     // If we haven't mapped VO bit for the address, it cannot be an object

src/util/object_forwarding.rs

@@ -150,7 +150,9 @@ pub fn read_forwarding_pointer<VM: VMBinding>(object: ObjectReference) -> Object
 
     // We write the forwarding poiner. We know it is an object reference.
     unsafe {
-        ObjectReference::from_raw_address(crate::util::Address::from_usize(
+        // We use "unchecked" convertion becasue we guarantee the forwarding pointer we stored
+        // previously is from a valid `ObjectReference` which is never zero.
+        ObjectReference::from_raw_address_unchecked(crate::util::Address::from_usize(
             VM::VMObjectModel::LOCAL_FORWARDING_POINTER_SPEC.load_atomic::<VM, usize>(
                 object,
                 Some(FORWARDING_POINTER_MASK),