Weak reference support (#145)

This adds weak reference support, and updates MMTk core to mmtk/mmtk-core#564.

* if weak reference is enabled in MMTk, we will add weak references during our tracing. Otherwise, still treat them as strong references.
* implement new changes in mmtk/mmtk-core#564
* add test with weak reference enabled (by default, it is disabled).

Author: qinsoon

.github/scripts/ci-test-weak-ref.sh

@@ -7,3 +7,5 @@ cd $cur
 ./ci-test-assertions.sh
 cd $cur
 ./ci-test-global-alloc-bit.sh
+cd $cur
+./ci-test-weak-ref.sh

.github/scripts/ci-test.sh

@@ -25,6 +25,19 @@ jobs:
       - name: Style checks
         run: ./.github/scripts/ci-style.sh
 
+  test-weak-ref:
+    runs-on: ubuntu-18.04
+    steps:
+      - uses: actions/checkout@v2
+      - name: Setup environments
+        run: |
+          ./.github/scripts/ci-checkout.sh
+          ./.github/scripts/ci-setup.sh
+
+      # Run the tests
+      - name: Dacapo Tests with Weak ref
+        run: ./.github/scripts/ci-test-weak-ref.sh
+
   msrv:
     runs-on: ubuntu-18.04
     steps:

.github/workflows/ci.yml

@@ -28,7 +28,7 @@ once_cell = "1.10.0"
 # - 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 = "0babba20290d3c4e4cdb2a83284aa7204c9a23cc" }
+mmtk = { git = "https://github.com/mmtk/mmtk-core.git", rev = "21176127d1f0493444fd901ea7277099a24bcecc" }
 # Uncomment the following to build locally
 # mmtk = { path = "../repos/mmtk-core" }
 

mmtk/Cargo.toml

@@ -138,7 +138,7 @@ pub struct InstanceKlass {
     // #endif
     pub idnum_allocated_count: u16,
     pub init_state: u8,
-    pub reference_type: u8,
+    pub reference_type: ReferenceType,
     pub this_class_index: u16,
     // #if INCLUDE_JVMTI
     pub jvmti_cached_class_field_map: OpaquePointer, // JvmtiCachedClassFieldMap*
@@ -154,6 +154,18 @@ pub struct InstanceKlass {
     pub fields: OpaquePointer,                 // Array<u2>*
 }
 
+#[repr(u8)]
+#[derive(Copy, Clone, Debug)]
+#[allow(dead_code)]
+pub enum ReferenceType {
+    None,    // Regular class
+    Other,   // Subclass of java/lang/ref/Reference, but not subclass of one of the classes below
+    Soft,    // Subclass of java/lang/ref/SoftReference
+    Weak,    // Subclass of java/lang/ref/WeakReference
+    Final,   // Subclass of java/lang/ref/FinalReference
+    Phantom, // Subclass of java/lang/ref/PhantomReference
+}
+
 impl InstanceKlass {
     const HEADER_SIZE: usize = mem::size_of::<Self>() / BYTES_IN_WORD;
     const VTABLE_START_OFFSET: usize = Self::HEADER_SIZE * BYTES_IN_WORD;

mmtk/src/abi.rs

@@ -179,18 +179,18 @@ pub extern "C" fn modify_check(object: ObjectReference) {
 }
 
 #[no_mangle]
-pub extern "C" fn add_weak_candidate(reff: ObjectReference, referent: ObjectReference) {
-    memory_manager::add_weak_candidate(&SINGLETON, reff, referent)
+pub extern "C" fn add_weak_candidate(reff: ObjectReference) {
+    memory_manager::add_weak_candidate(&SINGLETON, reff)
 }
 
 #[no_mangle]
-pub extern "C" fn add_soft_candidate(reff: ObjectReference, referent: ObjectReference) {
-    memory_manager::add_soft_candidate(&SINGLETON, reff, referent)
+pub extern "C" fn add_soft_candidate(reff: ObjectReference) {
+    memory_manager::add_soft_candidate(&SINGLETON, reff)
 }
 
 #[no_mangle]
-pub extern "C" fn add_phantom_candidate(reff: ObjectReference, referent: ObjectReference) {
-    memory_manager::add_phantom_candidate(&SINGLETON, reff, referent)
+pub extern "C" fn add_phantom_candidate(reff: ObjectReference) {
+    memory_manager::add_phantom_candidate(&SINGLETON, reff)
 }
 
 // The harness_begin()/end() functions are different than other API functions in terms of the thread state.

mmtk/src/api.rs

@@ -77,6 +77,7 @@ pub struct OpenJDK_Upcalls {
     pub number_of_mutators: extern "C" fn() -> usize,
     pub schedule_finalizer: extern "C" fn(),
     pub prepare_for_roots_re_scanning: extern "C" fn(),
+    pub enqueue_references: extern "C" fn(objects: *const ObjectReference, len: usize),
 }
 
 pub static mut UPCALLS: *const OpenJDK_Upcalls = null_mut();

mmtk/src/lib.rs

@@ -107,7 +107,39 @@ impl OopIterate for TypeArrayKlass {
 impl OopIterate for InstanceRefKlass {
     #[inline]
     fn oop_iterate(&self, oop: Oop, closure: &mut impl EdgeVisitor) {
+        use crate::abi::*;
+        use crate::api::{add_phantom_candidate, add_soft_candidate, add_weak_candidate};
         self.instance_klass.oop_iterate(oop, closure);
+
+        if Self::should_scan_weak_refs() {
+            let reference = ObjectReference::from(oop);
+            match self.instance_klass.reference_type {
+                ReferenceType::None => {
+                    panic!("oop_iterate on InstanceRefKlass with reference_type as None")
+                }
+                ReferenceType::Weak => add_weak_candidate(reference),
+                ReferenceType::Soft => add_soft_candidate(reference),
+                ReferenceType::Phantom => add_phantom_candidate(reference),
+                // Process these two types normally (as if they are strong refs)
+                // We will handle final reference later
+                ReferenceType::Final | ReferenceType::Other => {
+                    Self::process_ref_as_strong(oop, closure)
+                }
+            }
+        } else {
+            Self::process_ref_as_strong(oop, closure);
+        }
+    }
+}
+
+impl InstanceRefKlass {
+    #[inline]
+    fn should_scan_weak_refs() -> bool {
+        use SINGLETON;
+        !*SINGLETON.get_options().no_reference_types
+    }
+    #[inline]
+    fn process_ref_as_strong(oop: Oop, closure: &mut impl EdgeVisitor) {
         let referent_addr = Self::referent_address(oop);
         closure.visit_edge(referent_addr);
         let discovered_addr = Self::discovered_address(oop);

mmtk/src/object_scanning.rs

@@ -1,23 +1,24 @@
+use crate::abi::{InstanceRefKlass, Oop};
 use crate::OpenJDK;
+use crate::UPCALLS;
 use mmtk::util::opaque_pointer::VMWorkerThread;
 use mmtk::util::ObjectReference;
 use mmtk::vm::ReferenceGlue;
-use mmtk::TraceLocal;
 
 pub struct VMReferenceGlue {}
 
 impl ReferenceGlue<OpenJDK> for VMReferenceGlue {
-    fn set_referent(_reff: ObjectReference, _referent: ObjectReference) {
-        unimplemented!()
+    fn set_referent(reff: ObjectReference, referent: ObjectReference) {
+        let oop = Oop::from(reff);
+        unsafe { InstanceRefKlass::referent_address(oop).store(referent) };
     }
-    fn get_referent(_object: ObjectReference) -> ObjectReference {
-        unimplemented!()
+    fn get_referent(object: ObjectReference) -> ObjectReference {
+        let oop = Oop::from(object);
+        unsafe { InstanceRefKlass::referent_address(oop).load::<ObjectReference>() }
     }
-    fn process_reference<T: TraceLocal>(
-        _trace: &mut T,
-        _reference: ObjectReference,
-        _tls: VMWorkerThread,
-    ) -> ObjectReference {
-        unimplemented!()
+    fn enqueue_references(references: &[ObjectReference], _tls: VMWorkerThread) {
+        unsafe {
+            ((*UPCALLS).enqueue_references)(references.as_ptr(), references.len());
+        }
     }
 }

mmtk/src/reference_glue.rs

@@ -144,6 +144,7 @@ typedef struct {
     size_t (*number_of_mutators)();
     void (*schedule_finalizer)();
     void (*prepare_for_roots_re_scanning)();
+    void (*enqueue_references)(void** objects, size_t len);
 } OpenJDK_Upcalls;
 
 extern void openjdk_gc_init(OpenJDK_Upcalls *calls, size_t heap_size);

openjdk/mmtk.h

@@ -319,6 +319,27 @@ static void mmtk_prepare_for_roots_re_scanning() {
 #endif
 }
 
+static void mmtk_enqueue_references(void** objects, size_t len) {
+  if (len == 0) {
+    return;
+  }
+
+  MutexLocker x(Heap_lock);
+
+  oop prev = NULL;
+  for (size_t i = 0; i < len; i++) {
+    oop reff = (oop) objects[i];
+    if (prev != NULL) {
+      HeapAccess<AS_NO_KEEPALIVE>::oop_store_at(prev, java_lang_ref_Reference::discovered_offset, reff);
+    }
+    prev = reff;
+  }
+
+  oop old = Universe::swap_reference_pending_list(prev);
+  HeapAccess<AS_NO_KEEPALIVE>::oop_store_at(prev, java_lang_ref_Reference::discovered_offset, old);
+  assert(Universe::has_reference_pending_list(), "Reference pending list is empty after swap");
+}
+
 OpenJDK_Upcalls mmtk_upcalls = {
   mmtk_stop_all_mutators,
   mmtk_resume_mutators,
@@ -360,4 +381,5 @@ OpenJDK_Upcalls mmtk_upcalls = {
   mmtk_number_of_mutators,
   mmtk_schedule_finalizer,
   mmtk_prepare_for_roots_re_scanning,
+  mmtk_enqueue_references
 };

openjdk/mmtkUpcalls.cpp

@@ -76,6 +76,12 @@ void MMTkVMCompanionThread::run() {
       _reached_state = _threads_resumed;
       _lock->notify_all();
     }
+    {
+      MutexLocker x(Heap_lock);
+      if (Universe::has_reference_pending_list()) {
+        Heap_lock->notify_all();
+      }
+    }
   }
 }