Weak references. (#1459)

backend.native/tests/build.gradle

@@ -2076,6 +2076,11 @@ task memory_escape2(type: RunKonanTest) {
     source = "runtime/memory/escape2.kt"
 }
 
+task memory_weak0(type: RunKonanTest) {
+    goldValue = "Data(s=Hello)\nnull\nOK\n"
+    source = "runtime/memory/weak0.kt"
+}
+
 task mpp1(type: RunStandaloneKonanTest) {
     source = "codegen/mpp/mpp1.kt"
     flags = ['-tr', '-Xmulti-platform']

backend.native/tests/runtime/memory/weak0.kt

@@ -0,0 +1,35 @@
+package runtime.memory.weak0
+
+import kotlin.test.*
+import konan.ref.*
+
+data class Data(val s: String)
+
+fun localWeak(): WeakReference<Data>  {
+    val x = Data("Hello")
+    val weak = WeakReference(x)
+
+    println(weak.get())
+    return weak
+}
+
+fun multiWeak(): Array<WeakReference<Data>>  {
+    val x = Data("Hello")
+    val weaks = Array(100, { WeakReference(x) } )
+    weaks.forEach {
+        it -> if (it.get()?.s != "Hello") throw Error("bad reference")
+    }
+    return weaks
+}
+
+@Test fun runTest() {
+    val weak = localWeak()
+    val value = weak.get()
+    println(value?.toString())
+
+    val weaks = multiWeak()
+    weaks.forEach {
+        it -> if (it.get()?.s != null) throw Error("not null")
+    }
+    println("OK")
+}

runtime/src/main/cpp/Memory.cpp

@@ -464,7 +464,7 @@ void DeinitInstanceBody(const TypeInfo* typeInfo, void* body) {
 namespace {
 
 template<typename func>
-void traverseContainerObjectFields(ContainerHeader* container, func process) {
+inline void traverseContainerObjectFields(ContainerHeader* container, func process) {
   ObjHeader* obj = reinterpret_cast<ObjHeader*>(container + 1);
   for (int object = 0; object < container->objectCount(); object++) {
     const TypeInfo* typeInfo = obj->type_info();
@@ -485,7 +485,7 @@ void traverseContainerObjectFields(ContainerHeader* container, func process) {
 }
 
 template<typename func>
-void traverseContainerReferredObjects(ContainerHeader* container, func process) {
+inline void traverseContainerReferredObjects(ContainerHeader* container, func process) {
   traverseContainerObjectFields(container, [process](ObjHeader** location) {
     ObjHeader* ref = *location;
     if (ref != nullptr) process(ref);
@@ -510,9 +510,6 @@ inline void processFinalizerQueue(MemoryState* state) {
 #if TRACE_MEMORY
     state->containers->erase(container);
 #endif
-    if (!isAggregatingFrozenContainer(container))
-      runDeallocationHooks(container);
-
     CONTAINER_DESTROY_EVENT(state, container)
     konanFreeMemory(container);
     atomicAdd(&allocCount, -1);
@@ -835,8 +832,12 @@ MetaObjHeader* ObjHeader::createMetaObject(TypeInfo** location) {
 }
 
 void ObjHeader::destroyMetaObject(TypeInfo** location) {
-  TypeInfo* meta = *location;
+  MetaObjHeader* meta = *(reinterpret_cast<MetaObjHeader**>(location));
-  *location = nullptr;
+  *const_cast<const TypeInfo**>(location) = meta->typeInfo_;
+  if (meta->counter_ != nullptr) {
+    WeakReferenceCounterClear(meta->counter_);
+    UpdateRef(&meta->counter_, nullptr);
+  }
   konanFreeMemory(meta);
 }
 
@@ -890,29 +891,29 @@ void FreeAggregatingFrozenContainer(ContainerHeader* container) {
   MEMORY_LOG("Freeing subcontainers done\n");
 }
 
-void FreeContainer(ContainerHeader* header) {
+void FreeContainer(ContainerHeader* container) {
-  RuntimeAssert(!header->permanent(), "this kind of container shalln't be freed");
+  RuntimeAssert(!container->permanent(), "this kind of container shalln't be freed");
   auto state = memoryState;
 
-  CONTAINER_FREE_EVENT(state, header)
+  CONTAINER_FREE_EVENT(state, container)
 
-  if (isAggregatingFrozenContainer(header)) {
+  if (isAggregatingFrozenContainer(container)) {
-    FreeAggregatingFrozenContainer(header);
+    FreeAggregatingFrozenContainer(container);
     return;
+  } else {
+    runDeallocationHooks(container);
   }
 
   // Now let's clean all object's fields in this container.
-  traverseContainerObjectFields(header, [](ObjHeader** location) {
+  traverseContainerObjectFields(container, [](ObjHeader** location) {
     UpdateRef(location, nullptr);
   });
 
   // And release underlying memory.
-  if (!isFreeable(header)) {
+  if (isFreeable(container)) {
-    runDeallocationHooks(header);
+    container->setColor(CONTAINER_TAG_GC_BLACK);
-  } else {
+    if (!container->buffered())
-    header->setColor(CONTAINER_TAG_GC_BLACK);
+      scheduleDestroyContainer(state, container, false);
-    if (!header->buffered())
-      scheduleDestroyContainer(state, header, false);
   }
 }
 
@@ -1232,7 +1233,8 @@ void* GetReservedObjectTail(ObjHeader* obj) {
 void SetRef(ObjHeader** location, const ObjHeader* object) {
   MEMORY_LOG("SetRef *%p: %p\n", location, object)
   *const_cast<const ObjHeader**>(location) = object;
-  AddRef(object);
+  if (object != nullptr)
+    AddRef(object);
 }
 
 ObjHeader** GetReturnSlotIfArena(ObjHeader** returnSlot, ObjHeader** localSlot) {
@@ -1248,16 +1250,6 @@ ObjHeader** GetParamSlotIfArena(ObjHeader* param, ObjHeader** localSlot) {
   return reinterpret_cast<ObjHeader**>(reinterpret_cast<uintptr_t>(&chunk->arena) | ARENA_BIT);
 }
 
-void UpdateReturnRef(ObjHeader** returnSlot, const ObjHeader* object) {
-  if (isArenaSlot(returnSlot)) {
-    // Not a subject of reference counting.
-    if (object == nullptr || !isRefCounted(object)) return;
-    auto arena = initedArena(asArenaSlot(returnSlot));
-    returnSlot = arena->getSlot();
-  }
-  UpdateRef(returnSlot, object);
-}
-
 void UpdateRef(ObjHeader** location, const ObjHeader* object) {
   RuntimeAssert(!isArenaSlot(location), "must not be a slot");
   ObjHeader* old = *location;
@@ -1273,6 +1265,35 @@ void UpdateRef(ObjHeader** location, const ObjHeader* object) {
   }
 }
 
+void UpdateReturnRef(ObjHeader** returnSlot, const ObjHeader* object) {
+  if (isArenaSlot(returnSlot)) {
+    // Not a subject of reference counting.
+    if (object == nullptr || !isRefCounted(object)) return;
+    auto arena = initedArena(asArenaSlot(returnSlot));
+    returnSlot = arena->getSlot();
+  }
+  UpdateRef(returnSlot, object);
+}
+
+void UpdateRefIfNull(ObjHeader** location, const ObjHeader* object) {
+  if (object != nullptr) {
+#if KONAN_NO_THREADS
+    ObjHeader* old = *location;
+    if (old == nullptr) {
+      AddRef(object);
+      *const_cast<const ObjHeader**>(location) = object;
+    }
+#else
+    AddRef(object);
+    auto old = __sync_val_compare_and_swap(location, nullptr, const_cast<ObjHeader*>(object));
+    if (old != nullptr) {
+      // Failed to store, was not null.
+      ReleaseRef(object);
+    }
+#endif
+  }
+}
+
 void EnterFrame(ObjHeader** start, int parameters, int count) {
   MEMORY_LOG("EnterFrame %p .. %p\n", start, start + count + parameters)
 }

runtime/src/main/cpp/Memory.h

@@ -242,6 +242,8 @@ struct ArrayHeader {
 struct MetaObjHeader {
   // Pointer to the type info. Must be first, to match ArrayHeader and ObjHeader layout.
   const TypeInfo* typeInfo_;
+  // Strong reference to counter object.
+  ObjHeader* counter_;
 };
 
 inline uint32_t ArrayDataSizeBytes(const ArrayHeader* obj) {
@@ -394,6 +396,10 @@ bool HasReservedObjectTail(ObjHeader* obj) RUNTIME_NOTHROW;
 // Returns the pointer to the reserved space, `HasReservedObjectTail(obj)` must be true.
 void* GetReservedObjectTail(ObjHeader* obj) RUNTIME_NOTHROW;
 
+// Weak reference operations.
+// Atomically clears counter object reference.
+void WeakReferenceCounterClear(ObjHeader* counter);
+
 //
 // Object reference management.
 //
@@ -420,6 +426,8 @@ void* GetReservedObjectTail(ObjHeader* obj) RUNTIME_NOTHROW;
 void SetRef(ObjHeader** location, const ObjHeader* object) RUNTIME_NOTHROW;
 // Updates location.
 void UpdateRef(ObjHeader** location, const ObjHeader* object) RUNTIME_NOTHROW;
+// Updates location if it is null, atomically.
+void UpdateRefIfNull(ObjHeader** location, const ObjHeader* object) RUNTIME_NOTHROW;
 // Updates reference in return slot.
 void UpdateReturnRef(ObjHeader** returnSlot, const ObjHeader* object) RUNTIME_NOTHROW;
 // Optimization: release all references in range.

runtime/src/main/cpp/Weak.cpp

@@ -0,0 +1,85 @@
+/*
+ * Copyright 2010-2018 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+#include "Memory.h"
+
+namespace {
+
+// TODO: an ugly hack with fixed offsets.
+constexpr int referredOffset = 0;
+constexpr int lockOffset = sizeof(void*);
+
+#if !KONAN_NO_THREADS
+
+inline void lock(int32_t* address) {
+    while (__sync_val_compare_and_swap(address, 0, 1) == 1);
+}
+
+inline void unlock(int32_t* address) {
+    int old = __sync_val_compare_and_swap(address, 1, 0);
+    RuntimeAssert(old == 1, "Incorrect lock state");
+}
+
+#endif
+
+}  // namespace
+
+extern "C" {
+
+OBJ_GETTER(makeWeakReferenceCounter, void*);
+
+// See Weak.kt for implementation details.
+// Retrieve link on the counter object.
+OBJ_GETTER(Konan_getWeakReferenceCounter, ObjHeader* referred) {
+  MetaObjHeader* meta = referred->meta_object();
+  if (meta->counter_ == nullptr) {
+     ObjHolder counterHolder;
+     // Cast unneeded, just to emphasize we store an object reference as void*.
+     ObjHeader* counter = makeWeakReferenceCounter(reinterpret_cast<void*>(referred), counterHolder.slot());
+     UpdateRefIfNull(&meta->counter_, counter);
+  }
+  RETURN_OBJ(meta->counter_);
+}
+
+// Materialize a weak reference to either null or the real reference.
+OBJ_GETTER(Konan_WeakReferenceCounter_get, ObjHeader* counter) {
+  ObjHeader** referredAddress = reinterpret_cast<ObjHeader**>(reinterpret_cast<char*>(counter + 1) + referredOffset);
+#if KONAN_NO_THREADS
+  RETURN_OBJ(*referredAddress);
+#else
+  int32_t* lockAddress = reinterpret_cast<int32_t*>(reinterpret_cast<char*>(counter + 1) + lockOffset);
+  // Spinlock.
+  lock(lockAddress);
+  ObjHolder holder(*referredAddress);
+  unlock(lockAddress);
+  RETURN_OBJ(holder.obj());
+#endif
+}
+
+void WeakReferenceCounterClear(ObjHeader* counter) {
+  ObjHeader** referredAddress = reinterpret_cast<ObjHeader**>(reinterpret_cast<char*>(counter + 1) + referredOffset);
+  // Note, that we don't do UpdateRef here, as reference is weak.
+#if KONAN_NO_THREADS
+  *referredAddress = nullptr;
+#else
+  int32_t* lockAddress = reinterpret_cast<int32_t*>(reinterpret_cast<char*>(counter + 1) + lockOffset);
+  // Spinlock.
+  lock(lockAddress);
+  *referredAddress = nullptr;
+  unlock(lockAddress);
+#endif
+}
+
+}  // extern "C"

runtime/src/main/kotlin/konan/ref/Weak.kt

@@ -0,0 +1,52 @@
+/*
+ * Copyright 2010-2018 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package konan.ref
+
+/**
+ * Class WeakReference encapsulates weak reference to an object, which could be used to either
+ * retrieve a strong reference to an object, or return null, if object was already destoyed by
+ * the memory manager.
+ */
+class WeakReference<T> {
+    /**
+     * Creates a weak reference object pointing to an object. Weak reference doesn't prevent
+     * removing object, and is nullified once object is collected.
+     */
+    constructor(referred: T) {
+        if (referred == null) throw Error("Weak reference to null?")
+        pointer = getWeakReferenceCounter(referred)
+    }
+
+    /**
+     * Backing store for the object pointer, inaccessible directly.
+     */
+    @PublishedApi
+    internal var pointer: WeakReferenceCounter?
+
+    /**
+     * Clears reference to an object.
+     */
+    public fun clear() {
+        pointer = null
+    }
+}
+
+/**
+ * Returns either reference to an object or null, if it was collected.
+ */
+@Suppress("NON_PUBLIC_CALL_FROM_PUBLIC_INLINE")
+public inline fun <reified T> WeakReference<T>.get() = pointer?.get() as T?

runtime/src/main/kotlin/konan/ref/WeakPrivate.kt

@@ -0,0 +1,60 @@
+/*
+ * Copyright 2010-2018 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package konan.ref
+
+import kotlinx.cinterop.COpaquePointer
+import konan.internal.ExportForCppRuntime
+
+/**
+ *   Theory of operations:
+ *
+ *  Weak references in Kotlin/Native are implemented in the following way. Whenever weak reference to an
+ * object is created, we atomically modify type info pointer in the object to point into a metaobject.
+ * This metaobject contains a strong reference to the counter object (instance of WeakReferenceCounter class).
+ * Every other weak reference contains a strong reference to the counter object.
+ *
+ *         [weak1]  [weak2]
+ *             \      /
+ *             V     V
+ *     .......[Counter] <----
+ *     .                     |
+ *     .                     |
+ *      ->[Object] -> [Meta]-
+ *
+ *   References from weak reference objects to the counter and from the metaobject to the counter are strong,
+ *  and from the counter to the object is nullably weak. So whenever an object dies, if it has a metaobject,
+ *  it is traversed to find a counter object, and atomically nullify reference to the object. Afterward, all attempts
+ *  to get the object would yield null.
+ */
+
+// Clear holding the counter object, which refers to the actual object.
+internal class WeakReferenceCounter(var referred: COpaquePointer?) {
+    // Spinlock, potentially taken when materializing or removing 'referred' object.
+    var lock: Int = 0
+
+    @SymbolName("Konan_WeakReferenceCounter_get")
+    internal external fun get(): Any?
+}
+
+// Get a counter from non-null object.
+@SymbolName("Konan_getWeakReferenceCounter")
+external internal fun getWeakReferenceCounter(referent: Any): WeakReferenceCounter
+
+// Create a counter object.
+@ExportForCppRuntime
+internal fun makeWeakReferenceCounter(referred: COpaquePointer) = WeakReferenceCounter(referred)
+