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v8/include/cppgc/internal/persistent-node.h

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+// Copyright 2020 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef INCLUDE_CPPGC_INTERNAL_PERSISTENT_NODE_H_
+#define INCLUDE_CPPGC_INTERNAL_PERSISTENT_NODE_H_
+
+#include <array>
+#include <memory>
+#include <vector>
+
+#include "cppgc/internal/logging.h"
+#include "cppgc/trace-trait.h"
+#include "v8config.h"  // NOLINT(build/include_directory)
+
+namespace cppgc {
+namespace internal {
+
+class CrossThreadPersistentRegion;
+class FatalOutOfMemoryHandler;
+class RootVisitor;
+
+// PersistentNode represents a variant of two states:
+// 1) traceable node with a back pointer to the Persistent object;
+// 2) freelist entry.
+class PersistentNode final {
+ public:
+  PersistentNode() = default;
+
+  PersistentNode(const PersistentNode&) = delete;
+  PersistentNode& operator=(const PersistentNode&) = delete;
+
+  void InitializeAsUsedNode(void* owner, TraceRootCallback trace) {
+    CPPGC_DCHECK(trace);
+    owner_ = owner;
+    trace_ = trace;
+  }
+
+  void InitializeAsFreeNode(PersistentNode* next) {
+    next_ = next;
+    trace_ = nullptr;
+  }
+
+  void UpdateOwner(void* owner) {
+    CPPGC_DCHECK(IsUsed());
+    owner_ = owner;
+  }
+
+  PersistentNode* FreeListNext() const {
+    CPPGC_DCHECK(!IsUsed());
+    return next_;
+  }
+
+  void Trace(RootVisitor& root_visitor) const {
+    CPPGC_DCHECK(IsUsed());
+    trace_(root_visitor, owner_);
+  }
+
+  bool IsUsed() const { return trace_; }
+
+  void* owner() const {
+    CPPGC_DCHECK(IsUsed());
+    return owner_;
+  }
+
+ private:
+  // PersistentNode acts as a designated union:
+  // If trace_ != nullptr, owner_ points to the corresponding Persistent handle.
+  // Otherwise, next_ points to the next freed PersistentNode.
+  union {
+    void* owner_ = nullptr;
+    PersistentNode* next_;
+  };
+  TraceRootCallback trace_ = nullptr;
+};
+
+class V8_EXPORT PersistentRegionBase {
+  using PersistentNodeSlots = std::array<PersistentNode, 256u>;
+
+ public:
+  // Clears Persistent fields to avoid stale pointers after heap teardown.
+  ~PersistentRegionBase();
+
+  PersistentRegionBase(const PersistentRegionBase&) = delete;
+  PersistentRegionBase& operator=(const PersistentRegionBase&) = delete;
+
+  void Iterate(RootVisitor&);
+
+  size_t NodesInUse() const;
+
+  void ClearAllUsedNodes();
+
+ protected:
+  explicit PersistentRegionBase(const FatalOutOfMemoryHandler& oom_handler);
+
+  PersistentNode* TryAllocateNodeFromFreeList(void* owner,
+                                              TraceRootCallback trace) {
+    PersistentNode* node = nullptr;
+    if (V8_LIKELY(free_list_head_)) {
+      node = free_list_head_;
+      free_list_head_ = free_list_head_->FreeListNext();
+      CPPGC_DCHECK(!node->IsUsed());
+      node->InitializeAsUsedNode(owner, trace);
+      nodes_in_use_++;
+    }
+    return node;
+  }
+
+  void FreeNode(PersistentNode* node) {
+    CPPGC_DCHECK(node);
+    CPPGC_DCHECK(node->IsUsed());
+    node->InitializeAsFreeNode(free_list_head_);
+    free_list_head_ = node;
+    CPPGC_DCHECK(nodes_in_use_ > 0);
+    nodes_in_use_--;
+  }
+
+  PersistentNode* RefillFreeListAndAllocateNode(void* owner,
+                                                TraceRootCallback trace);
+
+ private:
+  template <typename PersistentBaseClass>
+  void ClearAllUsedNodes();
+
+  void RefillFreeList();
+
+  std::vector<std::unique_ptr<PersistentNodeSlots>> nodes_;
+  PersistentNode* free_list_head_ = nullptr;
+  size_t nodes_in_use_ = 0;
+  const FatalOutOfMemoryHandler& oom_handler_;
+
+  friend class CrossThreadPersistentRegion;
+};
+
+// Variant of PersistentRegionBase that checks whether the allocation and
+// freeing happens only on the thread that created the region.
+class V8_EXPORT PersistentRegion final : public PersistentRegionBase {
+ public:
+  explicit PersistentRegion(const FatalOutOfMemoryHandler&);
+  // Clears Persistent fields to avoid stale pointers after heap teardown.
+  ~PersistentRegion() = default;
+
+  PersistentRegion(const PersistentRegion&) = delete;
+  PersistentRegion& operator=(const PersistentRegion&) = delete;
+
+  V8_INLINE PersistentNode* AllocateNode(void* owner, TraceRootCallback trace) {
+    CPPGC_DCHECK(IsCreationThread());
+    auto* node = TryAllocateNodeFromFreeList(owner, trace);
+    if (V8_LIKELY(node)) return node;
+
+    // Slow path allocation allows for checking thread correspondence.
+    CPPGC_CHECK(IsCreationThread());
+    return RefillFreeListAndAllocateNode(owner, trace);
+  }
+
+  V8_INLINE void FreeNode(PersistentNode* node) {
+    CPPGC_DCHECK(IsCreationThread());
+    PersistentRegionBase::FreeNode(node);
+  }
+
+ private:
+  bool IsCreationThread();
+
+  int creation_thread_id_;
+};
+
+// CrossThreadPersistent uses PersistentRegionBase but protects it using this
+// lock when needed.
+class V8_EXPORT PersistentRegionLock final {
+ public:
+  PersistentRegionLock();
+  ~PersistentRegionLock();
+
+  static void AssertLocked();
+};
+
+// Variant of PersistentRegionBase that checks whether the PersistentRegionLock
+// is locked.
+class V8_EXPORT CrossThreadPersistentRegion final
+    : protected PersistentRegionBase {
+ public:
+  explicit CrossThreadPersistentRegion(const FatalOutOfMemoryHandler&);
+  // Clears Persistent fields to avoid stale pointers after heap teardown.
+  ~CrossThreadPersistentRegion();
+
+  CrossThreadPersistentRegion(const CrossThreadPersistentRegion&) = delete;
+  CrossThreadPersistentRegion& operator=(const CrossThreadPersistentRegion&) =
+      delete;
+
+  V8_INLINE PersistentNode* AllocateNode(void* owner, TraceRootCallback trace) {
+    PersistentRegionLock::AssertLocked();
+    auto* node = TryAllocateNodeFromFreeList(owner, trace);
+    if (V8_LIKELY(node)) return node;
+
+    return RefillFreeListAndAllocateNode(owner, trace);
+  }
+
+  V8_INLINE void FreeNode(PersistentNode* node) {
+    PersistentRegionLock::AssertLocked();
+    PersistentRegionBase::FreeNode(node);
+  }
+
+  void Iterate(RootVisitor&);
+
+  size_t NodesInUse() const;
+
+  void ClearAllUsedNodes();
+};
+
+}  // namespace internal
+
+}  // namespace cppgc
+
+#endif  // INCLUDE_CPPGC_INTERNAL_PERSISTENT_NODE_H_