1 //==- llvm/ADT/IntrusiveRefCntPtr.h - Smart Refcounting Pointer --*- C++ -*-==// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 // 9 // This file defines the RefCountedBase, ThreadSafeRefCountedBase, and 10 // IntrusiveRefCntPtr classes. 11 // 12 // IntrusiveRefCntPtr is a smart pointer to an object which maintains a 13 // reference count. (ThreadSafe)RefCountedBase is a mixin class that adds a 14 // refcount member variable and methods for updating the refcount. An object 15 // that inherits from (ThreadSafe)RefCountedBase deletes itself when its 16 // refcount hits zero. 17 // 18 // For example: 19 // 20 // class MyClass : public RefCountedBase<MyClass> {}; 21 // 22 // void foo() { 23 // // Constructing an IntrusiveRefCntPtr increases the pointee's refcount by 24 // // 1 (from 0 in this case). 25 // IntrusiveRefCntPtr<MyClass> Ptr1(new MyClass()); 26 // 27 // // Copying an IntrusiveRefCntPtr increases the pointee's refcount by 1. 28 // IntrusiveRefCntPtr<MyClass> Ptr2(Ptr1); 29 // 30 // // Constructing an IntrusiveRefCntPtr has no effect on the object's 31 // // refcount. After a move, the moved-from pointer is null. 32 // IntrusiveRefCntPtr<MyClass> Ptr3(std::move(Ptr1)); 33 // assert(Ptr1 == nullptr); 34 // 35 // // Clearing an IntrusiveRefCntPtr decreases the pointee's refcount by 1. 36 // Ptr2.reset(); 37 // 38 // // The object deletes itself when we return from the function, because 39 // // Ptr3's destructor decrements its refcount to 0. 40 // } 41 // 42 // You can use IntrusiveRefCntPtr with isa<T>(), dyn_cast<T>(), etc.: 43 // 44 // IntrusiveRefCntPtr<MyClass> Ptr(new MyClass()); 45 // OtherClass *Other = dyn_cast<OtherClass>(Ptr); // Ptr.get() not required 46 // 47 // IntrusiveRefCntPtr works with any class that 48 // 49 // - inherits from (ThreadSafe)RefCountedBase, 50 // - has Retain() and Release() methods, or 51 // - specializes IntrusiveRefCntPtrInfo. 52 // 53 //===----------------------------------------------------------------------===// 54 55 #ifndef LLVM_ADT_INTRUSIVEREFCNTPTR_H 56 #define LLVM_ADT_INTRUSIVEREFCNTPTR_H 57 58 #include <atomic> 59 #include <cassert> 60 #include <cstddef> 61 62 namespace llvm { 63 64 /// A CRTP mixin class that adds reference counting to a type. 65 /// 66 /// The lifetime of an object which inherits from RefCountedBase is managed by 67 /// calls to Release() and Retain(), which increment and decrement the object's 68 /// refcount, respectively. When a Release() call decrements the refcount to 0, 69 /// the object deletes itself. 70 template <class Derived> class RefCountedBase { 71 mutable unsigned RefCount = 0; 72 73 public: 74 RefCountedBase() = default; 75 RefCountedBase(const RefCountedBase &) {} 76 77 void Retain() const { ++RefCount; } 78 79 void Release() const { 80 assert(RefCount > 0 && "Reference count is already zero."); 81 if (--RefCount == 0) 82 delete static_cast<const Derived *>(this); 83 } 84 }; 85 86 /// A thread-safe version of \c RefCountedBase. 87 template <class Derived> class ThreadSafeRefCountedBase { 88 mutable std::atomic<int> RefCount; 89 90 protected: 91 ThreadSafeRefCountedBase() : RefCount(0) {} 92 93 public: 94 void Retain() const { RefCount.fetch_add(1, std::memory_order_relaxed); } 95 96 void Release() const { 97 int NewRefCount = RefCount.fetch_sub(1, std::memory_order_acq_rel) - 1; 98 assert(NewRefCount >= 0 && "Reference count was already zero."); 99 if (NewRefCount == 0) 100 delete static_cast<const Derived *>(this); 101 } 102 }; 103 104 /// Class you can specialize to provide custom retain/release functionality for 105 /// a type. 106 /// 107 /// Usually specializing this class is not necessary, as IntrusiveRefCntPtr 108 /// works with any type which defines Retain() and Release() functions -- you 109 /// can define those functions yourself if RefCountedBase doesn't work for you. 110 /// 111 /// One case when you might want to specialize this type is if you have 112 /// - Foo.h defines type Foo and includes Bar.h, and 113 /// - Bar.h uses IntrusiveRefCntPtr<Foo> in inline functions. 114 /// 115 /// Because Foo.h includes Bar.h, Bar.h can't include Foo.h in order to pull in 116 /// the declaration of Foo. Without the declaration of Foo, normally Bar.h 117 /// wouldn't be able to use IntrusiveRefCntPtr<Foo>, which wants to call 118 /// T::Retain and T::Release. 119 /// 120 /// To resolve this, Bar.h could include a third header, FooFwd.h, which 121 /// forward-declares Foo and specializes IntrusiveRefCntPtrInfo<Foo>. Then 122 /// Bar.h could use IntrusiveRefCntPtr<Foo>, although it still couldn't call any 123 /// functions on Foo itself, because Foo would be an incomplete type. 124 template <typename T> struct IntrusiveRefCntPtrInfo { 125 static void retain(T *obj) { obj->Retain(); } 126 static void release(T *obj) { obj->Release(); } 127 }; 128 129 /// A smart pointer to a reference-counted object that inherits from 130 /// RefCountedBase or ThreadSafeRefCountedBase. 131 /// 132 /// This class increments its pointee's reference count when it is created, and 133 /// decrements its refcount when it's destroyed (or is changed to point to a 134 /// different object). 135 template <typename T> class IntrusiveRefCntPtr { 136 T *Obj = nullptr; 137 138 public: 139 using element_type = T; 140 141 explicit IntrusiveRefCntPtr() = default; 142 IntrusiveRefCntPtr(T *obj) : Obj(obj) { retain(); } 143 IntrusiveRefCntPtr(const IntrusiveRefCntPtr &S) : Obj(S.Obj) { retain(); } 144 IntrusiveRefCntPtr(IntrusiveRefCntPtr &&S) : Obj(S.Obj) { S.Obj = nullptr; } 145 146 template <class X> 147 IntrusiveRefCntPtr(IntrusiveRefCntPtr<X> &&S) : Obj(S.get()) { 148 S.Obj = nullptr; 149 } 150 151 template <class X> 152 IntrusiveRefCntPtr(const IntrusiveRefCntPtr<X> &S) : Obj(S.get()) { 153 retain(); 154 } 155 156 ~IntrusiveRefCntPtr() { release(); } 157 158 IntrusiveRefCntPtr &operator=(IntrusiveRefCntPtr S) { 159 swap(S); 160 return *this; 161 } 162 163 T &operator*() const { return *Obj; } 164 T *operator->() const { return Obj; } 165 T *get() const { return Obj; } 166 explicit operator bool() const { return Obj; } 167 168 void swap(IntrusiveRefCntPtr &other) { 169 T *tmp = other.Obj; 170 other.Obj = Obj; 171 Obj = tmp; 172 } 173 174 void reset() { 175 release(); 176 Obj = nullptr; 177 } 178 179 void resetWithoutRelease() { Obj = nullptr; } 180 181 private: 182 void retain() { 183 if (Obj) 184 IntrusiveRefCntPtrInfo<T>::retain(Obj); 185 } 186 187 void release() { 188 if (Obj) 189 IntrusiveRefCntPtrInfo<T>::release(Obj); 190 } 191 192 template <typename X> friend class IntrusiveRefCntPtr; 193 }; 194 195 template <class T, class U> 196 inline bool operator==(const IntrusiveRefCntPtr<T> &A, 197 const IntrusiveRefCntPtr<U> &B) { 198 return A.get() == B.get(); 199 } 200 201 template <class T, class U> 202 inline bool operator!=(const IntrusiveRefCntPtr<T> &A, 203 const IntrusiveRefCntPtr<U> &B) { 204 return A.get() != B.get(); 205 } 206 207 template <class T, class U> 208 inline bool operator==(const IntrusiveRefCntPtr<T> &A, U *B) { 209 return A.get() == B; 210 } 211 212 template <class T, class U> 213 inline bool operator!=(const IntrusiveRefCntPtr<T> &A, U *B) { 214 return A.get() != B; 215 } 216 217 template <class T, class U> 218 inline bool operator==(T *A, const IntrusiveRefCntPtr<U> &B) { 219 return A == B.get(); 220 } 221 222 template <class T, class U> 223 inline bool operator!=(T *A, const IntrusiveRefCntPtr<U> &B) { 224 return A != B.get(); 225 } 226 227 template <class T> 228 bool operator==(std::nullptr_t A, const IntrusiveRefCntPtr<T> &B) { 229 return !B; 230 } 231 232 template <class T> 233 bool operator==(const IntrusiveRefCntPtr<T> &A, std::nullptr_t B) { 234 return B == A; 235 } 236 237 template <class T> 238 bool operator!=(std::nullptr_t A, const IntrusiveRefCntPtr<T> &B) { 239 return !(A == B); 240 } 241 242 template <class T> 243 bool operator!=(const IntrusiveRefCntPtr<T> &A, std::nullptr_t B) { 244 return !(A == B); 245 } 246 247 // Make IntrusiveRefCntPtr work with dyn_cast, isa, and the other idioms from 248 // Casting.h. 249 template <typename From> struct simplify_type; 250 251 template <class T> struct simplify_type<IntrusiveRefCntPtr<T>> { 252 using SimpleType = T *; 253 254 static SimpleType getSimplifiedValue(IntrusiveRefCntPtr<T> &Val) { 255 return Val.get(); 256 } 257 }; 258 259 template <class T> struct simplify_type<const IntrusiveRefCntPtr<T>> { 260 using SimpleType = /*const*/ T *; 261 262 static SimpleType getSimplifiedValue(const IntrusiveRefCntPtr<T> &Val) { 263 return Val.get(); 264 } 265 }; 266 267 } // end namespace llvm 268 269 #endif // LLVM_ADT_INTRUSIVEREFCNTPTR_H 270