1 //===----------------------------------------------------------------------===// 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 #include <__config> 10 #ifdef _LIBCPP_DEPRECATED_ABI_LEGACY_LIBRARY_DEFINITIONS_FOR_INLINE_FUNCTIONS 11 # define _LIBCPP_SHARED_PTR_DEFINE_LEGACY_INLINE_FUNCTIONS 12 #endif 13 14 #include <memory> 15 16 #ifndef _LIBCPP_HAS_NO_THREADS 17 # include <mutex> 18 # include <thread> 19 # if defined(__ELF__) && defined(_LIBCPP_LINK_PTHREAD_LIB) 20 # pragma comment(lib, "pthread") 21 # endif 22 #endif 23 24 #include "include/atomic_support.h" 25 26 _LIBCPP_BEGIN_NAMESPACE_STD 27 28 const allocator_arg_t allocator_arg = allocator_arg_t(); 29 30 bad_weak_ptr::~bad_weak_ptr() noexcept {} 31 32 const char* 33 bad_weak_ptr::what() const noexcept 34 { 35 return "bad_weak_ptr"; 36 } 37 38 __shared_count::~__shared_count() 39 { 40 } 41 42 __shared_weak_count::~__shared_weak_count() 43 { 44 } 45 46 #if defined(_LIBCPP_SHARED_PTR_DEFINE_LEGACY_INLINE_FUNCTIONS) 47 void 48 __shared_count::__add_shared() noexcept 49 { 50 __libcpp_atomic_refcount_increment(__shared_owners_); 51 } 52 53 bool 54 __shared_count::__release_shared() noexcept 55 { 56 if (__libcpp_atomic_refcount_decrement(__shared_owners_) == -1) 57 { 58 __on_zero_shared(); 59 return true; 60 } 61 return false; 62 } 63 64 void 65 __shared_weak_count::__add_shared() noexcept 66 { 67 __shared_count::__add_shared(); 68 } 69 70 void 71 __shared_weak_count::__add_weak() noexcept 72 { 73 __libcpp_atomic_refcount_increment(__shared_weak_owners_); 74 } 75 76 void 77 __shared_weak_count::__release_shared() noexcept 78 { 79 if (__shared_count::__release_shared()) 80 __release_weak(); 81 } 82 83 #endif // _LIBCPP_SHARED_PTR_DEFINE_LEGACY_INLINE_FUNCTIONS 84 85 void 86 __shared_weak_count::__release_weak() noexcept 87 { 88 // NOTE: The acquire load here is an optimization of the very 89 // common case where a shared pointer is being destructed while 90 // having no other contended references. 91 // 92 // BENEFIT: We avoid expensive atomic stores like XADD and STREX 93 // in a common case. Those instructions are slow and do nasty 94 // things to caches. 95 // 96 // IS THIS SAFE? Yes. During weak destruction, if we see that we 97 // are the last reference, we know that no-one else is accessing 98 // us. If someone were accessing us, then they would be doing so 99 // while the last shared / weak_ptr was being destructed, and 100 // that's undefined anyway. 101 // 102 // If we see anything other than a 0, then we have possible 103 // contention, and need to use an atomicrmw primitive. 104 // The same arguments don't apply for increment, where it is legal 105 // (though inadvisable) to share shared_ptr references between 106 // threads, and have them all get copied at once. The argument 107 // also doesn't apply for __release_shared, because an outstanding 108 // weak_ptr::lock() could read / modify the shared count. 109 if (__libcpp_atomic_load(&__shared_weak_owners_, _AO_Acquire) == 0) 110 { 111 // no need to do this store, because we are about 112 // to destroy everything. 113 //__libcpp_atomic_store(&__shared_weak_owners_, -1, _AO_Release); 114 __on_zero_shared_weak(); 115 } 116 else if (__libcpp_atomic_refcount_decrement(__shared_weak_owners_) == -1) 117 __on_zero_shared_weak(); 118 } 119 120 __shared_weak_count* 121 __shared_weak_count::lock() noexcept 122 { 123 long object_owners = __libcpp_atomic_load(&__shared_owners_); 124 while (object_owners != -1) 125 { 126 if (__libcpp_atomic_compare_exchange(&__shared_owners_, 127 &object_owners, 128 object_owners+1)) 129 return this; 130 } 131 return nullptr; 132 } 133 134 const void* 135 __shared_weak_count::__get_deleter(const type_info&) const noexcept 136 { 137 return nullptr; 138 } 139 140 #if !defined(_LIBCPP_HAS_NO_THREADS) 141 142 static constexpr std::size_t __sp_mut_count = 16; 143 static constinit __libcpp_mutex_t mut_back[__sp_mut_count] = 144 { 145 _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, 146 _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, 147 _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, 148 _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER 149 }; 150 151 _LIBCPP_CONSTEXPR __sp_mut::__sp_mut(void* p) noexcept 152 : __lx(p) 153 { 154 } 155 156 void 157 __sp_mut::lock() noexcept 158 { 159 auto m = static_cast<__libcpp_mutex_t*>(__lx); 160 unsigned count = 0; 161 while (!__libcpp_mutex_trylock(m)) 162 { 163 if (++count > 16) 164 { 165 __libcpp_mutex_lock(m); 166 break; 167 } 168 this_thread::yield(); 169 } 170 } 171 172 void 173 __sp_mut::unlock() noexcept 174 { 175 __libcpp_mutex_unlock(static_cast<__libcpp_mutex_t*>(__lx)); 176 } 177 178 __sp_mut& 179 __get_sp_mut(const void* p) 180 { 181 static constinit __sp_mut muts[__sp_mut_count] = { 182 &mut_back[ 0], &mut_back[ 1], &mut_back[ 2], &mut_back[ 3], 183 &mut_back[ 4], &mut_back[ 5], &mut_back[ 6], &mut_back[ 7], 184 &mut_back[ 8], &mut_back[ 9], &mut_back[10], &mut_back[11], 185 &mut_back[12], &mut_back[13], &mut_back[14], &mut_back[15] 186 }; 187 return muts[hash<const void*>()(p) & (__sp_mut_count-1)]; 188 } 189 190 #endif // !defined(_LIBCPP_HAS_NO_THREADS) 191 192 void* 193 align(size_t alignment, size_t size, void*& ptr, size_t& space) 194 { 195 void* r = nullptr; 196 if (size <= space) 197 { 198 char* p1 = static_cast<char*>(ptr); 199 char* p2 = reinterpret_cast<char*>(reinterpret_cast<size_t>(p1 + (alignment - 1)) & -alignment); 200 size_t d = static_cast<size_t>(p2 - p1); 201 if (d <= space - size) 202 { 203 r = p2; 204 ptr = r; 205 space -= d; 206 } 207 } 208 return r; 209 } 210 211 _LIBCPP_END_NAMESPACE_STD 212