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