libcxx/src/memory.cpp

   1 //===------------------------ memory.cpp ----------------------------------===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is dual licensed under the MIT and the University of Illinois Open
   6 // Source Licenses. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9
  10 #define _LIBCPP_BUILDING_MEMORY
  11 #include "memory"
  12 #ifndef _LIBCPP_HAS_NO_THREADS
  13 #include "mutex"
  14 #include "thread"
  15 #endif
  16 #include "include/atomic_support.h"
  17
  18 _LIBCPP_BEGIN_NAMESPACE_STD
  19
  20 const allocator_arg_t allocator_arg = allocator_arg_t();
  21
  22 bad_weak_ptr::~bad_weak_ptr() _NOEXCEPT {}
  23
  24 const char*
  25 bad_weak_ptr::what() const _NOEXCEPT
  26 {
  27     return "bad_weak_ptr";
  28 }
  29
  30 __shared_count::~__shared_count()
  31 {
  32 }
  33
  34 __shared_weak_count::~__shared_weak_count()
  35 {
  36 }
  37
  38 #if defined(_LIBCPP_DEPRECATED_ABI_LEGACY_LIBRARY_DEFINITIONS_FOR_INLINE_FUNCTIONS)
  39 void
  40 __shared_count::__add_shared() _NOEXCEPT
  41 {
  42     __libcpp_atomic_refcount_increment(__shared_owners_);
  43 }
  44
  45 bool
  46 __shared_count::__release_shared() _NOEXCEPT
  47 {
  48     if (__libcpp_atomic_refcount_decrement(__shared_owners_) == -1)
  49     {
  50         __on_zero_shared();
  51         return true;
  52     }
  53     return false;
  54 }
  55
  56 void
  57 __shared_weak_count::__add_shared() _NOEXCEPT
  58 {
  59     __shared_count::__add_shared();
  60 }
  61
  62 void
  63 __shared_weak_count::__add_weak() _NOEXCEPT
  64 {
  65     __libcpp_atomic_refcount_increment(__shared_weak_owners_);
  66 }
  67
  68 void
  69 __shared_weak_count::__release_shared() _NOEXCEPT
  70 {
  71     if (__shared_count::__release_shared())
  72         __release_weak();
  73 }
  74
  75 #endif // _LIBCPP_DEPRECATED_ABI_LEGACY_LIBRARY_DEFINITIONS_FOR_INLINE_FUNCTIONS
  76
  77 void
  78 __shared_weak_count::__release_weak() _NOEXCEPT
  79 {
  80     // NOTE: The acquire load here is an optimization of the very
  81     // common case where a shared pointer is being destructed while
  82     // having no other contended references.
  83     //
  84     // BENEFIT: We avoid expensive atomic stores like XADD and STREX
  85     // in a common case.  Those instructions are slow and do nasty
  86     // things to caches.
  87     //
  88     // IS THIS SAFE?  Yes.  During weak destruction, if we see that we
  89     // are the last reference, we know that no-one else is accessing
  90     // us. If someone were accessing us, then they would be doing so
  91     // while the last shared / weak_ptr was being destructed, and
  92     // that's undefined anyway.
  93     //
  94     // If we see anything other than a 0, then we have possible
  95     // contention, and need to use an atomicrmw primitive.
  96     // The same arguments don't apply for increment, where it is legal
  97     // (though inadvisable) to share shared_ptr references between
  98     // threads, and have them all get copied at once.  The argument
  99     // also doesn't apply for __release_shared, because an outstanding
 100     // weak_ptr::lock() could read / modify the shared count.
 101     if (__libcpp_atomic_load(&__shared_weak_owners_, _AO_Acquire) == 0)
 102     {
 103         // no need to do this store, because we are about
 104         // to destroy everything.
 105         //__libcpp_atomic_store(&__shared_weak_owners_, -1, _AO_Release);
 106         __on_zero_shared_weak();
 107     }
 108     else if (__libcpp_atomic_refcount_decrement(__shared_weak_owners_) == -1)
 109         __on_zero_shared_weak();
 110 }
 111
 112 __shared_weak_count*
 113 __shared_weak_count::lock() _NOEXCEPT
 114 {
 115     long object_owners = __libcpp_atomic_load(&__shared_owners_);
 116     while (object_owners != -1)
 117     {
 118         if (__libcpp_atomic_compare_exchange(&__shared_owners_,
 119                                              &object_owners,
 120                                              object_owners+1))
 121             return this;
 122     }
 123     return nullptr;
 124 }
 125
 126 #if !defined(_LIBCPP_NO_RTTI) || !defined(_LIBCPP_BUILD_STATIC)
 127
 128 const void*
 129 __shared_weak_count::__get_deleter(const type_info&) const _NOEXCEPT
 130 {
 131     return nullptr;
 132 }
 133
 134 #endif  // _LIBCPP_NO_RTTI
 135
 136 #if !defined(_LIBCPP_HAS_NO_ATOMIC_HEADER)
 137
 138 _LIBCPP_SAFE_STATIC static const std::size_t __sp_mut_count = 16;
 139 _LIBCPP_SAFE_STATIC static __libcpp_mutex_t mut_back[__sp_mut_count] =
 140 {
 141     _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER,
 142     _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER,
 143     _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER,
 144     _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER
 145 };
 146
 147 _LIBCPP_CONSTEXPR __sp_mut::__sp_mut(void* p) _NOEXCEPT
 148    : __lx(p)
 149 {
 150 }
 151
 152 void
 153 __sp_mut::lock() _NOEXCEPT
 154 {
 155     auto m = static_cast<__libcpp_mutex_t*>(__lx);
 156     unsigned count = 0;
 157     while (!__libcpp_mutex_trylock(m))
 158     {
 159         if (++count > 16)
 160         {
 161             __libcpp_mutex_lock(m);
 162             break;
 163         }
 164         this_thread::yield();
 165     }
 166 }
 167
 168 void
 169 __sp_mut::unlock() _NOEXCEPT
 170 {
 171     __libcpp_mutex_unlock(static_cast<__libcpp_mutex_t*>(__lx));
 172 }
 173
 174 __sp_mut&
 175 __get_sp_mut(const void* p)
 176 {
 177     static __sp_mut muts[__sp_mut_count]
 178     {
 179         &mut_back[ 0], &mut_back[ 1], &mut_back[ 2], &mut_back[ 3],
 180         &mut_back[ 4], &mut_back[ 5], &mut_back[ 6], &mut_back[ 7],
 181         &mut_back[ 8], &mut_back[ 9], &mut_back[10], &mut_back[11],
 182         &mut_back[12], &mut_back[13], &mut_back[14], &mut_back[15]
 183     };
 184     return muts[hash<const void*>()(p) & (__sp_mut_count-1)];
 185 }
 186
 187 #endif // !defined(_LIBCPP_HAS_NO_ATOMIC_HEADER)
 188
 189 void
 190 declare_reachable(void*)
 191 {
 192 }
 193
 194 void
 195 declare_no_pointers(char*, size_t)
 196 {
 197 }
 198
 199 void
 200 undeclare_no_pointers(char*, size_t)
 201 {
 202 }
 203
 204 #if !defined(_LIBCPP_ABI_POINTER_SAFETY_ENUM_TYPE)
 205 pointer_safety get_pointer_safety() _NOEXCEPT
 206 {
 207     return pointer_safety::relaxed;
 208 }
 209 #endif
 210
 211 void*
 212 __undeclare_reachable(void* p)
 213 {
 214     return p;
 215 }
 216
 217 void*
 218 align(size_t alignment, size_t size, void*& ptr, size_t& space)
 219 {
 220     void* r = nullptr;
 221     if (size <= space)
 222     {
 223         char* p1 = static_cast<char*>(ptr);
 224         char* p2 = reinterpret_cast<char*>(reinterpret_cast<size_t>(p1 + (alignment - 1)) & -alignment);
 225         size_t d = static_cast<size_t>(p2 - p1);
 226         if (d <= space - size)
 227         {
 228             r = p2;
 229             ptr = r;
 230             space -= d;
 231         }
 232     }
 233     return r;
 234 }
 235
 236 _LIBCPP_END_NAMESPACE_STD