contrib/libc++/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 namespace
  21 {
  22
  23 // NOTE: Relaxed and acq/rel atomics (for increment and decrement respectively)
  24 // should be sufficient for thread safety.
  25 // See https://llvm.org/bugs/show_bug.cgi?id=22803
  26 template <class T>
  27 inline T
  28 increment(T& t) _NOEXCEPT
  29 {
  30     return __libcpp_atomic_add(&t, 1, _AO_Relaxed);
  31 }
  32
  33 template <class T>
  34 inline T
  35 decrement(T& t) _NOEXCEPT
  36 {
  37     return __libcpp_atomic_add(&t, -1, _AO_Acq_Rel);
  38 }
  39
  40 }  // namespace
  41
  42 const allocator_arg_t allocator_arg = allocator_arg_t();
  43
  44 bad_weak_ptr::~bad_weak_ptr() _NOEXCEPT {}
  45
  46 const char*
  47 bad_weak_ptr::what() const _NOEXCEPT
  48 {
  49     return "bad_weak_ptr";
  50 }
  51
  52 __shared_count::~__shared_count()
  53 {
  54 }
  55
  56 void
  57 __shared_count::__add_shared() _NOEXCEPT
  58 {
  59     increment(__shared_owners_);
  60 }
  61
  62 bool
  63 __shared_count::__release_shared() _NOEXCEPT
  64 {
  65     if (decrement(__shared_owners_) == -1)
  66     {
  67         __on_zero_shared();
  68         return true;
  69     }
  70     return false;
  71 }
  72
  73 __shared_weak_count::~__shared_weak_count()
  74 {
  75 }
  76
  77 void
  78 __shared_weak_count::__add_shared() _NOEXCEPT
  79 {
  80     __shared_count::__add_shared();
  81 }
  82
  83 void
  84 __shared_weak_count::__add_weak() _NOEXCEPT
  85 {
  86     increment(__shared_weak_owners_);
  87 }
  88
  89 void
  90 __shared_weak_count::__release_shared() _NOEXCEPT
  91 {
  92     if (__shared_count::__release_shared())
  93         __release_weak();
  94 }
  95
  96 void
  97 __shared_weak_count::__release_weak() _NOEXCEPT
  98 {
  99     // NOTE: The acquire load here is an optimization of the very
 100     // common case where a shared pointer is being destructed while
 101     // having no other contended references.
 102     //
 103     // BENEFIT: We avoid expensive atomic stores like XADD and STREX
 104     // in a common case.  Those instructions are slow and do nasty
 105     // things to caches.
 106     //
 107     // IS THIS SAFE?  Yes.  During weak destruction, if we see that we
 108     // are the last reference, we know that no-one else is accessing
 109     // us. If someone were accessing us, then they would be doing so
 110     // while the last shared / weak_ptr was being destructed, and
 111     // that's undefined anyway.
 112     //
 113     // If we see anything other than a 0, then we have possible
 114     // contention, and need to use an atomicrmw primitive.
 115     // The same arguments don't apply for increment, where it is legal
 116     // (though inadvisable) to share shared_ptr references between
 117     // threads, and have them all get copied at once.  The argument
 118     // also doesn't apply for __release_shared, because an outstanding
 119     // weak_ptr::lock() could read / modify the shared count.
 120     if (__libcpp_atomic_load(&__shared_weak_owners_, _AO_Acquire) == 0)
 121     {
 122         // no need to do this store, because we are about
 123         // to destroy everything.
 124         //__libcpp_atomic_store(&__shared_weak_owners_, -1, _AO_Release);
 125         __on_zero_shared_weak();
 126     }
 127     else if (decrement(__shared_weak_owners_) == -1)
 128         __on_zero_shared_weak();
 129 }
 130
 131 __shared_weak_count*
 132 __shared_weak_count::lock() _NOEXCEPT
 133 {
 134     long object_owners = __libcpp_atomic_load(&__shared_owners_);
 135     while (object_owners != -1)
 136     {
 137         if (__libcpp_atomic_compare_exchange(&__shared_owners_,
 138                                              &object_owners,
 139                                              object_owners+1))
 140             return this;
 141     }
 142     return 0;
 143 }
 144
 145 #if !defined(_LIBCPP_NO_RTTI) || !defined(_LIBCPP_BUILD_STATIC)
 146
 147 const void*
 148 __shared_weak_count::__get_deleter(const type_info&) const _NOEXCEPT
 149 {
 150     return 0;
 151 }
 152
 153 #endif  // _LIBCPP_NO_RTTI
 154
 155 #if !defined(_LIBCPP_HAS_NO_ATOMIC_HEADER)
 156
 157 _LIBCPP_SAFE_STATIC static const std::size_t __sp_mut_count = 16;
 158 _LIBCPP_SAFE_STATIC static __libcpp_mutex_t mut_back[__sp_mut_count] =
 159 {
 160     _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER,
 161     _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER,
 162     _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER,
 163     _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER, _LIBCPP_MUTEX_INITIALIZER
 164 };
 165
 166 _LIBCPP_CONSTEXPR __sp_mut::__sp_mut(void* p) _NOEXCEPT
 167    : __lx(p)
 168 {
 169 }
 170
 171 void
 172 __sp_mut::lock() _NOEXCEPT
 173 {
 174     auto m = static_cast<__libcpp_mutex_t*>(__lx);
 175     unsigned count = 0;
 176     while (__libcpp_mutex_trylock(m) != 0)
 177     {
 178         if (++count > 16)
 179         {
 180             __libcpp_mutex_lock(m);
 181             break;
 182         }
 183         this_thread::yield();
 184     }
 185 }
 186
 187 void
 188 __sp_mut::unlock() _NOEXCEPT
 189 {
 190     __libcpp_mutex_unlock(static_cast<__libcpp_mutex_t*>(__lx));
 191 }
 192
 193 __sp_mut&
 194 __get_sp_mut(const void* p)
 195 {
 196     static __sp_mut muts[__sp_mut_count]
 197     {
 198         &mut_back[ 0], &mut_back[ 1], &mut_back[ 2], &mut_back[ 3],
 199         &mut_back[ 4], &mut_back[ 5], &mut_back[ 6], &mut_back[ 7],
 200         &mut_back[ 8], &mut_back[ 9], &mut_back[10], &mut_back[11],
 201         &mut_back[12], &mut_back[13], &mut_back[14], &mut_back[15]
 202     };
 203     return muts[hash<const void*>()(p) & (__sp_mut_count-1)];
 204 }
 205
 206 #endif // !defined(_LIBCPP_HAS_NO_ATOMIC_HEADER)
 207
 208 void
 209 declare_reachable(void*)
 210 {
 211 }
 212
 213 void
 214 declare_no_pointers(char*, size_t)
 215 {
 216 }
 217
 218 void
 219 undeclare_no_pointers(char*, size_t)
 220 {
 221 }
 222
 223 #if !defined(_LIBCPP_ABI_POINTER_SAFETY_ENUM_TYPE)
 224 pointer_safety get_pointer_safety() _NOEXCEPT
 225 {
 226     return pointer_safety::relaxed;
 227 }
 228 #endif
 229
 230 void*
 231 __undeclare_reachable(void* p)
 232 {
 233     return p;
 234 }
 235
 236 void*
 237 align(size_t alignment, size_t size, void*& ptr, size_t& space)
 238 {
 239     void* r = nullptr;
 240     if (size <= space)
 241     {
 242         char* p1 = static_cast<char*>(ptr);
 243         char* p2 = reinterpret_cast<char*>(reinterpret_cast<size_t>(p1 + (alignment - 1)) & -alignment);
 244         size_t d = static_cast<size_t>(p2 - p1);
 245         if (d <= space - size)
 246         {
 247             r = p2;
 248             ptr = r;
 249             space -= d;
 250         }
 251     }
 252     return r;
 253 }
 254
 255 _LIBCPP_END_NAMESPACE_STD