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      1 // Copyright 2012 the V8 project authors. All rights reserved.
      2 // Redistribution and use in source and binary forms, with or without
      3 // modification, are permitted provided that the following conditions are
      4 // met:
      5 //
      6 //     * Redistributions of source code must retain the above copyright
      7 //       notice, this list of conditions and the following disclaimer.
      8 //     * Redistributions in binary form must reproduce the above
      9 //       copyright notice, this list of conditions and the following
     10 //       disclaimer in the documentation and/or other materials provided
     11 //       with the distribution.
     12 //     * Neither the name of Google Inc. nor the names of its
     13 //       contributors may be used to endorse or promote products derived
     14 //       from this software without specific prior written permission.
     15 //
     16 // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
     17 // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
     18 // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
     19 // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
     20 // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
     21 // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
     22 // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
     23 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
     24 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
     25 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
     26 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
     27 
     28 // emulates google3/base/once.h
     29 //
     30 // This header is intended to be included only by v8's internal code. Users
     31 // should not use this directly.
     32 //
     33 // This is basically a portable version of pthread_once().
     34 //
     35 // This header declares:
     36 // * A type called OnceType.
     37 // * A macro V8_DECLARE_ONCE() which declares a (global) variable of type
     38 //   OnceType.
     39 // * A function CallOnce(OnceType* once, void (*init_func)()).
     40 //   This function, when invoked multiple times given the same OnceType object,
     41 //   will invoke init_func on the first call only, and will make sure none of
     42 //   the calls return before that first call to init_func has finished.
     43 //
     44 // Additionally, the following features are supported:
     45 // * A macro V8_ONCE_INIT which is expanded into the expression used to
     46 //   initialize a OnceType. This is only useful when clients embed a OnceType
     47 //   into a structure of their own and want to initialize it statically.
     48 // * The user can provide a parameter which CallOnce() forwards to the
     49 //   user-provided function when it is called. Usage example:
     50 //     CallOnce(&my_once, &MyFunctionExpectingIntArgument, 10);
     51 // * This implementation guarantees that OnceType is a POD (i.e. no static
     52 //   initializer generated).
     53 //
     54 // This implements a way to perform lazy initialization.  It's more efficient
     55 // than using mutexes as no lock is needed if initialization has already
     56 // happened.
     57 //
     58 // Example usage:
     59 //   void Init();
     60 //   V8_DECLARE_ONCE(once_init);
     61 //
     62 //   // Calls Init() exactly once.
     63 //   void InitOnce() {
     64 //     CallOnce(&once_init, &Init);
     65 //   }
     66 //
     67 // Note that if CallOnce() is called before main() has begun, it must
     68 // only be called by the thread that will eventually call main() -- that is,
     69 // the thread that performs dynamic initialization.  In general this is a safe
     70 // assumption since people don't usually construct threads before main() starts,
     71 // but it is technically not guaranteed.  Unfortunately, Win32 provides no way
     72 // whatsoever to statically-initialize its synchronization primitives, so our
     73 // only choice is to assume that dynamic initialization is single-threaded.
     74 
     75 #ifndef V8_ONCE_H_
     76 #define V8_ONCE_H_
     77 
     78 #include "atomicops.h"
     79 
     80 namespace v8 {
     81 namespace internal {
     82 
     83 typedef AtomicWord OnceType;
     84 
     85 #define V8_ONCE_INIT 0
     86 
     87 #define V8_DECLARE_ONCE(NAME) ::v8::internal::OnceType NAME
     88 
     89 enum {
     90   ONCE_STATE_UNINITIALIZED = 0,
     91   ONCE_STATE_EXECUTING_FUNCTION = 1,
     92   ONCE_STATE_DONE = 2
     93 };
     94 
     95 typedef void (*NoArgFunction)();
     96 typedef void (*PointerArgFunction)(void* arg);
     97 
     98 template <typename T>
     99 struct OneArgFunction {
    100   typedef void (*type)(T);
    101 };
    102 
    103 void CallOnceImpl(OnceType* once, PointerArgFunction init_func, void* arg);
    104 
    105 inline void CallOnce(OnceType* once, NoArgFunction init_func) {
    106   if (Acquire_Load(once) != ONCE_STATE_DONE) {
    107     CallOnceImpl(once, reinterpret_cast<PointerArgFunction>(init_func), NULL);
    108   }
    109 }
    110 
    111 
    112 template <typename Arg>
    113 inline void CallOnce(OnceType* once,
    114     typename OneArgFunction<Arg*>::type init_func, Arg* arg) {
    115   if (Acquire_Load(once) != ONCE_STATE_DONE) {
    116     CallOnceImpl(once, reinterpret_cast<PointerArgFunction>(init_func),
    117         static_cast<void*>(arg));
    118   }
    119 }
    120 
    121 } }  // namespace v8::internal
    122 
    123 #endif  // V8_ONCE_H_
    124