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      1 // Copyright (C) 2011 The Android Open Source Project
      2 // All rights reserved.
      3 //
      4 // Redistribution and use in source and binary forms, with or without
      5 // modification, are permitted provided that the following conditions
      6 // are met:
      7 // 1. Redistributions of source code must retain the above copyright
      8 //    notice, this list of conditions and the following disclaimer.
      9 // 2. Redistributions in binary form must reproduce the above copyright
     10 //    notice, this list of conditions and the following disclaimer in the
     11 //    documentation and/or other materials provided with the distribution.
     12 // 3. Neither the name of the project nor the names of its contributors
     13 //    may be used to endorse or promote products derived from this software
     14 //    without specific prior written permission.
     15 //
     16 // THIS SOFTWARE IS PROVIDED BY THE PROJECT AND CONTRIBUTORS ``AS IS'' AND
     17 // ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
     18 // IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
     19 // ARE DISCLAIMED.  IN NO EVENT SHALL THE PROJECT OR CONTRIBUTORS BE LIABLE
     20 // FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
     21 // DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
     22 // OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
     23 // HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
     24 // LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
     25 // OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     26 // SUCH DAMAGE.
     27 //
     28 // One-time construction C++ runtime support
     29 // See "3.3.2 One-time Construction API" of the Itanium C++ ABI reference
     30 // And "3.2.3 Guard variables and the one-time construction API" in the ARM C++ ABI reference.
     31 
     32 /* Note that the ARM C++ ABI defines the size of each guard variable
     33  * as 32-bit, while the generic/Itanium one defines it as 64-bit.
     34  *
     35  * Also the ARM C++ ABI uses the least-significant bit to indicate
     36  * completion, while the generic/Itanium one uses the least-significant
     37  * byte. In all cases the corresponding item is set to value '1'
     38  *
     39  * We will treat guard variables here as 32-bit values, even on x86,
     40  * given that this representation is compatible with compiler-generated
     41  * variables that are 64-bits on little-endian systems. This makes the
     42  * code simpler and slightly more efficient
     43  */
     44 
     45 #include <stddef.h>
     46 #include <pthread.h>
     47 
     48 /* In this implementation, we use a single global mutex+condvar pair.
     49  *
     50  * Pros: portable and doesn't require playing with futexes, atomics
     51  *       and memory barriers.
     52  *
     53  * Cons: Slower than necessary.
     54  */
     55 static pthread_mutex_t  sMutex = PTHREAD_RECURSIVE_MUTEX_INITIALIZER;
     56 static pthread_cond_t   sCond  = PTHREAD_COND_INITIALIZER;
     57 
     58 
     59 extern "C" int __cxa_guard_acquire(int volatile * gv)
     60 {
     61     pthread_mutex_lock(&sMutex);
     62     for (;;) {
     63         // while gv points to a volatile value, we use the
     64         // previous pthread_mutex_lock or pthread_cond_wait
     65         // as a trivial memory barrier
     66         int guard = *gv;
     67         if ((guard & 1) != 0) {
     68             /* already initialized - return 0 */
     69             pthread_mutex_unlock(&sMutex);
     70             return 0;
     71         }
     72 
     73         // we use bit 8 to indicate that the guard value is being
     74         // initialized, and bit 9 to indicate that there is another
     75         // thread waiting for its completion.
     76         if ((guard & 0x100) == 0) {
     77             // nobody is initializing this yet, so mark the guard value
     78             // first. and allow initialization to proceed.
     79             *gv = 0x100;
     80             pthread_mutex_unlock(&sMutex);
     81             return 1;
     82         }
     83 
     84         // already being initialized by amother thread,
     85         // we must indicate that there is a waiter, then
     86         // wait to be woken up before trying again.
     87         *gv = guard | 0x200;
     88         pthread_cond_wait(&sCond, &sMutex);
     89     }
     90 }
     91 
     92 extern "C" void __cxa_guard_release(int volatile * gv)
     93 {
     94     pthread_mutex_lock(&sMutex);
     95 
     96     int guard = *gv;
     97     // this indicates initialization for our two ABIs.
     98     *gv = 0x1;
     99     if ((guard & 0x200) != 0)
    100         pthread_cond_broadcast(&sCond);
    101 
    102     pthread_mutex_unlock(&sMutex);
    103 }
    104 
    105 extern "C" void __cxa_guard_abort(int volatile * gv)
    106 {
    107     pthread_mutex_lock(&sMutex);
    108 
    109     int guard = *gv;
    110     *gv = 0;
    111     if ((guard & 0x200) != 0)
    112         pthread_cond_broadcast(&sCond);
    113 
    114     pthread_mutex_unlock(&sMutex);
    115 }
    116