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      1 /*
      2  * Copyright (C) 2011 The Android Open Source Project
      3  * All rights reserved.
      4  *
      5  * Redistribution and use in source and binary forms, with or without
      6  * modification, are permitted provided that the following conditions
      7  * are met:
      8  *  * Redistributions of source code must retain the above copyright
      9  *    notice, this list of conditions and the following disclaimer.
     10  *  * Redistributions in binary form must reproduce the above copyright
     11  *    notice, this list of conditions and the following disclaimer in
     12  *    the documentation and/or other materials provided with the
     13  *    distribution.
     14  *
     15  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
     16  * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
     17  * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
     18  * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
     19  * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
     20  * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
     21  * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
     22  * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
     23  * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
     24  * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
     25  * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     26  * SUCH DAMAGE.
     27  */
     28 
     29 
     30 /* The purpose of this file is to export a small set of atomic-related
     31  * functions from the C library, to ensure binary ABI compatibility for
     32  * the NDK.
     33  *
     34  * These functions were initially exposed by the NDK through <sys/atomics.h>,
     35  * which was unfortunate because their implementation didn't provide any
     36  * memory barriers at all.
     37  *
     38  * This wasn't a problem for the platform code that used them, because it
     39  * used explicit barrier instructions around them. On the other hand, it means
     40  * that any NDK-generated machine code that linked against them would not
     41  * perform correctly when running on multi-core devices.
     42  *
     43  * To fix this, the platform code was first modified to not use any of these
     44  * functions (everything is now inlined through assembly statements, see
     45  * libc/private/bionic_arm_inline.h and the headers it includes.
     46  *
     47  * The functions here are thus only for the benefit of NDK applications,
     48  * and now includes full memory barriers to prevent any random memory ordering
     49  * issue from cropping.
     50  *
     51  * Note that we also provide an updated <sys/atomics.h> header that defines
     52  * always_inlined versions of the functions that use the GCC builtin
     53  * intrinsics to perform the same thing.
     54  *
     55  * NOTE: There is no need for a similar file for non-ARM platforms.
     56  */
     57 
     58 /* DO NOT INCLUDE <sys/atomics.h> HERE ! */
     59 
     60 int
     61 __atomic_cmpxchg(int old, int _new, volatile int *ptr)
     62 {
     63     /* We must return 0 on success */
     64     return __sync_val_compare_and_swap(ptr, old, _new) != old;
     65 }
     66 
     67 int
     68 __atomic_swap(int _new, volatile int *ptr)
     69 {
     70     int prev;
     71     do {
     72         prev = *ptr;
     73     } while (__sync_val_compare_and_swap(ptr, prev, _new) != prev);
     74     return prev;
     75 }
     76 
     77 int
     78 __atomic_dec(volatile int *ptr)
     79 {
     80   return __sync_fetch_and_sub (ptr, 1);
     81 }
     82 
     83 int
     84 __atomic_inc(volatile int *ptr)
     85 {
     86   return __sync_fetch_and_add (ptr, 1);
     87 }
     88