libatomic/cas_n.c

   1 /* Copyright (C) 2012-2020 Free Software Foundation, Inc.
   2    Contributed by Richard Henderson <rth@redhat.com>.
   3
   4    This file is part of the GNU Atomic Library (libatomic).
   5
   6    Libatomic is free software; you can redistribute it and/or modify it
   7    under the terms of the GNU General Public License as published by
   8    the Free Software Foundation; either version 3 of the License, or
   9    (at your option) any later version.
  10
  11    Libatomic is distributed in the hope that it will be useful, but WITHOUT ANY
  12    WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
  13    FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
  14    more details.
  15
  16    Under Section 7 of GPL version 3, you are granted additional
  17    permissions described in the GCC Runtime Library Exception, version
  18    3.1, as published by the Free Software Foundation.
  19
  20    You should have received a copy of the GNU General Public License and
  21    a copy of the GCC Runtime Library Exception along with this program;
  22    see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
  23    <http://www.gnu.org/licenses/>.  */
  24
  25 #include "libatomic_i.h"
  26
  27
  28 /* If we support the builtin, just use it.  */
  29 #if !DONE && defined(atomic_compare_exchange_n)
  30 bool
  31 SIZE(libat_compare_exchange) (UTYPE *mptr, UTYPE *eptr, UTYPE newval,
  32                               int smodel, int fmodel UNUSED)
  33 {
  34   if (maybe_specialcase_relaxed(smodel))
  35     return atomic_compare_exchange_n (mptr, eptr, newval, false,
  36                                       __ATOMIC_RELAXED, __ATOMIC_RELAXED);
  37   else if (maybe_specialcase_acqrel(smodel))
  38     return atomic_compare_exchange_n (mptr, eptr, newval, false,
  39                                       __ATOMIC_ACQ_REL, __ATOMIC_RELAXED);
  40   else
  41     return atomic_compare_exchange_n (mptr, eptr, newval, false,
  42                                       __ATOMIC_SEQ_CST, __ATOMIC_RELAXED);
  43 }
  44
  45 #define DONE 1
  46 #endif /* HAVE_ATOMIC_CAS */
  47
  48
  49 /* If this type is not larger than word-sized, fall back to a word-sized
  50    compare-and-swap loop, possibly assisted by the OS.  */
  51 #if !DONE && N <= WORDSIZE && defined(atomic_compare_exchange_w)
  52 bool
  53 SIZE(libat_compare_exchange) (UTYPE *mptr, UTYPE *eptr, UTYPE newval,
  54                               int smodel, int fmodel)
  55 {
  56   UWORD mask, shift, weval, woldval, wnewval, t, *wptr;
  57
  58   pre_barrier (smodel);
  59
  60   if (N < WORDSIZE)
  61     {
  62       wptr = (UWORD *)((uintptr_t)mptr & -WORDSIZE);
  63       shift = (((uintptr_t)mptr % WORDSIZE) * CHAR_BIT) ^ SIZE(INVERT_MASK);
  64       mask = SIZE(MASK) << shift;
  65     }
  66   else
  67     {
  68       wptr = (UWORD *)mptr;
  69       shift = 0;
  70       mask = -1;
  71     }
  72
  73   weval = (UWORD)*eptr << shift;
  74   wnewval = (UWORD)newval << shift;
  75   woldval = __atomic_load_n (wptr, __ATOMIC_RELAXED);
  76   do
  77     {
  78       if ((woldval & mask) != weval)
  79         goto failure;
  80       t = (woldval & ~mask) | wnewval;
  81     }
  82   while (!atomic_compare_exchange_w (wptr, &woldval, t, true,
  83                                      __ATOMIC_RELAXED, __ATOMIC_RELAXED));
  84   post_barrier (smodel);
  85   return true;
  86
  87  failure:
  88   *eptr = woldval >> shift;
  89   post_barrier (fmodel);
  90   return false;
  91 }
  92
  93 #define DONE 1
  94 #endif /* HAVE_ATOMIC_CAS && N <= WORDSIZE */
  95
  96
  97 /* Otherwise, fall back to some sort of protection mechanism.  */
  98 #if !DONE
  99 bool
 100 SIZE(libat_compare_exchange) (UTYPE *mptr, UTYPE *eptr, UTYPE newval,
 101                               int smodel, int fmodel UNUSED)
 102 {
 103   UTYPE oldval;
 104   UWORD magic;
 105   bool ret;
 106
 107   pre_seq_barrier (smodel);
 108   magic = protect_start (mptr);
 109
 110   oldval = *mptr;
 111   ret = (oldval == *eptr);
 112   if (ret)
 113     *mptr = newval;
 114   else
 115     *eptr = oldval;
 116
 117   protect_end (mptr, magic);
 118   post_seq_barrier (smodel);
 119
 120   return ret;
 121 }
 122 #endif
 123
 124 EXPORT_ALIAS (SIZE(compare_exchange));