libatomic/cas_n.c

   1 /* Copyright (C) 2012-2024 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 #define LAT_CAS_N
  26 #include "libatomic_i.h"
  27
  28
  29 /* If we support the builtin, just use it.  */
  30 #if !DONE && defined(atomic_compare_exchange_n)
  31 bool
  32 SIZE(libat_compare_exchange) (UTYPE *mptr, UTYPE *eptr, UTYPE newval,
  33                               int smodel, int fmodel UNUSED)
  34 {
  35   if (maybe_specialcase_relaxed(smodel))
  36     return atomic_compare_exchange_n (mptr, eptr, newval, false,
  37                                       __ATOMIC_RELAXED, __ATOMIC_RELAXED);
  38   else if (maybe_specialcase_acqrel(smodel))
  39     return atomic_compare_exchange_n (mptr, eptr, newval, false,
  40                                       __ATOMIC_ACQ_REL, __ATOMIC_RELAXED);
  41   else
  42     return atomic_compare_exchange_n (mptr, eptr, newval, false,
  43                                       __ATOMIC_SEQ_CST, __ATOMIC_RELAXED);
  44 }
  45
  46 #define DONE 1
  47 #endif /* HAVE_ATOMIC_CAS */
  48
  49
  50 /* If this type is not larger than word-sized, fall back to a word-sized
  51    compare-and-swap loop, possibly assisted by the OS.  */
  52 #if !DONE && N <= WORDSIZE && defined(atomic_compare_exchange_w)
  53 bool
  54 SIZE(libat_compare_exchange) (UTYPE *mptr, UTYPE *eptr, UTYPE newval,
  55                               int smodel, int fmodel)
  56 {
  57   UWORD mask, shift, weval, woldval, wnewval, t, *wptr;
  58
  59   pre_barrier (smodel);
  60
  61   if (N < WORDSIZE)
  62     {
  63       wptr = (UWORD *)((uintptr_t)mptr & -WORDSIZE);
  64       shift = (((uintptr_t)mptr % WORDSIZE) * CHAR_BIT) ^ SIZE(INVERT_MASK);
  65       mask = SIZE(MASK) << shift;
  66     }
  67   else
  68     {
  69       wptr = (UWORD *)mptr;
  70       shift = 0;
  71       mask = -1;
  72     }
  73
  74   weval = (UWORD)*eptr << shift;
  75   wnewval = (UWORD)newval << shift;
  76   woldval = __atomic_load_n (wptr, __ATOMIC_RELAXED);
  77   do
  78     {
  79       if ((woldval & mask) != weval)
  80         goto failure;
  81       t = (woldval & ~mask) | wnewval;
  82     }
  83   while (!atomic_compare_exchange_w (wptr, &woldval, t, true,
  84                                      __ATOMIC_RELAXED, __ATOMIC_RELAXED));
  85   post_barrier (smodel);
  86   return true;
  87
  88  failure:
  89   *eptr = woldval >> shift;
  90   post_barrier (fmodel);
  91   return false;
  92 }
  93
  94 #define DONE 1
  95 #endif /* HAVE_ATOMIC_CAS && N <= WORDSIZE */
  96
  97
  98 /* Otherwise, fall back to some sort of protection mechanism.  */
  99 #if !DONE
 100 bool
 101 SIZE(libat_compare_exchange) (UTYPE *mptr, UTYPE *eptr, UTYPE newval,
 102                               int smodel, int fmodel UNUSED)
 103 {
 104   UTYPE oldval;
 105   UWORD magic;
 106   bool ret;
 107
 108   pre_seq_barrier (smodel);
 109   magic = protect_start (mptr);
 110
 111   oldval = *mptr;
 112   ret = (oldval == *eptr);
 113   if (ret)
 114     *mptr = newval;
 115   else
 116     *eptr = oldval;
 117
 118   protect_end (mptr, magic);
 119   post_seq_barrier (smodel);
 120
 121   return ret;
 122 }
 123 #endif
 124
 125 EXPORT_ALIAS (SIZE(compare_exchange));
 126 #undef LAT_CAS_N