1  /* Copyright (C) 2012-2023 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));