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 && SIZE(HAVE_ATOMIC_EXCHANGE)
      30  UTYPE
      31  SIZE(libat_exchange) (UTYPE *mptr, UTYPE newval, int smodel)
      32  {
      33    if (maybe_specialcase_relaxed(smodel))
      34      return __atomic_exchange_n (mptr, newval, __ATOMIC_RELAXED);
      35    else if (maybe_specialcase_acqrel(smodel))
      36      return __atomic_exchange_n (mptr, newval, __ATOMIC_ACQ_REL);
      37    else
      38      return __atomic_exchange_n (mptr, newval, __ATOMIC_SEQ_CST);
      39  }
      40  
      41  #define DONE 1
      42  #endif /* HAVE_ATOMIC_EXCHANGE */
      43  
      44  
      45  #if !DONE && defined(atomic_compare_exchange_n)
      46  UTYPE
      47  SIZE(libat_exchange) (UTYPE *mptr, UTYPE newval, int smodel)
      48  {
      49    UTYPE oldval;
      50  
      51    pre_barrier (smodel);
      52  
      53    oldval = *mptr;
      54    while (!atomic_compare_exchange_n (mptr, &oldval, newval, true,
      55  				     __ATOMIC_RELAXED, __ATOMIC_RELAXED))
      56      continue;
      57  
      58    post_barrier (smodel);
      59  
      60    return oldval;
      61  }
      62  
      63  #define DONE 1
      64  #endif /* atomic_compare_exchange_n */
      65  
      66  
      67  /* If this type is smaller than word-sized, fall back to a word-sized
      68     compare-and-swap loop.  */
      69  #if !DONE && N <= WORDSIZE && defined(atomic_compare_exchange_w)
      70  UTYPE
      71  SIZE(libat_exchange) (UTYPE *mptr, UTYPE newval, int smodel)
      72  {
      73    UWORD mask, shift, woldval, wnewval, t, *wptr;
      74  
      75    pre_barrier (smodel);
      76  
      77    if (N < WORDSIZE)
      78      {
      79        wptr = (UWORD *)((uintptr_t)mptr & -WORDSIZE);
      80        shift = (((uintptr_t)mptr % WORDSIZE) * CHAR_BIT) ^ SIZE(INVERT_MASK);
      81        mask = SIZE(MASK) << shift;
      82      }
      83    else
      84      {
      85        wptr = (UWORD *)mptr;
      86        shift = 0;
      87        mask = -1;
      88      }
      89  
      90    wnewval = (UWORD)newval << shift;
      91    woldval = __atomic_load_n (wptr, __ATOMIC_RELAXED);
      92    do
      93      {
      94        t = (woldval & ~mask) | wnewval;
      95      }
      96    while (!atomic_compare_exchange_w (wptr, &woldval, t, true,
      97  				     __ATOMIC_RELAXED, __ATOMIC_RELAXED));
      98  
      99    post_barrier (smodel);
     100    return woldval >> shift;
     101  }
     102  
     103  #define DONE 1
     104  #endif /* HAVE_ATOMIC_CAS && N < WORDSIZE */
     105  
     106  
     107  /* Otherwise, fall back to some sort of protection mechanism.  */
     108  #if !DONE
     109  UTYPE
     110  SIZE(libat_exchange) (UTYPE *mptr, UTYPE newval, int smodel UNUSED)
     111  {
     112    UTYPE oldval;
     113    UWORD magic;
     114  
     115    pre_seq_barrier (smodel);
     116    magic = protect_start (mptr);
     117  
     118    oldval = *mptr;
     119    *mptr = newval;
     120  
     121    protect_end (mptr, magic);
     122    post_seq_barrier (smodel);
     123  
     124    return oldval;
     125  }
     126  #endif
     127  
     128  EXPORT_ALIAS (SIZE(exchange));