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_LDST)
      30  UTYPE
      31  SIZE(libat_load) (UTYPE *mptr, int smodel)
      32  {
      33    if (maybe_specialcase_relaxed(smodel))
      34      return __atomic_load_n (mptr, __ATOMIC_RELAXED);
      35    else if (maybe_specialcase_acqrel(smodel))
      36      /* Note that REL and ACQ_REL are not valid for loads.  */
      37      return __atomic_load_n (mptr, __ATOMIC_ACQUIRE);
      38    else
      39      return __atomic_load_n (mptr, __ATOMIC_SEQ_CST);
      40  }
      41  
      42  #define DONE 1
      43  #endif /* HAVE_ATOMIC_LOAD */
      44  
      45  
      46  /* If we have compare-and-swap, use it to swap 0 with 0 and as a side
      47     effect load the original value.  */
      48  #if !DONE && defined(atomic_compare_exchange_n)
      49  UTYPE
      50  SIZE(libat_load) (UTYPE *mptr, int smodel)
      51  {
      52    UTYPE t = 0;
      53  
      54    if (maybe_specialcase_relaxed(smodel))
      55      atomic_compare_exchange_n (mptr, &t, 0, true,
      56  			       __ATOMIC_RELAXED, __ATOMIC_RELAXED);
      57    else if (maybe_specialcase_acqrel(smodel))
      58      atomic_compare_exchange_n (mptr, &t, 0, true,
      59  			       __ATOMIC_ACQ_REL, __ATOMIC_ACQ_REL);
      60    else
      61      atomic_compare_exchange_n (mptr, &t, 0, true,
      62                                 __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);
      63  
      64    return t;
      65  }
      66  
      67  #define DONE 1
      68  #endif /* atomic_compare_exchange_n */
      69  
      70  
      71  /* Similar, but only assume a word-sized compare-and-swap.  */
      72  #if !DONE && N < WORDSIZE && defined(atomic_compare_exchange_w)
      73  UTYPE
      74  SIZE(libat_load) (UTYPE *mptr, int smodel)
      75  {
      76    UWORD shift, t, *wptr;
      77  
      78    pre_barrier (smodel);
      79  
      80    wptr = (UWORD *)((uintptr_t)mptr & -WORDSIZE);
      81    shift = (((uintptr_t)mptr % WORDSIZE) * CHAR_BIT) ^ SIZE(INVERT_MASK);
      82  
      83    /* Exchange 0 with 0, placing the old value of *WPTR in T.  */
      84    t = 0;
      85    atomic_compare_exchange_w (wptr, &t, 0);
      86  
      87    post_barrier (smodel);
      88    return t >> shift;
      89  }
      90  
      91  #define DONE 1
      92  #endif /* HAVE_ATOMIC_CAS && N < WORDSIZE */
      93  
      94  
      95  /* Otherwise, fall back to some sort of protection mechanism.  */
      96  #if !DONE
      97  UTYPE
      98  SIZE(libat_load) (UTYPE *mptr, int smodel)
      99  {
     100    UTYPE ret;
     101    UWORD magic;
     102  
     103    pre_seq_barrier (smodel);
     104    magic = protect_start (mptr);
     105  
     106    ret = *mptr;
     107  
     108    protect_end (mptr, magic);
     109    post_seq_barrier (smodel);
     110  
     111    return ret;
     112  }
     113  #endif
     114  
     115  EXPORT_ALIAS (SIZE(load));