libatomic/load_n.c

   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));