1/* Copyright (C) 2012-2015 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) 30UTYPE 31SIZE(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) 49UTYPE 50SIZE(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) 73UTYPE 74SIZE(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 97UTYPE 98SIZE(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 115EXPORT_ALIAS (SIZE(load)); 116