libatomic/glfree.c

   1 /* Copyright (C) 2012-2021 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 /* Accesses with a power-of-two size are not lock-free if we don't have an
  28    integer type of this size or if they are not naturally aligned.  They
  29    are lock-free if such a naturally aligned access is always lock-free
  30    according to the compiler, which requires that both atomic loads and CAS
  31    are available.
  32    In all other cases, we fall through to LARGER (see below).  */
  33 #define EXACT(N)                                                \
  34   do {                                                          \
  35     if (!C2(HAVE_INT,N)) break;                                 \
  36     if ((uintptr_t)ptr & (N - 1)) break;                        \
  37     if (__atomic_always_lock_free(N, 0)) return true;           \
  38     if (!C2(MAYBE_HAVE_ATOMIC_CAS_,N)) break;                   \
  39     if (C2(FAST_ATOMIC_LDST_,N)) return true;                   \
  40   } while (0)
  41
  42
  43 /* We next check to see if an access of a larger size is lock-free.  We use
  44    a similar check as in EXACT, except that we also check that the alignment
  45    of the access is so that the data to be accessed is completely covered
  46    by the larger access.  */
  47 #define LARGER(N)                                               \
  48   do {                                                          \
  49     uintptr_t r = (uintptr_t)ptr & (N - 1);                     \
  50     if (!C2(HAVE_INT,N)) break;                                 \
  51     if (!C2(FAST_ATOMIC_LDST_,N)) break;                        \
  52     if (!C2(MAYBE_HAVE_ATOMIC_CAS_,N)) break;                   \
  53     if (r + n <= N) return true;                                \
  54   } while (0)
  55
  56
  57 /* Note that this can return that a size/alignment is not lock-free even if
  58    all the operations that we use to implement the respective accesses provide
  59    lock-free forward progress as specified in C++14:  Users likely expect
  60    "lock-free" to also mean "fast", which is why we do not return true if, for
  61    example, we implement loads with this size/alignment using a CAS.  */
  62 bool
  63 libat_is_lock_free (size_t n, void *ptr)
  64 {
  65   switch (n)
  66     {
  67     case 0:                             return true;
  68     case 1:             EXACT(1);       goto L4;
  69     case 2:             EXACT(2);       goto L4;
  70     case 4:             EXACT(4);       goto L8;
  71     case 8:             EXACT(8);       goto L16;
  72     case 16:            EXACT(16);      break;
  73
  74     case 3: L4:         LARGER(4);      /* FALLTHRU */
  75     case 5 ... 7: L8:   LARGER(8);      /* FALLTHRU */
  76     case 9 ... 15: L16: LARGER(16);     break;
  77     }
  78
  79   return false;
  80 }
  81
  82 EXPORT_ALIAS (is_lock_free);