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/* Copyright (C) 2012-2019 Free Software Foundation, Inc.
Contributed by Richard Henderson <>.
This file is part of the GNU Atomic Library (libatomic).
Libatomic is free software; you can redistribute it and/or modify it
under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3 of the License, or
(at your option) any later version.
Libatomic is distributed in the hope that it will be useful, but WITHOUT ANY
WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
FOR A PARTICULAR PURPOSE. See the GNU General Public License for
more details.
Under Section 7 of GPL version 3, you are granted additional
permissions described in the GCC Runtime Library Exception, version
3.1, as published by the Free Software Foundation.
You should have received a copy of the GNU General Public License and
a copy of the GCC Runtime Library Exception along with this program;
see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
<>. */
#include "libatomic_i.h"
/* Accesses with a power-of-two size are not lock-free if we don't have an
integer type of this size or if they are not naturally aligned. They
are lock-free if such a naturally aligned access is always lock-free
according to the compiler, which requires that both atomic loads and CAS
are available.
In all other cases, we fall through to LARGER (see below). */
#define EXACT(N) \
do { \
if (!C2(HAVE_INT,N)) break; \
if ((uintptr_t)ptr & (N - 1)) break; \
if (__atomic_always_lock_free(N, 0)) return true; \
if (!C2(MAYBE_HAVE_ATOMIC_CAS_,N)) break; \
if (C2(FAST_ATOMIC_LDST_,N)) return true; \
} while (0)
/* We next check to see if an access of a larger size is lock-free. We use
a similar check as in EXACT, except that we also check that the alignment
of the access is so that the data to be accessed is completely covered
by the larger access. */
#define LARGER(N) \
do { \
uintptr_t r = (uintptr_t)ptr & (N - 1); \
if (!C2(HAVE_INT,N)) break; \
if (!C2(FAST_ATOMIC_LDST_,N)) break; \
if (!C2(MAYBE_HAVE_ATOMIC_CAS_,N)) break; \
if (r + n <= N) return true; \
} while (0)
/* Note that this can return that a size/alignment is not lock-free even if
all the operations that we use to implement the respective accesses provide
lock-free forward progress as specified in C++14: Users likely expect
"lock-free" to also mean "fast", which is why we do not return true if, for
example, we implement loads with this size/alignment using a CAS. */
libat_is_lock_free (size_t n, void *ptr)
switch (n)
case 0: return true;
case 1: EXACT(1); goto L4;
case 2: EXACT(2); goto L4;
case 4: EXACT(4); goto L8;
case 8: EXACT(8); goto L16;
case 16: EXACT(16); break;
case 3: L4: LARGER(4); /* FALLTHRU */
case 5 ... 7: L8: LARGER(8); /* FALLTHRU */
case 9 ... 15: L16: LARGER(16); break;
return false;
EXPORT_ALIAS (is_lock_free);