83 lines
3.1 KiB
C
83 lines
3.1 KiB
C
/* Copyright (C) 2012-2018 Free Software Foundation, Inc.
|
|
Contributed by Richard Henderson <rth@redhat.com>.
|
|
|
|
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
|
|
<http://www.gnu.org/licenses/>. */
|
|
|
|
#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. */
|
|
bool
|
|
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);
|