-/* Copyright (C) 2012-2017 Free Software Foundation, Inc.
+/* Copyright (C) 2012-2020 Free Software Foundation, Inc.
Contributed by Richard Henderson <rth@redhat.com>.
This file is part of the GNU Atomic Library (libatomic).
#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)) 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(HAVE_ATOMIC_LDST_,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)
{