| /* Copyright (C) 2012-2021 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); |