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fbd26352 | 1 | /* Copyright (C) 2012-2019 Free Software Foundation, Inc. |
6db1d2ca | 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 | ||
d5f5fa27 | 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). */ | |
6db1d2ca | 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; \ | |
d5f5fa27 | 38 | if (!C2(MAYBE_HAVE_ATOMIC_CAS_,N)) break; \ |
39 | if (C2(FAST_ATOMIC_LDST_,N)) return true; \ | |
6db1d2ca | 40 | } while (0) |
41 | ||
42 | ||
d5f5fa27 | 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. */ | |
6db1d2ca | 47 | #define LARGER(N) \ |
48 | do { \ | |
49 | uintptr_t r = (uintptr_t)ptr & (N - 1); \ | |
50 | if (!C2(HAVE_INT,N)) break; \ | |
d5f5fa27 | 51 | if (!C2(FAST_ATOMIC_LDST_,N)) break; \ |
6db1d2ca | 52 | if (!C2(MAYBE_HAVE_ATOMIC_CAS_,N)) break; \ |
53 | if (r + n <= N) return true; \ | |
54 | } while (0) | |
55 | ||
56 | ||
d5f5fa27 | 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. */ | |
6db1d2ca | 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); |