83 lines
3.1 KiB
C
83 lines
3.1 KiB
C
/* Copyright (C) 2012-2021 Free Software Foundation, Inc.
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Contributed by Richard Henderson <rth@redhat.com>.
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This file is part of the GNU Atomic Library (libatomic).
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Libatomic is free software; you can redistribute it and/or modify it
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under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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Libatomic is distributed in the hope that it will be useful, but WITHOUT ANY
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WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
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FOR A PARTICULAR PURPOSE. See the GNU General Public License for
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more details.
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Under Section 7 of GPL version 3, you are granted additional
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permissions described in the GCC Runtime Library Exception, version
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3.1, as published by the Free Software Foundation.
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You should have received a copy of the GNU General Public License and
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a copy of the GCC Runtime Library Exception along with this program;
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see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
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<http://www.gnu.org/licenses/>. */
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#include "libatomic_i.h"
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/* Accesses with a power-of-two size are not lock-free if we don't have an
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integer type of this size or if they are not naturally aligned. They
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are lock-free if such a naturally aligned access is always lock-free
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according to the compiler, which requires that both atomic loads and CAS
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are available.
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In all other cases, we fall through to LARGER (see below). */
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#define EXACT(N) \
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do { \
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if (!C2(HAVE_INT,N)) break; \
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if ((uintptr_t)ptr & (N - 1)) break; \
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if (__atomic_always_lock_free(N, 0)) return true; \
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if (!C2(MAYBE_HAVE_ATOMIC_CAS_,N)) break; \
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if (C2(FAST_ATOMIC_LDST_,N)) return true; \
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} while (0)
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/* We next check to see if an access of a larger size is lock-free. We use
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a similar check as in EXACT, except that we also check that the alignment
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of the access is so that the data to be accessed is completely covered
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by the larger access. */
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#define LARGER(N) \
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do { \
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uintptr_t r = (uintptr_t)ptr & (N - 1); \
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if (!C2(HAVE_INT,N)) break; \
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if (!C2(FAST_ATOMIC_LDST_,N)) break; \
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if (!C2(MAYBE_HAVE_ATOMIC_CAS_,N)) break; \
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if (r + n <= N) return true; \
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} while (0)
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/* Note that this can return that a size/alignment is not lock-free even if
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all the operations that we use to implement the respective accesses provide
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lock-free forward progress as specified in C++14: Users likely expect
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"lock-free" to also mean "fast", which is why we do not return true if, for
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example, we implement loads with this size/alignment using a CAS. */
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bool
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libat_is_lock_free (size_t n, void *ptr)
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{
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switch (n)
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{
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case 0: return true;
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case 1: EXACT(1); goto L4;
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case 2: EXACT(2); goto L4;
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case 4: EXACT(4); goto L8;
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case 8: EXACT(8); goto L16;
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case 16: EXACT(16); break;
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case 3: L4: LARGER(4); /* FALLTHRU */
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case 5 ... 7: L8: LARGER(8); /* FALLTHRU */
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case 9 ... 15: L16: LARGER(16); break;
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}
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return false;
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}
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EXPORT_ALIAS (is_lock_free);
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