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0025f0042f
As usual, we use C11 semantics, and emulate it if <stdatomic.h> is not available. It's a bit messy with __sync_val_compare_and_swap(). We assume it has "strong" semantics (it can't fail sporadically), but I'm not sure if this is really the case. On the other hand, weak semantics don't seem to be possible, since the builtin can't distinguish between the two failure cases that could occur. Also, to match the C11 interface, use of gcc builtins is unavoidable. Add a check to the build system to make sure the compiler supports them (although I don't think there's any compiler which supports __sync_*, but not these extensions). Needed for the following commit.
89 lines
2.8 KiB
C
89 lines
2.8 KiB
C
/*
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* This file is part of mpv.
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* Copyright (c) 2013 Stefano Pigozzi <stefano.pigozzi@gmail.com>
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*
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* mpv is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* mpv is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License along
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* with mpv. If not, see <http://www.gnu.org/licenses/>.
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*/
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#ifndef MP_ATOMICS_H
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#define MP_ATOMICS_H
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#include <inttypes.h>
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#include "config.h"
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#define HAVE_ATOMICS 1
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#if HAVE_STDATOMIC
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#include <stdatomic.h>
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#else
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// Emulate the parts of C11 stdatomic.h needed by mpv.
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// Still relies on gcc/clang atomic builtins.
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typedef struct { volatile unsigned long v; } atomic_ulong;
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typedef struct { volatile int v; } atomic_int;
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typedef struct { volatile _Bool v; } atomic_bool;
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typedef struct { volatile long long v; } atomic_llong;
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typedef struct { volatile uint_least32_t v; } atomic_uint_least32_t;
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typedef struct { volatile unsigned long long v; } atomic_ullong;
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#define ATOMIC_VAR_INIT(x) \
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{.v = (x)}
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#if HAVE_ATOMIC_BUILTINS
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#define atomic_load(p) \
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__atomic_load_n(&(p)->v, __ATOMIC_SEQ_CST)
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#define atomic_store(p, val) \
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__atomic_store_n(&(p)->v, val, __ATOMIC_SEQ_CST)
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#define atomic_fetch_add(a, b) \
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__atomic_fetch_add(&(a)->v, b, __ATOMIC_SEQ_CST)
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#define atomic_compare_exchange_strong(a, b, c) \
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__atomic_compare_exchange_n(&(a)->v, b, c, 0, __ATOMIC_SEQ_CST, \
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__ATOMIC_SEQ_CST)
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#elif HAVE_SYNC_BUILTINS
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#define atomic_load(p) \
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__sync_fetch_and_add(&(p)->v, 0)
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#define atomic_store(p, val) \
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(__sync_synchronize(), (p)->v = (val), __sync_synchronize())
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#define atomic_fetch_add(a, b) \
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__sync_fetch_and_add(&(a)->v, b)
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// Assumes __sync_val_compare_and_swap is "strong" (using the C11 meaning).
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#define atomic_compare_exchange_strong(p, old, new) \
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({ __typeof__((p)->v) val_ = __sync_val_compare_and_swap(&(p)->v, *(old), new); \
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bool ok_ = val_ == *(old); \
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if (!ok_) *(old) = val_; \
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ok_; })
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#else
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// This is extremely wrong. The build system actually disables code that has
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// a serious dependency on working atomics, so this is barely ok.
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#define atomic_load(p) ((p)->v)
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#define atomic_store(p, val) ((p)->v = (val))
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#define atomic_fetch_add(a, b) (((a)->v += (b)) - (b))
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#define atomic_compare_exchange_strong(p, old, new) \
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((p)->v == *(old) ? ((p)->v = (new), 1) : (*(old) = (p)->v, 0))
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#undef HAVE_ATOMICS
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#define HAVE_ATOMICS 0
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#endif /* no atomics */
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#endif /* else HAVE_STDATOMIC */
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#endif
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