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9a16f2653b
If the compiler doesn’t provide modern (C++11) atomic builtins (which is now quite unlikely), we implement our own using the `__sync_synchronize()` memory barrier. As Behdad and others have pointed out, though, the implementation didn’t follow the same semantics as we use with the C++11 builtins — `__ATOMIC_SEQ_CST`. Fix the use of memory barriers to provide `__ATOMIC_SEQ_CST` semantics. In particular, this fixes the following common pattern: ``` GObject *obj = my_object_new (); g_atomic_pointer_set (&shared_ptr, obj); ``` Previously this would have expanded to: ``` GObject *obj = my_object_new (); *shared_ptr = obj; __sync_synchronize (); ``` While the compiler would not have reordered the stores to `obj` and `shared_ptr` within the code on one thread (due to the dependency between them), the memory system might have made the write to `shared_ptr` visible to other threads before the write to `obj` — if they then dereferenced `shared_ptr` before seeing the write to `obj`, that would be a bug. Instead, the expansion is now: ``` GObject *obj = my_object_new (); __sync_synchronize (); *shared_ptr = obj; ``` This ensures that the write to `obj` is visible to all threads before any write to `shared_ptr` is visible to any threads. For completeness, `__sync_synchronize()` is augmented with a compiler barrier to ensure that no loads/stores can be reordered locally before or after it. Tested by disabling the C++11 atomic implementation and running: ``` meson test --repeat 1000 atomic atomic-test ``` Signed-off-by: Philip Withnall <withnall@endlessm.com> Fixes: #1449
306 lines
16 KiB
C
306 lines
16 KiB
C
/*
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* Copyright © 2011 Ryan Lortie
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*
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* This library is free software; you can redistribute it and/or
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* modify it under the terms of the GNU Lesser General Public
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* License as published by the Free Software Foundation; either
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* version 2.1 of the License, or (at your option) any later version.
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*
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* This library is distributed in the hope that it will be useful, but
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* WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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* Lesser General Public License for more details.
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*
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* You should have received a copy of the GNU Lesser General Public
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* License along with this library; if not, see <http://www.gnu.org/licenses/>.
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*
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* Author: Ryan Lortie <desrt@desrt.ca>
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*/
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#ifndef __G_ATOMIC_H__
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#define __G_ATOMIC_H__
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#if !defined (__GLIB_H_INSIDE__) && !defined (GLIB_COMPILATION)
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#error "Only <glib.h> can be included directly."
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#endif
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#include <glib/gtypes.h>
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G_BEGIN_DECLS
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GLIB_AVAILABLE_IN_ALL
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gint g_atomic_int_get (const volatile gint *atomic);
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GLIB_AVAILABLE_IN_ALL
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void g_atomic_int_set (volatile gint *atomic,
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gint newval);
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GLIB_AVAILABLE_IN_ALL
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void g_atomic_int_inc (volatile gint *atomic);
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GLIB_AVAILABLE_IN_ALL
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gboolean g_atomic_int_dec_and_test (volatile gint *atomic);
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GLIB_AVAILABLE_IN_ALL
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gboolean g_atomic_int_compare_and_exchange (volatile gint *atomic,
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gint oldval,
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gint newval);
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GLIB_AVAILABLE_IN_ALL
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gint g_atomic_int_add (volatile gint *atomic,
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gint val);
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GLIB_AVAILABLE_IN_2_30
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guint g_atomic_int_and (volatile guint *atomic,
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guint val);
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GLIB_AVAILABLE_IN_2_30
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guint g_atomic_int_or (volatile guint *atomic,
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guint val);
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GLIB_AVAILABLE_IN_ALL
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guint g_atomic_int_xor (volatile guint *atomic,
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guint val);
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GLIB_AVAILABLE_IN_ALL
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gpointer g_atomic_pointer_get (const volatile void *atomic);
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GLIB_AVAILABLE_IN_ALL
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void g_atomic_pointer_set (volatile void *atomic,
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gpointer newval);
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GLIB_AVAILABLE_IN_ALL
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gboolean g_atomic_pointer_compare_and_exchange (volatile void *atomic,
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gpointer oldval,
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gpointer newval);
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GLIB_AVAILABLE_IN_ALL
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gssize g_atomic_pointer_add (volatile void *atomic,
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gssize val);
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GLIB_AVAILABLE_IN_2_30
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gsize g_atomic_pointer_and (volatile void *atomic,
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gsize val);
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GLIB_AVAILABLE_IN_2_30
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gsize g_atomic_pointer_or (volatile void *atomic,
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gsize val);
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GLIB_AVAILABLE_IN_ALL
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gsize g_atomic_pointer_xor (volatile void *atomic,
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gsize val);
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GLIB_DEPRECATED_IN_2_30_FOR(g_atomic_int_add)
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gint g_atomic_int_exchange_and_add (volatile gint *atomic,
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gint val);
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G_END_DECLS
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#if defined(G_ATOMIC_LOCK_FREE) && defined(__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4)
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/* We prefer the new C11-style atomic extension of GCC if available */
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#if defined(__ATOMIC_SEQ_CST)
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#define g_atomic_int_get(atomic) \
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(G_GNUC_EXTENSION ({ \
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G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gint)); \
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gint gaig_temp; \
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(void) (0 ? *(atomic) ^ *(atomic) : 1); \
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__atomic_load ((gint *)(atomic), &gaig_temp, __ATOMIC_SEQ_CST); \
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(gint) gaig_temp; \
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}))
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#define g_atomic_int_set(atomic, newval) \
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(G_GNUC_EXTENSION ({ \
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G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gint)); \
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gint gais_temp = (gint) (newval); \
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(void) (0 ? *(atomic) ^ (newval) : 1); \
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__atomic_store ((gint *)(atomic), &gais_temp, __ATOMIC_SEQ_CST); \
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}))
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#define g_atomic_pointer_get(atomic) \
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(G_GNUC_EXTENSION ({ \
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G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gpointer)); \
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gpointer gapg_temp; \
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__atomic_load ((gpointer *)(atomic), &gapg_temp, __ATOMIC_SEQ_CST); \
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gapg_temp; \
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}))
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#define g_atomic_pointer_set(atomic, newval) \
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(G_GNUC_EXTENSION ({ \
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G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gpointer)); \
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gpointer gaps_temp = (gpointer)(newval); \
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(void) (0 ? (gpointer) *(atomic) : NULL); \
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__atomic_store ((gpointer *)(atomic), &gaps_temp, __ATOMIC_SEQ_CST); \
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}))
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#else /* defined(__ATOMIC_SEQ_CST) */
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/* We want to achieve __ATOMIC_SEQ_CST semantics here. See
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* https://en.cppreference.com/w/c/atomic/memory_order#Constants. For load
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* operations, that means performing an *acquire*:
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* > A load operation with this memory order performs the acquire operation on
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* > the affected memory location: no reads or writes in the current thread can
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* > be reordered before this load. All writes in other threads that release
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* > the same atomic variable are visible in the current thread.
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*
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* “no reads or writes in the current thread can be reordered before this load”
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* is implemented using a compiler barrier (a no-op `__asm__` section) to
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* prevent instruction reordering. Writes in other threads are synchronised
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* using `__sync_synchronize()`. It’s unclear from the GCC documentation whether
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* `__sync_synchronize()` acts as a compiler barrier, hence our explicit use of
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* one.
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*
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* For store operations, `__ATOMIC_SEQ_CST` means performing a *release*:
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* > A store operation with this memory order performs the release operation:
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* > no reads or writes in the current thread can be reordered after this store.
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* > All writes in the current thread are visible in other threads that acquire
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* > the same atomic variable (see Release-Acquire ordering below) and writes
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* > that carry a dependency into the atomic variable become visible in other
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* > threads that consume the same atomic (see Release-Consume ordering below).
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*
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* “no reads or writes in the current thread can be reordered after this store”
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* is implemented using a compiler barrier to prevent instruction reordering.
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* “All writes in the current thread are visible in other threads” is implemented
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* using `__sync_synchronize()`; similarly for “writes that carry a dependency”.
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*/
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#define g_atomic_int_get(atomic) \
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(G_GNUC_EXTENSION ({ \
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gint gaig_result; \
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G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gint)); \
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(void) (0 ? *(atomic) ^ *(atomic) : 1); \
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gaig_result = (gint) *(atomic); \
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__sync_synchronize (); \
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__asm__ __volatile__ ("" : : : "memory"); \
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gaig_result; \
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}))
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#define g_atomic_int_set(atomic, newval) \
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(G_GNUC_EXTENSION ({ \
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G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gint)); \
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(void) (0 ? *(atomic) ^ (newval) : 1); \
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__sync_synchronize (); \
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__asm__ __volatile__ ("" : : : "memory"); \
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*(atomic) = (newval); \
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}))
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#define g_atomic_pointer_get(atomic) \
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(G_GNUC_EXTENSION ({ \
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gpointer gapg_result; \
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G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gpointer)); \
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gapg_result = (gpointer) *(atomic); \
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__sync_synchronize (); \
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__asm__ __volatile__ ("" : : : "memory"); \
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gapg_result; \
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}))
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#define g_atomic_pointer_set(atomic, newval) \
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(G_GNUC_EXTENSION ({ \
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G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gpointer)); \
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(void) (0 ? (gpointer) *(atomic) : NULL); \
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__sync_synchronize (); \
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__asm__ __volatile__ ("" : : : "memory"); \
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*(atomic) = (__typeof__ (*(atomic))) (gsize) (newval); \
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}))
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#endif /* !defined(__ATOMIC_SEQ_CST) */
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#define g_atomic_int_inc(atomic) \
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(G_GNUC_EXTENSION ({ \
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G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gint)); \
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(void) (0 ? *(atomic) ^ *(atomic) : 1); \
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(void) __sync_fetch_and_add ((atomic), 1); \
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}))
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#define g_atomic_int_dec_and_test(atomic) \
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(G_GNUC_EXTENSION ({ \
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G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gint)); \
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(void) (0 ? *(atomic) ^ *(atomic) : 1); \
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__sync_fetch_and_sub ((atomic), 1) == 1; \
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}))
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#define g_atomic_int_compare_and_exchange(atomic, oldval, newval) \
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(G_GNUC_EXTENSION ({ \
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G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gint)); \
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(void) (0 ? *(atomic) ^ (newval) ^ (oldval) : 1); \
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__sync_bool_compare_and_swap ((atomic), (oldval), (newval)) ? TRUE : FALSE; \
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}))
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#define g_atomic_int_add(atomic, val) \
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(G_GNUC_EXTENSION ({ \
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G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gint)); \
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(void) (0 ? *(atomic) ^ (val) : 1); \
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(gint) __sync_fetch_and_add ((atomic), (val)); \
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}))
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#define g_atomic_int_and(atomic, val) \
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(G_GNUC_EXTENSION ({ \
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G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gint)); \
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(void) (0 ? *(atomic) ^ (val) : 1); \
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(guint) __sync_fetch_and_and ((atomic), (val)); \
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}))
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#define g_atomic_int_or(atomic, val) \
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(G_GNUC_EXTENSION ({ \
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G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gint)); \
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(void) (0 ? *(atomic) ^ (val) : 1); \
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(guint) __sync_fetch_and_or ((atomic), (val)); \
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}))
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#define g_atomic_int_xor(atomic, val) \
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(G_GNUC_EXTENSION ({ \
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G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gint)); \
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(void) (0 ? *(atomic) ^ (val) : 1); \
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(guint) __sync_fetch_and_xor ((atomic), (val)); \
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}))
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#define g_atomic_pointer_compare_and_exchange(atomic, oldval, newval) \
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(G_GNUC_EXTENSION ({ \
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G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gpointer)); \
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(void) (0 ? (gpointer) *(atomic) : NULL); \
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__sync_bool_compare_and_swap ((atomic), (oldval), (newval)) ? TRUE : FALSE; \
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}))
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#define g_atomic_pointer_add(atomic, val) \
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(G_GNUC_EXTENSION ({ \
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G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gpointer)); \
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(void) (0 ? (gpointer) *(atomic) : NULL); \
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(void) (0 ? (val) ^ (val) : 1); \
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(gssize) __sync_fetch_and_add ((atomic), (val)); \
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}))
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#define g_atomic_pointer_and(atomic, val) \
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(G_GNUC_EXTENSION ({ \
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G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gpointer)); \
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(void) (0 ? (gpointer) *(atomic) : NULL); \
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(void) (0 ? (val) ^ (val) : 1); \
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(gsize) __sync_fetch_and_and ((atomic), (val)); \
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}))
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#define g_atomic_pointer_or(atomic, val) \
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(G_GNUC_EXTENSION ({ \
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G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gpointer)); \
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(void) (0 ? (gpointer) *(atomic) : NULL); \
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(void) (0 ? (val) ^ (val) : 1); \
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(gsize) __sync_fetch_and_or ((atomic), (val)); \
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}))
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#define g_atomic_pointer_xor(atomic, val) \
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(G_GNUC_EXTENSION ({ \
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G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gpointer)); \
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(void) (0 ? (gpointer) *(atomic) : NULL); \
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(void) (0 ? (val) ^ (val) : 1); \
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(gsize) __sync_fetch_and_xor ((atomic), (val)); \
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}))
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#else /* defined(G_ATOMIC_LOCK_FREE) && defined(__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4) */
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#define g_atomic_int_get(atomic) \
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(g_atomic_int_get ((gint *) (atomic)))
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#define g_atomic_int_set(atomic, newval) \
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(g_atomic_int_set ((gint *) (atomic), (gint) (newval)))
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#define g_atomic_int_compare_and_exchange(atomic, oldval, newval) \
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(g_atomic_int_compare_and_exchange ((gint *) (atomic), (oldval), (newval)))
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#define g_atomic_int_add(atomic, val) \
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(g_atomic_int_add ((gint *) (atomic), (val)))
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#define g_atomic_int_and(atomic, val) \
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(g_atomic_int_and ((guint *) (atomic), (val)))
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#define g_atomic_int_or(atomic, val) \
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(g_atomic_int_or ((guint *) (atomic), (val)))
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#define g_atomic_int_xor(atomic, val) \
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(g_atomic_int_xor ((guint *) (atomic), (val)))
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#define g_atomic_int_inc(atomic) \
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(g_atomic_int_inc ((gint *) (atomic)))
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#define g_atomic_int_dec_and_test(atomic) \
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(g_atomic_int_dec_and_test ((gint *) (atomic)))
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#define g_atomic_pointer_get(atomic) \
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(g_atomic_pointer_get (atomic))
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#define g_atomic_pointer_set(atomic, newval) \
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(g_atomic_pointer_set ((atomic), (gpointer) (newval)))
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#define g_atomic_pointer_compare_and_exchange(atomic, oldval, newval) \
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(g_atomic_pointer_compare_and_exchange ((atomic), (gpointer) (oldval), (gpointer) (newval)))
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#define g_atomic_pointer_add(atomic, val) \
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(g_atomic_pointer_add ((atomic), (gssize) (val)))
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#define g_atomic_pointer_and(atomic, val) \
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(g_atomic_pointer_and ((atomic), (gsize) (val)))
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#define g_atomic_pointer_or(atomic, val) \
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(g_atomic_pointer_or ((atomic), (gsize) (val)))
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#define g_atomic_pointer_xor(atomic, val) \
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(g_atomic_pointer_xor ((atomic), (gsize) (val)))
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#endif /* defined(__GNUC__) && defined(G_ATOMIC_OP_USE_GCC_BUILTINS) */
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#endif /* __G_ATOMIC_H__ */
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