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gatomic: Fix false positive with Clang+TSAN

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__atomic_load_8 and friends do not exist under clang. Use the generic
__atomic_load variant instead that are documented here:
https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html

These have the additional benefit that the exact size of gint (4 bytes)
or gpointer (4 or 8 bytes) no longer have to be checked.

I initially tried `__typeof__(*(atomic)) val;`, but that caused warnings
in Clang (-Wincompatible-pointer-types-discards-qualifiers) when
"atomic" points to a volatile variable. Aside from that, it is
apparently not supported everywhere, see the g_has_typeof macro.
Another reason not to use it are new warnings under Clang, including:

    glib/deprecated/gthread-deprecated.c:683:11: warning: incompatible pointer types initializing 'typeof (*(&mutex->mutex.mutex))' (aka 'union _GMutex *') with an expression of type 'GRecMutex *' (aka 'struct _GRecMutex *') [-Wincompatible-pointer-types]
              g_atomic_pointer_set (&mutex->mutex.mutex, result);

Hence, cast the atomic variable to gint/gpointer pointers, the size was
already statically asserted so the cast should be safe.

The macros use a (hopefully) rare "gaps_temp" name instead of something
like "val" to avoid an issue with GCC builds:

    glib/tests/once.c:123:test_once4: assertion failed (val == "foo"): (NULL == "foo")

Closes #1843
This commit is contained in:
Peter Wu 2019-07-26 00:55:52 +01:00
parent 17be9e5c4c
commit a3d90c0726
2 changed files with 11 additions and 42 deletions
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9
glib/gatomic.c
View File

@ -96,15 +96,6 @@
#if defined (__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4)
#if defined(__ATOMIC_SEQ_CST) && !defined(__clang__)
/* The implementation used in this code path in gatomic.h assumes
* 4-byte int */
G_STATIC_ASSERT (sizeof (gint) == 4);
/* The implementations in gatomic.h assume 4- or 8-byte pointers */
G_STATIC_ASSERT (sizeof (void *) == 4 || sizeof (void *) == 8);
#endif
/**
* g_atomic_int_get:
* @atomic: a pointer to a #gint or #guint

44
glib/gatomic.h
View File

@ -85,61 +85,39 @@ G_END_DECLS
#if defined(G_ATOMIC_LOCK_FREE) && defined(__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4)
/* We prefer the new C11-style atomic extension of GCC if available */
#if defined(__ATOMIC_SEQ_CST) && !defined(__clang__)
/* This assumes sizeof(int) is 4: gatomic.c statically
* asserts that (using G_STATIC_ASSERT at top-level in a header was
* problematic, see #730932) */
#if defined(__ATOMIC_SEQ_CST)
#define g_atomic_int_get(atomic) \
(G_GNUC_EXTENSION ({ \
G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gint)); \
gint gaig_temp; \
(void) (0 ? *(atomic) ^ *(atomic) : 1); \
(gint) __atomic_load_4 ((atomic), __ATOMIC_SEQ_CST); \
__atomic_load ((gint *)(atomic), &gaig_temp, __ATOMIC_SEQ_CST); \
(gint) gaig_temp; \
}))
#define g_atomic_int_set(atomic, newval) \
(G_GNUC_EXTENSION ({ \
G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gint)); \
gint gais_temp = (gint) (newval); \
(void) (0 ? *(atomic) ^ (newval) : 1); \
__atomic_store_4 ((atomic), (newval), __ATOMIC_SEQ_CST); \
__atomic_store ((gint *)(atomic), &gais_temp, __ATOMIC_SEQ_CST); \
}))
#if GLIB_SIZEOF_VOID_P == 8
#define g_atomic_pointer_get(atomic) \
(G_GNUC_EXTENSION ({ \
G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gpointer)); \
guint64 gapg_temp = __atomic_load_8 ((atomic), __ATOMIC_SEQ_CST); \
(gpointer) gapg_temp; \
gpointer gapg_temp; \
__atomic_load ((gpointer *)(atomic), &gapg_temp, __ATOMIC_SEQ_CST); \
gapg_temp; \
}))
#define g_atomic_pointer_set(atomic, newval) \
(G_GNUC_EXTENSION ({ \
G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gpointer)); \
gpointer gaps_temp = (gpointer)(newval); \
(void) (0 ? (gpointer) *(atomic) : NULL); \
__atomic_store_8 ((atomic), (gsize) (newval), __ATOMIC_SEQ_CST); \
__atomic_store ((gpointer *)(atomic), &gaps_temp, __ATOMIC_SEQ_CST); \
}))
#else /* GLIB_SIZEOF_VOID_P == 8 */
/* This assumes that if sizeof(void *) is not 8, then it is 4:
* gatomic.c statically asserts that (using G_STATIC_ASSERT
* at top-level in a header was problematic, see #730932) */
#define g_atomic_pointer_get(atomic) \
(G_GNUC_EXTENSION ({ \
G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gpointer)); \
guint32 gapg_temp = __atomic_load_4 ((atomic), __ATOMIC_SEQ_CST); \
(gpointer) gapg_temp; \
}))
#define g_atomic_pointer_set(atomic, newval) \
(G_GNUC_EXTENSION ({ \
G_STATIC_ASSERT (sizeof *(atomic) == sizeof (gpointer)); \
(void) (0 ? (gpointer) *(atomic) : NULL); \
__atomic_store_4 ((atomic), (gsize) (newval), __ATOMIC_SEQ_CST); \
}))
#endif /* GLIB_SIZEOF_VOID_P == 8 */
#else /* defined(__ATOMIC_SEQ_CST) */
#define g_atomic_int_get(atomic) \