atomic: drop __sync builtins

The correctness of the stdatomic.h emulation via the __sync builtins is
questionable, and we've been relying on exact stdatomic semantics for a
while, so just get rid of it. Compilers which support __sync but not
stdatomic.h will use to the slow mutex fallback.

Not sure about the __atomic builtins. It doesn't seem to harm either, so
leave it for now.

2 files changed, 1 insertions, 28 deletions

diff --git a/osdep/atomic.h b/osdep/atomic.h
index 9028a504eb..50f4f403da 100644
--- a/osdep/atomic.h
+++ b/osdep/atomic.h
@@ -61,25 +61,6 @@ typedef struct { volatile unsigned long long v; } atomic_ullong;
     __atomic_compare_exchange_n(&(a)->v, b, c, 0, __ATOMIC_SEQ_CST, \
     __ATOMIC_SEQ_CST)
 
-#elif HAVE_SYNC_BUILTINS
-
-#define atomic_load(p) \
-    __sync_fetch_and_add(&(p)->v, 0)
-#define atomic_store(p, val) \
-    (__sync_synchronize(), (p)->v = (val), __sync_synchronize())
-#define atomic_fetch_add(a, b) \
-    __sync_fetch_and_add(&(a)->v, b)
-#define atomic_fetch_and(a, b) \
-    __sync_fetch_and_and(&(a)->v, b)
-#define atomic_fetch_or(a, b) \
-    __sync_fetch_and_or(&(a)->v, b)
-// Assumes __sync_val_compare_and_swap is "strong" (using the C11 meaning).
-#define atomic_compare_exchange_strong(p, old, new) \
-    ({ __typeof__((p)->v) val_ = __sync_val_compare_and_swap(&(p)->v, *(old), new); \
-       bool ok_ = val_ == *(old);       \
-       if (!ok_) *(old) = val_;         \
-       ok_; })
-
 #elif defined(__GNUC__)
 
 #include <pthread.h>
diff --git a/wscript b/wscript
index a7ec0a9c3d..6ebc3f9fd0 100644
--- a/wscript
+++ b/wscript
@@ -179,19 +179,11 @@ main_dependencies = [
                 'test = __atomic_add_fetch(&test, 1, __ATOMIC_SEQ_CST)')),
         'deps_neg': [ 'stdatomic' ],
     }, {
-        'name': 'sync-builtins',
-        'desc': 'compiler support for __sync built-ins',
-        'func': check_statement('stdint.h',
-                    'int64_t test = 0;'
-                    '__typeof__(test) x = ({int a = 1; a;});'
-                    'test = __sync_add_and_fetch(&test, 1)'),
-        'deps_neg': [ 'stdatomic', 'atomic-builtins' ],
-    }, {
         'name': 'atomics',
         'desc': 'stdatomic.h support or emulation',
         'func': check_true,
         'req': True,
-        'deps_any': ['stdatomic', 'atomic-builtins', 'sync-builtins', 'gnuc'],
+        'deps_any': ['stdatomic', 'atomic-builtins', 'gnuc'],
     }, {
         'name': 'c11-tls',
         'desc': 'C11 TLS support',