[Qemu-devel] [PATCH for-1.7] atomic.h: Fix build with clang

[Peter Maydell]

clang defines __ATOMIC_SEQ_CST but its implementation of the
__atomic_exchange() builtin differs from that of gcc. Move the
__clang__ branch of the ifdef ladder to the top and fix its
implementation (there is no such builtin as __sync_exchange),
so we can compile with clang again.

Signed-off-by: Peter Maydell <address@hidden>
---
I don't have access to a Linux box with clang, so this is only
tested on MacOSX clang, but I believe it to be both required
and correct for all clang platforms.

 include/qemu/atomic.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/qemu/atomic.h b/include/qemu/atomic.h
index 0aa8913..492bce1 100644
--- a/include/qemu/atomic.h
+++ b/include/qemu/atomic.h
@@ -168,14 +168,14 @@
 #endif
 
 #ifndef atomic_xchg
-#ifdef __ATOMIC_SEQ_CST
+#if defined(__clang__)
+#define atomic_xchg(ptr, i)    __sync_swap(ptr, i)
+#elif defined(__ATOMIC_SEQ_CST)
 #define atomic_xchg(ptr, i)    ({                           \
     typeof(*ptr) _new = (i), _old;                          \
     __atomic_exchange(ptr, &_new, &_old, __ATOMIC_SEQ_CST); \
     _old;                                                   \
 })
-#elif defined __clang__
-#define atomic_xchg(ptr, i)    __sync_exchange(ptr, i)
 #else
 /* __sync_lock_test_and_set() is documented to be an acquire barrier only.  */
 #define atomic_xchg(ptr, i)    (smp_mb(), __sync_lock_test_and_set(ptr, i))
-- 
1.7.11.4