x86: atomic64: Improve cmpxchg8b()

Rewrite cmpxchg8b() to not use %edi register but a generic "+m"
constraint, to increase compiler freedom in code generation and
possibly better code.

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
LKML-Reference: <alpine.LFD.2.01.0907021653030.3210@localhost.localdomain>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 7 insertions, 12 deletions

diff --git a/arch/x86/lib/atomic64_32.c b/arch/x86/lib/atomic64_32.c
index afa5d444918b..5fc1e2caa544 100644
--- a/arch/x86/lib/atomic64_32.c
+++ b/arch/x86/lib/atomic64_32.c
@@ -6,19 +6,14 @@
 
 static inline u64 cmpxchg8b(u64 *ptr, u64 old, u64 new)
 {
-        asm volatile(
-
-                LOCK_PREFIX "cmpxchg8b (%[ptr])\n"
-
-                     :          "=A" (old)
-
-                     : [ptr]    "D" (ptr),
-                                "A" (old),
-                                "b" (ll_low(new)),
-                                "c" (ll_high(new))
-
-                     : "memory");
+        u32 low = new;
+        u32 high = new >> 32;
 
+        asm volatile(
+                LOCK_PREFIX "cmpxchg8b %1\n"
+                     : "+A" (old), "+m" (*ptr)
+                     :  "b" (low),  "c" (high)
+                     );
         return old;
 }