x86: atomic64: Improve atomic64_read()

Optimize atomic64_read() as a special open-coded
cmpxchg8b variant. This generates nicer code:

arch/x86/lib/atomic64_32.o:

   text	   data	    bss	    dec	    hex	filename
    435	      0	      0	    435	    1b3	atomic64_32.o.before
    431	      0	      0	    431	    1af	atomic64_32.o.after

md5:
   bd8ab95e69c93518578bfaf0ea3be4d9  atomic64_32.o.before.asm
   2bdfd4bd1f6b7b61b7fc127aef90ce3b  atomic64_32.o.after.asm

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: NIngo Molnar <mingo@elte.hu>

arch/x86/lib/atomic64_32.c

@@ -62,9 +62,17 @@ void atomic64_set(atomic64_t *ptr, u64 new_val)
  */
 u64 atomic64_read(atomic64_t *ptr)
 {
-	u64 old = 1LL << 32;
+	u64 res;
 
-	return cmpxchg8b(&ptr->counter, old, old);
+	asm volatile(
+		"mov %%ebx, %%eax\n\t"
+		"mov %%ecx, %%edx\n\t"
+		LOCK_PREFIX "cmpxchg8b %1\n"
+			: "+A" (res)
+			: "m" (*ptr)
+		);
+
+	return res;
 }
 
 /**