x86: atomic64 assembly improvements

In the "xchg" implementation, %ebx and %ecx don't need to be copied
into %eax and %edx respectively (this is only necessary when desiring
to only read the stored value).

In the "add_unless" implementation, swapping the use of %ecx and %esi
for passing arguments allows %esi to become an input only (i.e.
permitting the register to be re-used to address the same object
without reload).

In "{add,sub}_return", doing the initial read64 through the passed in
%ecx decreases a register dependency.

In "inc_not_zero", a branch can be eliminated by or-ing together the
two halves of the current (64-bit) value, and code size can be further
reduced by adjusting the arithmetic slightly.

v2: Undo the folding of "xchg" and "set".

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Link: http://lkml.kernel.org/r/4F19A2BC020000780006E0DC@nat28.tlf.novell.com
Cc: Luca Barbieri <luca@luca-barbieri.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

arch/x86/lib/atomic64_cx8_32.S

@@ -55,8 +55,6 @@ ENDPROC(atomic64_set_cx8)
 ENTRY(atomic64_xchg_cx8)
 	CFI_STARTPROC
 
-	movl %ebx, %eax
-	movl %ecx, %edx
 1:
 	LOCK_PREFIX
 	cmpxchg8b (%esi)
@@ -78,7 +76,7 @@ ENTRY(atomic64_\func\()_return_cx8)
 	movl %edx, %edi
 	movl %ecx, %ebp
 
-	read64 %ebp
+	read64 %ecx
 1:
 	movl %eax, %ebx
 	movl %edx, %ecx
@@ -159,23 +157,22 @@ ENTRY(atomic64_add_unless_cx8)
 	SAVE ebx
 /* these just push these two parameters on the stack */
 	SAVE edi
-	SAVE esi
+	SAVE ecx
 
-	movl %ecx, %ebp
-	movl %eax, %esi
+	movl %eax, %ebp
 	movl %edx, %edi
 
-	read64 %ebp
+	read64 %esi
 1:
 	cmpl %eax, 0(%esp)
 	je 4f
 2:
 	movl %eax, %ebx
 	movl %edx, %ecx
-	addl %esi, %ebx
+	addl %ebp, %ebx
 	adcl %edi, %ecx
 	LOCK_PREFIX
-	cmpxchg8b (%ebp)
+	cmpxchg8b (%esi)
 	jne 1b
 
 	movl $1, %eax
@@ -199,13 +196,13 @@ ENTRY(atomic64_inc_not_zero_cx8)
 
 	read64 %esi
 1:
-	testl %eax, %eax
-	je 4f
-2:
+	movl %eax, %ecx
+	orl %edx, %ecx
+	jz 3f
 	movl %eax, %ebx
-	movl %edx, %ecx
+	xorl %ecx, %ecx
 	addl $1, %ebx
-	adcl $0, %ecx
+	adcl %edx, %ecx
 	LOCK_PREFIX
 	cmpxchg8b (%esi)
 	jne 1b
@@ -214,9 +211,5 @@ ENTRY(atomic64_inc_not_zero_cx8)
 3:
 	RESTORE ebx
 	ret
-4:
-	testl %edx, %edx
-	jne 2b
-	jmp 3b
 	CFI_ENDPROC
 ENDPROC(atomic64_inc_not_zero_cx8)