linux-kernel - [PATCH] x86/asm: Use CC_SET/CC_OUT in percpu_cmpxchg8b

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Date:   Tue,  5 Jun 2018 18:39:09 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>, x86@...nel.org
Subject: [PATCH] x86/asm: Use CC_SET/CC_OUT in percpu_cmpxchg8b_double

Use CC_SET(z)/CC_OUT(z) instead of explicit setz instruction.
Using these two defines, the compiler that supports generation of
condition code outputs from inline assembly flags generates e.g.:

  cmpxchg8b %fs:(%esi)
  jne    172255 <__kmalloc+0x65>

instead of

  cmpxchg8b %fs:(%esi)
  sete   %al
  test   %al,%al
  je     172255 <__kmalloc+0x65>

Note that older compilers now generate

  cmpxchg8b %fs:(%esi)
  sete   %cl
  test   %cl,%cl
  je     173a85 <__kmalloc+0x65>

since we have to mark that cmpxchg8b instruction outputs to %eax
register and this way clobbers the value in the register.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: x86@...nel.org
---
 arch/x86/include/asm/percpu.h | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/percpu.h b/arch/x86/include/asm/percpu.h
index a06b07399d17..e9202a0de8f0 100644
--- a/arch/x86/include/asm/percpu.h
+++ b/arch/x86/include/asm/percpu.h
@@ -450,9 +450,10 @@ do {									\
 	bool __ret;							\
 	typeof(pcp1) __o1 = (o1), __n1 = (n1);				\
 	typeof(pcp2) __o2 = (o2), __n2 = (n2);				\
-	asm volatile("cmpxchg8b "__percpu_arg(1)"\n\tsetz %0\n\t"	\
-		    : "=a" (__ret), "+m" (pcp1), "+m" (pcp2), "+d" (__o2) \
-		    :  "b" (__n1), "c" (__n2), "a" (__o1));		\
+	asm volatile("cmpxchg8b "__percpu_arg(1)			\
+		     CC_SET(z)						\
+		     : CC_OUT(z) (__ret), "+m" (pcp1), "+m" (pcp2), "+a" (__o1), "+d" (__o2) \
+		     : "b" (__n1), "c" (__n2));				\
 	__ret;								\
 })
 
-- 
2.17.1