linux-kernel - [PATCH 19/19] x86/asm: Save an instruction in DECLARE

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Date:	Thu, 25 Jun 2015 18:44:13 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH 19/19] x86/asm: Save an instruction in DECLARE_ARGS users

From: George Spelvin <linux@...izon.com>

Before, the code to do RDTSC looked like:

  rdtsc
  shl $0x20,%rdx
  mov %eax,%eax
  or  %rdx,%rax

The "mov %eax,%eax" is required to clear the high 32 bits of %rax.

By declaring low and high as 64-bit variables, the code is simplified
to:

  rdtsc
  shl $0x20,%rdx
  or  %rdx,%rax

Yes, it's a 2-byte instruction that's not on a critical path, but there
are principles to be upheld.

Every user of EAX_EDX_RET has been checked. I tried to check users of
EAX_EDX_ARGS, but there weren't any, so I deleted it to be safe.

(There's no benefit to making "high" 64 bits, but it was the simplest
way to proceed.)

Signed-off-by: George Spelvin <linux@...izon.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: jacob.jun.pan@...ux.intel.com
Cc: x86-ml <x86@...nel.org>
Link: http://lkml.kernel.org/r/20150618075906.4615.qmail@ns.horizon.com
Signed-off-by: Borislav Petkov <bp@...e.de>
---
 arch/x86/include/asm/msr.h | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
index 02bdd6c65017..131eec2ca137 100644
--- a/arch/x86/include/asm/msr.h
+++ b/arch/x86/include/asm/msr.h
@@ -47,14 +47,13 @@ static inline unsigned long long native_read_tscp(unsigned int *aux)
  * it means rax *or* rdx.
  */
 #ifdef CONFIG_X86_64
-#define DECLARE_ARGS(val, low, high)	unsigned low, high
-#define EAX_EDX_VAL(val, low, high)	((low) | ((u64)(high) << 32))
-#define EAX_EDX_ARGS(val, low, high)	"a" (low), "d" (high)
+/* Using 64-bit values saves one instruction clearing the high half of low */
+#define DECLARE_ARGS(val, low, high)	unsigned long low, high
+#define EAX_EDX_VAL(val, low, high)	((low) | (high) << 32)
 #define EAX_EDX_RET(val, low, high)	"=a" (low), "=d" (high)
 #else
 #define DECLARE_ARGS(val, low, high)	unsigned long long val
 #define EAX_EDX_VAL(val, low, high)	(val)
-#define EAX_EDX_ARGS(val, low, high)	"A" (val)
 #define EAX_EDX_RET(val, low, high)	"=A" (val)
 #endif
 
-- 
2.3.5

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