linux-kernel - [tip:x86/asm] x86/asm/tsc: Save an instruction in DECLARE

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Date:	Mon, 6 Jul 2015 08:45:48 -0700
From:	tip-bot for George Spelvin <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	luto@...capital.net, bp@...e.de, hpa@...or.com, mingo@...nel.org,
	brgerst@...il.com, dvlasenk@...hat.com, bp@...en8.de,
	linux-kernel@...r.kernel.org, peterz@...radead.org,
	tglx@...utronix.de, torvalds@...ux-foundation.org,
	linux@...izon.com
Subject: [tip:x86/asm] x86/asm/tsc:
  Save an instruction in DECLARE_ARGS users

Commit-ID:  5a33fcb8d991209bac0a266ab499e4b53d116cdd
Gitweb:     http://git.kernel.org/tip/5a33fcb8d991209bac0a266ab499e4b53d116cdd
Author:     George Spelvin <linux@...izon.com>
AuthorDate: Thu, 25 Jun 2015 18:44:13 +0200
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Mon, 6 Jul 2015 15:23:30 +0200

x86/asm/tsc: Save an instruction in DECLARE_ARGS users

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>
Signed-off-by: Borislav Petkov <bp@...e.de>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: jacob.jun.pan@...ux.intel.com
Link: http://lkml.kernel.org/r/20150618075906.4615.qmail@ns.horizon.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 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 02bdd6c..131eec2 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
 
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