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Subject[PATCH 19/19] x86/asm: Save an instruction in DECLARE_ARGS users
Date
From: George Spelvin <linux@horizon.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@horizon.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: jacob.jun.pan@linux.intel.com
Cc: x86-ml <x86@kernel.org>
Link: http://lkml.kernel.org/r/20150618075906.4615.qmail@ns.horizon.com
Signed-off-by: Borislav Petkov <bp@suse.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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