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@xxxxxxxxxxx> --- I came across this while in the middle of hacking on some other code that isn't booting yet, so I haven't exactly tested it thoroughly. I *did* check both the Intel and AMD specifications to ensure that this was a safe optimization, and did user-space tests on both Intel and AMD processors. But it's simple enough I'm willing to risk public embarrassment. 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 de36f22e..d6b8a80d 100644 --- a/arch/x86/include/asm/msr.h +++ b/arch/x86/include/asm/msr.h @@ -46,14 +46,13 @@ 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 -- 2.1.4 -- To unsubscribe from this list: send the line "unsubscribe linux-x86_64" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
