ref: 1c30bd451e1813f1cf9f9db4d7c1a4f3f95d1e37 dir: /sys/src/libc/amd64/cycles.s/
TEXT cycles(SB),1,$0 /* time stamp counter; cycles since power up */ RDTSC MOVL AX, 0(RARG) /* lo */ MOVL DX, 4(RARG) /* hi */ RET