ref: 8a6a354e22521e7d2d14b8b4c2d1b7a65a0c8d5e 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