In the absence of SWAP (which should be there - even the ATtiny10 has it), you'd want a shift left (SWAP does the equivalent of a rotate, but that's an "optimization" assuming that the original values is less than 16.) a shift-left is just adding to yourself:

```
mov x1, x2
add x1, x1 ;;; *2
add x1, x1 ;;; *4
add x1, x1 ;;; *8
add x1, x1 ;;; *16
sub x1, x2 ;;; result = Y*15
sub x1, x2 ;;; result = Y*14
```

(if you look at the instruction set manual, you'll discover that "ROL r" encodes to the same instruction as "ADC r,r", and "LSL r" encodes the same as "ADD r,r")