Peter,

From an other hand, decrementing the word variable for AVR gives us:

sbiw r24,1
brne label

But the same fragment for 8051 will be much more:

clr c
mov a,r6
subb a,#1
mov r6,a
mov a,r7
subb a,#0
mov r7,a
jnz label

So your example proves nothing.

Best regards,
George