There's quite a bit of stuff on AVR assembler at http://www.avr-asm-tutorial.net/avr_en/
westfw:
Are you sure now? i *= 5 compiles to:12e: 9c 01 movw r18, r24
130: 22 0f add r18, r18
132: 33 1f adc r19, r19
134: 22 0f add r18, r18
136: 33 1f adc r19, r19
138: 28 0f add r18, r24
13a: 39 1f adc r19, r25
(that's (i+i)+(i+i)+i )
I can't see a way of beating that. Just for fun I tried a few other factors too, and GCC always comes up with extremely efficient code.
Ah, how I don't miss the days of writing assembler. 