Parthasaradhi Nayani wrote: > >> From: Nicholas Vinen <h...@x256.org> >> > > For example, things like "unsigned char x, y; > >> x = y>>4" could >> use the nibble swap instruction rather than four shifts, >> and things like >> > > Shifting a byte or int right or left must push in 00s from the other side so > swapping a nibble is not the right thing to do. So is the case with other > examples. Correct me if I am wrong. > > Nayani > > Yes, it has to blank the top 4 bits, but I believe it's still faster to swap the nibble and do that than four shifts. Something like:
SWAP r1 LDI $15, r2 AND r2, r1 This is three instructions and three cycles, as opposed to: LSR r1 LSR r1 LSR r1 LSR r1 which is four instructions and cycles. The former requires a spare register but that generally isn't a problem. This is just an example. I didn't note them down at the time but I saw the compiler doing a lot of things the "long way" when there was a simple, faster, smaller way to do it. The case of accessing some of the bytes in a larger type via shifting was particularly annoying. Perhaps a union would have solved that, but it seems silly to have to resort to doing it that way. Now that I've signed up to this list, if and when I come across avr-gcc missing obvious optimisations I'll report them. Nicholas
_______________________________________________ AVR-GCC-list mailing list AVR-GCC-list@nongnu.org http://lists.nongnu.org/mailman/listinfo/avr-gcc-list
