[Bug target/50256] AVR GCC - several unnecessary register moves

["gjl at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50256

--- Comment #6 from Georg-Johann Lay <gjl at gcc dot gnu.org> 2011-09-04 22:19:53 UTC ---
You don't need R20: Simply use %D0 which is cleared, anyway. As %0 is early
clobber, it's not an input and you can clear is at the beginning.

You don't need to clear R4/R5 (similar R6/R7): Just rearrange multiplications
and use (note R6 is (implicitely) 0 at that time)
   mul %A1,%C2
   movw r4,r0
instead of
   mul %A1,%C2
   add r4,r0
   adc r5,r1

You don't need to move to answer by hand; just use %A0 instead of R5 etc. and
you save moves and register footprint (notice that this interferes with
previous hint because you change registers even/odd; it's up to you to work it
out and find smartest way of your assembler).

Finally, you could let the compiler allocate temporary registers for you, i.e.
a 16-bit instead of R2/R3 etc. The compiler knows better which registers are
best and will try to use call-clobbered registers instead of expensive
call-used ones.

All in all, you will get a much greater performance gain by tweaking you code
than the compiler could ever do by saving some poor register moves ;-)