Georg-Johann Lay wrote:
> The current implementation of parity in avr-libc compiles
>
> #include <stdint.h>
> #include <util/parity.h>
>
> uint8_t pari1 (uint8_t val)
> {
> return parity_even_bit (val);
> }
>
> with -Os to
>
> pari1:
> /* #APP */
> mov __tmp_reg__,r24
> swap r24
> eor r24,__tmp_reg__
> mov __tmp_reg__,r24
> lsr r24
> lsr r24
> eor r24,__tmp_reg__
> /* #NOAPP */
> ldi r25,lo8(0)
> adiw r24,1
> asr r25
> ror r24
> andi r24,lo8(1)
> ret
>
> which are 12 instructions, 13 ticks and 2 regs.
>
> Parity can be implemented with 9 instructions
> and 9 ticks and one reg. Instead of r24 any d-reg
> can be used:
>
> ;; r24 = parity8 (r24)
> ;; clobbers: __tmp_reg__
> parity8:
> ;; parity is in r24[0..7]
> mov __tmp_reg__, r24
> swap __tmp_reg__
> eor r24, __tmp_reg__
> ;; parity is in r24[0..3]
> subi r24, -4
> andi r24, -5
> subi r24, -6
> ;; parity is in r24[0,3]
> sbrc r24, 3
> inc r24
> ;; parity is in r24[0]
> andi r24, 1
>
> An implementation similar to the original avr-libc
> using one instruction/tick more is
>
> mov __tmp_reg__,r24
> swap r24
> eor r24,__tmp_reg__
> mov __tmp_reg__,r24
> lsr r24
> lsr r24
> eor r24,__tmp_reg__
> lsr r24
> sbci r24,0
> andi r24,1
IIRC you're required to clear r25 even if the return type is uint8_t,
i.e., the return type needs to be upgraded to "int".
I think this happens because, since most expressions are upgraded to int
anyway, it is cheaper to do it in one place than in all call sites.
--
Paulo Marques
Software Development Department - Grupo PIE, S.A.
Phone: +351 252 290600, Fax: +351 252 290601
Web: www.grupopie.com
"Very funny Scotty. Now beam up my clothes."
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