[avr-gcc-list] [avr-libc]: Smarter parity implementation

[Georg-Johann Lay]

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

Regards.