On Thursday 21 April 2005 15:58, Joerg Wunsch wrote:
> As Dmitry K. wrote:
> > I have look 'ffs' from newlib (1.12.0). It take up to 800 (!)
> > clocks. Reason: shift is included to loop.
>
> The slightly modified BSD code (counter reduced to 8 bits) yields
>
> sbiw r24,0
> breq .L1
> ldi r18,lo8(1)
> .L9:
> sbrc r24,0
> rjmp .L8
> lsr r25
> ror r24
> subi r18,lo8(-(1))
> rjmp .L9
> .L8:
> mov r24,r18
> clr r25
> .L1:
> ret
>
> This makes 6 clocks per cycle, so up to ~ 100 clocks max.
Misprint: 7 clocks per cycle (rjmp).
> I'm interested in seeing Dmitry's code...
I have use byte-width shift:
#define val_lo r24
#define val_hi r25
#define cntr r30
...
ffs:
ldi cntr, 1
tst val_lo
brne 2f
or val_lo, val_hi
breq 9f
ldi cntr, 8
1: inc cntr
2: ror val_lo
brcc 1b
mov r24, cntr
clr r25
9: ret
Summary: 6+4*7+3+2 = 39 clocks (max), without call/ret.
ffsl: the same idea --> 18 words.
ffsll: movw+sbiw usage --> 26 words.
Dmitry.
_______________________________________________
AVR-libc-dev mailing list
[email protected]
http://lists.nongnu.org/mailman/listinfo/avr-libc-dev
