Re: [avr-gcc-list] dev and mod may not be optimized

[David Brown]

Dmitry K. wrote:
> On Wednesday 12 December 2007 01:16, Albert Andras wrote:
> > ----- Original Message -----
> > From: "Nathan Moore" <address@hidden>
> >
> > > unsigned char a, b, c;
> > > c = some_input_function();
> > > LABEL:
> > > a = c/10;
> > > b = c%10;
> > > ...
> > >
> > > *****************************************
> > > LABEL:
> > > mov r24,r18
> > > ldi r22,lo8(10)
> > > call __udivmodqi4
> > > mov r19,r24
> > > .LM436:
> > > mov r24,r18
> > >  call __udivmodqi4
> > >
> > > If I'm understanding __udivmodqi4 it produces the results of both of
> > > these with one call.
> > Acording to:
> > http://www.nongnu.org/avr-libc/user-manual/structdiv__t.html
> >
> > you could do:
> >
> > div_t t;
> >       t=div(c/10);
> >       a=t.quot;
> >       b=t.rem;
>
> Alas, the usage of div() function is more space and more time
> expansive. The reason is that div() uses an another function:
> __divmodhi4(), i.e. 16-bits.
>
> Compare two programs:
>   prog1:  150 bytes, 238 clocks
>   prog2:  212 bytes, 316 clocks
>
> Regards,
> Dmitry.
>
> -------------------------------------------------------------
> volatile unsigned char a, b, v = 123;
> int main ()
> {
>     unsigned char c = v;
>     a = c / 10;
>     b = c % 10;
>     return 0;
> }
> -------------------------------------------------------------
> #include <stdlib.h>
> volatile unsigned char a, b, v = 123;
> int main ()
> {
>     div_t d;
>     d = div (v, 10);
>     a = d.quot;
>     b = d.rem;
>     return 0;
> }
> --------------------------------------------------------------

How about:

volatile unsigned char a, b, v = 123;
int main ()
{
    unsigned char c = v;
    a = c / 10;
    b = c - a * 10;
    return 0;
}

It's a little inelegant, but multiplying by 10 and then subtracting is 
much smaller and faster than doing a second division.

mvh.,

David