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

[Dmitry K.]

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;
}
--------------------------------------------------------------