Re: [avr-gcc-list] Problem with delay loop

[David Brown]

This is probably in the FAQ somewhere - if not, it should be!

The compiler is smart enough to figure out that your delay function does 
no useful work - thus the optimiser does not generate any code.  This is 
correct compilation - it's your code that is wrong.  The difference is 
that the newer version of the compiler is smarter than the older version 
(or newer makefiles have higher optimisation enabled by default).

The correct way to write such a loop is:

void delay(unsigned int del_cnt) {
        volatile unsigned int n = del_cnt;
        while (n--);
}

(Incidentally, *never* use "unsigned" as a shorthand - write the proper 
type name "unsigned int".  The same applies to "long", "short", 
"signed", etc.)

If you don't know how to use "volatile", there should be plenty of 
references on the web (including the archives for this mailing list), 
which will give you a better explanation.

mvh.,

David


Royce Pereira wrote:
> Hi all,
>
> In the latest WinAVR (avr-gcc (GCC) 4.1.2 (WinAVR 20070525) I found this.
>
> Check this out:
> //======================
> void delay(unsigned del_cnt)
>     {
>        while(del_cnt--);
>
>        return;
>     }
> //=======================
>
> Compiles as (from the .lss file):
>
> //=======================
> void delay(word cnt)
>     {
>   2aa:  08 95           ret
>
>        while(cnt--);
>
>        return;
>     }
> //=======================
>
> The loop is not executed !!
>
> Where as in an older version (WinAVR-20060421)
>
> It compiles correctly as:
> //==========================================
> void delay(word cnt)
>     {
>        while(cnt--);
>   286:  01 97           sbiw    r24, 0x01       ; 1
>   288:  2f ef           ldi     r18, 0xFF       ; 255
>   28a:  8f 3f           cpi     r24, 0xFF       ; 255
>   28c:  92 07           cpc     r25, r18
>   28e:  d9 f7           brne    .-10            ; 0x286 <delay>
>   290:  08 95           ret
>
>
>        return;
>     }
> //=======================================
>
> As a result none of my delays used in my older programs work when recompiled...
>
> Thanks,
>
> --Royce