Re: [avr-gcc-list] GCC 3.0.2 unsigned int bug?

[Carsten Beth]

Hi Chris!

Maybe you are compiling with optimization switch on. Then the compiler shortens
the loop, actualy the following program do (except from the timing) the same
thing:

void delay(void)
{
        unsigned int d;

        d = 0x1000;
        d = 0;
}

and also

void delay(void)
{
        unsigned int d;

        d = 0;
}

do the same.

You can try something like this:

#define nop()   __asm__ __volatile__ ("nop")

void delay(void)
{
        unsigned int d;

        for( d = 0; d < 0x1000; d++ )
              nop();
}


Carsten




Chris Elmquist wrote:

>
> I'm having trouble understanding this:
>
> void delay(void)
> {
>         unsigned int d;
>
>         d = 0x1000;
>         while (d) d--;
> }
>
> generates this code (using avr-objdump --disassemble):
>
> 0000004a <delay>:
>   4a:   80 e0           ldi     r24, 0x00       ; 0
>   4c:   90 e1           ldi     r25, 0x10       ; 16
>   4e:   80 50           subi    r24, 0x00       ; 0
>   50:   94 40           sbci    r25, 0x04       ; 4
>   52:   e9 f7           brne    .-6             ; 0x4e
>   54:   08 95           ret
>
> I can clearly see that the MSB goes in r25 and the LSB in r24
> but then why does it subtract 0x04 from the MSB each iteration?
>
> Suspecting maybe a disassembler problem, the '94 40' opcode
> indeed would subtract 0x40 from the MSB wouldn't it?
>
> Am I missing something obvious here or is this a bug?