Re: Incorrect code

you didn't assign anything to ptr1. so *ptr1++ = *ptr2++; is undefined.


On Sat, Aug 9, 2014 at 3:51 PM, Thomas D. Dean <[email protected]>
wrote:

> Is this a problem with -Os?
>
> It looks like the compiler has decided that cs is always zero in the code
> below.  The cs calculation appears correct.
>
> In spi_copy.c
>
> ...
> uint8_t spi_copy_in(uint8_t *dest, uint8_t *source, unsigned int max_len) {
>   uint8_t cs;
>   uint8_t len;
>   uint8_t *ptr1, *ptr2;
>
>   cs = 0;
>   len = source[0];
>   if (len > max_len) return 0xff;
>   ptr2 = &source[1];
>   do {
>         cs += *ptr2++;
>   } while (--len);
>   if (cs == 0) {
>         len = source[0] - 1; // do not need the checksum
>         ptr2 = &source[1];
>         do {
>           *ptr1++ = *ptr2++;
>         } while (--len);
>   }
>   return cs;
> }
> ...
>
> > make
> ...
> avr-gcc -Os -Wall -Wmissing-prototypes  -fno-strict-aliasing -Wall
> -I../include -I../../Class -mmcu=atmega128 -c -o spi_copy.o
> ../../Class/spi_copy.c
> ...
>
> > avr-objdump -d ng02.elf | more
> ...
>
> 0000077c <spi_copy_in>:
>  77c:   fb 01           movw    r30, r22
>  77e:   20 81           ld      r18, Z
>  780:   82 2f           mov     r24, r18
>  782:   90 e0           ldi     r25, 0x00       ; 0
>  784:   48 17           cp      r20, r24
>  786:   59 07           cpc     r21, r25
>  788:   38 f0           brcs    .+14            ; 0x798 <spi_copy_in+0x1c>
>  78a:   31 96           adiw    r30, 0x01       ; 1
>  78c:   80 e0           ldi     r24, 0x00       ; 0
>  78e:   91 91           ld      r25, Z+
>  790:   89 0f           add     r24, r25
>  792:   21 50           subi    r18, 0x01       ; 1
>  794:   e1 f7           brne    .-8             ; 0x78e <spi_copy_in+0x12>
>  796:   08 95           ret
>  798:   8f ef           ldi     r24, 0xFF       ; 255
>  79a:   08 95           ret
>
> Tom Dean
>
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>

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