Re: [avr-gcc-list] Incorrect code

[Erik Christiansen]

On 09.08.14 05:51, Thomas D. Dean wrote:
> Is this a problem with -Os?

Good question. Is the second "if" restored with optimisation off?

> It looks like the compiler has decided that cs is always zero in the code
> below.

It's nearly midnight here, and I'm viewing the code from down under, but
ISTM that it thinks cs can never be zero.

> The cs calculation appears correct.

Yep.

> 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;
> }

...
                                       # The way I see it:
> 0000077c <spi_copy_in>:
                                       # len = source[0];
>  77c:   fb 01           movw    r30, r22
>  77e:   20 81           ld      r18, Z
                                       # if (len > max_len) return 0xff;
>  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>
                                       # ptr2 = &source[1];
>  78a:   31 96           adiw    r30, 0x01       ; 1
                                       # cs = 0;
>  78c:   80 e0           ldi     r24, 0x00       ; 0
                                       # cs += *ptr2++;
>  78e:   91 91           ld      r25, Z+
>  790:   89 0f           add     r24, r25
                                       # while (--len);
>  792:   21 50           subi    r18, 0x01       ; 1
>  794:   e1 f7           brne    .-8             ; 0x78e <spi_copy_in+0x12>
               # Who took Tom's cheese?
               # Does it deduce that cs cannot ever be zero?
>  796:   08 95           ret
>  798:   8f ef           ldi     r24, 0xFF       ; 255
>  79a:   08 95           ret

That looks scary. Which version is it? (Others will ask. ;-)

Erik

-- 
On the basis of evidence we may be sure that we are wrong                     
but we can never be sure that we are right.        - Richard Feynman