Incorrect code

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