Re: [avr-gcc-list] avr-gcc (GCC) 3.4.5 Bug: copying structure through pointer will destroy the pointer

[Joerg Wunsch]

address@hidden wrote:

> I must have mixed up some of my testing results. Thanks to
> lfmorrison from AVRfreaks.

Here's the smallest reproducible test case:

#define STRUCTSIZE 5
struct a {
        char z[STRUCTSIZE];
};

extern struct a *ap;
extern struct a b;

void
bug(void)
{
        *ap++ = b;
}

It results in:

bug:
/* prologue: frame size=0 */
/* prologue end (size=0) */
        lds r24,ap
        lds r25,(ap)+1
        mov r31,r25
        mov r30,r24
        ldi r26,lo8(b)
        ldi r27,hi8(b)
        ldi r18,lo8(5)
.L2:
        ld r0,X+
        st Z+,r0
        subi r18,lo8(-(-1))
        adiw r24,5
        sts (ap)+1,r25
        sts ap,r24
        tst r18
        brne .L2
/* epilogue: frame size=0 */
        ret

By reduzing STRUCTSIZE to 4, the bug disappears, as the implied memory
copy operation will be unrolled:

bug:
/* prologue: frame size=0 */
/* prologue end (size=0) */
        lds r30,ap
        lds r31,(ap)+1
        lds r24,b
        lds r25,(b)+1
        lds r26,(b)+2
        lds r27,(b)+3
        st Z+,r24
        st Z+,r25
        st Z+,r26
        st Z+,r27
        sts (ap)+1,r31
        sts ap,r30
/* epilogue: frame size=0 */
        ret

-- 
cheers, J"org               .-.-.   --... ...--   -.. .  DL8DTL

http://www.sax.de/~joerg/                        NIC: JW11-RIPE
Never trust an operating system you don't have sources for. ;-)