Hi.

I've a reduced test case showing that a function like:

struct x {
 uint8_t a;
 uint16_t b;
};

f(struct x *y)
...

will expect that 'y' is correctly aligned: it uses 'ldrh' to access y->b and ldrh documentation says "if the memory is not halfworld-aligned and no data abort occurs, the value written to the destination register is unpredictable."

Test case:

typedef unsigned char		uint8_t;
typedef unsigned short int	uint16_t;

struct x {
  uint16_t d;
  uint8_t c;
};

int f(struct x *y)
{
  diag_printf("y=%p y->c=%d(0x%x) y->d=%d(0x%x)\n\r",y,y->c,y->c,y->d,y->d);
}

int main(int argc,char **argv )
{
  void *s=malloc(sizeof(struct x)+1);
  struct x *y;

  s++;   // <- problem origin

  y=s;
  y->c=0xAA;
  y->d=0xBBCC;
  f(y);

  while(1);
}

Without "s++;":

y=0x000088e0 y->c=170(0xaa) y->d=48076(0xbbcc)

With "s++;":

y=0x000088e1 y->c=170(0xaa) y->d=-872415045(0xcc0000bb)

Generated code with -Os is:

int f(struct x *y)
{
40000698:	b500      	push	{lr}
  diag_printf("y=%p y->c=%d(0x%x) y->d=%d(0x%x)\n\r",y,y->c,y->c,y->d,y->d);
4000069a:	7883      	ldrb	r3, [r0, #2]
4000069c:	8802      	ldrh	r2, [r0, #0] <--- *HERE*
4000069e:	b082      	sub	sp, #8
400006a0:	9200      	str	r2, [sp, #0]
400006a2:	9201      	str	r2, [sp, #4]
400006a4:	1c01      	mov	r1, r0		(add r1, r0, #0)
400006a6:	1c1a      	mov	r2, r3		(add r2, r3, #0)
400006a8:	4802      	ldr	r0, [pc, #8]	(400006b4 <.text+0x674>)
400006aa:	ff4ff002 	bl	4000354c <diag_printf>
}
400006ae:	b002      	add	sp, #8
400006b0:	bc02      	pop	{r1}
400006b2:	4708      	bx	r1
400006b4:	98e4      	ldr	r0, [sp, #912]
400006b6:	4000      	and	r0, r0

Tested on arm-elf-gcc 3.4.4, 3.4.6, 4.1.1, the latest two with and without the patch related to bug 29373.

Thanks for your help.