[Bug c/29938] New: [3.4.4/3.4.6/4.1.1] ARM structure pointer alignment problem with optimization

["bernard dot fouche at kuantic dot com" <gcc-bugzilla at gcc dot gnu dot org>]

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.


-- 
           Summary: [3.4.4/3.4.6/4.1.1] ARM structure pointer alignment
                    problem with optimization
           Product: gcc
           Version: 4.1.1
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: bernard dot fouche at kuantic dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29938