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

[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]

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

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

[rearnsha at gcc dot gnu dot org]

------- Comment #1 from rearnsha at gcc dot gnu dot org  2006-11-22 10:50 -------
The compiler expects 'y' to be correctly aligned because the ABI says that it
can expect this to be so.  If you write a program that then provides an
unaligned value, then the behaviour is undefined and you can get any result the
compiler feels like giving you.

Hint: don't increment void* pointers (it's a gcc extension that permits this
anyway) and then try to assign them to a structure.  Malloc provides memory
that is correctly aligned for direct assignment to any structure type.


-- 

rearnsha at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


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

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

[bernard dot fouche at kuantic dot com]

------- Comment #2 from bernard dot fouche at kuantic dot com  2006-11-22 11:32 -------
Subject: Re:  [3.4.4/3.4.6/4.1.1] ARM structure pointer alignment
 problem with optimization

rearnsha at gcc dot gnu dot org wrote:
> ------- Comment #1 from rearnsha at gcc dot gnu dot org  2006-11-22 10:50 -------
> The compiler expects 'y' to be correctly aligned because the ABI says that it
> can expect this to be so.  If you write a program that then provides an
> unaligned value, then the behaviour is undefined and you can get any result the
> compiler feels like giving you.
>
> Hint: don't increment void* pointers (it's a gcc extension that permits this
> anyway) and then try to assign them to a structure.  Malloc provides memory
> that is correctly aligned for direct assignment to any structure type.
>
>
>   
Sorry for having taken your time for nothing. I'll read the ABI spec 
better next time. Too bad my target does not have alignment traps, I've 
fallen in the problem while building a packet with an odd size header.

Regards,

 Bernard


-- 


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