Re: Possible Bug

[Paolo Bonzini <bonzini at gnu dot org>]

On 03/28/2011 01:06 PM, Richard Guenther wrote:
> > /* GCC uses 8-byte loads and register passing even though sizeof = 6 */
> > typedef struct __attribute__((__packed__))
> > {
> >    unsigned chr            :16;
> >    unsigned loc            :32;
> > } GENOME_LOC_TYPE_2;
> >
> > //#define GENOME_LOC_TYPE GENOME_LOC_TYPE_1
> > #define GENOME_LOC_TYPE GENOME_LOC_TYPE_2
> >
> > static __attribute__((__noclone__,__noinline__))
> > int f(GENOME_LOC_TYPE x)
> > {
> >   return x.loc;
> > }
> >
> > GENOME_LOC_TYPE h;
> > GENOME_LOC_TYPE *g =&h;
> >
> > int
> > main()
> > {
> >   printf ("%d %d\n", sizeof (GENOME_LOC_TYPE),
> >                      __alignof__(GENOME_LOC_TYPE));
> >   return f(*g);
> > }
> >
> >
> > Both definitions have a (sizeof = 6, alignof = 1) but GCC loads the second
> > with an 8-byte load.  It's really an ugly bug if I understood it correctly,
> > because I would have expected the second struct to have sizeof = 8.  The two
> > final bytes are not padding, they are what's left of the unsigned int from
> > which the bitfields are carved.  If that were the correct fix for the bug,
> > it would be a change to the ABI.
>
> At expansion time we have the following for the call argument:
>
>   <mem_ref 0x7ffff7ff9118
>      type<record_type 0x7ffff5b295e8 GENOME_LOC_TYPE_2 packed type_0 BLK
>          size<integer_cst 0x7ffff5b256b8 constant 48>
>          unit size<integer_cst 0x7ffff5b25708 constant 6>
>          align 8 symtab 0 alias set -1 canonical type 0x7ffff5b29540
>
> which looks ok to me.

It already isn't, why is the alignment 8 if __alignof__ 
(GENOME_LOC_TYPE_2) is 1?

The other question is a layout question, should the packed attribute 
affect the removal of padding from the last bitfield element?  That's a 
very different kind of padding, and it affects whether the size of the 
struct should be 6 or 8?  Note this is slightly different from the 
problem in -Wpacked-bitfield-compat.

In fact, should the poster's desired layout (the same as 
GENOME_LOC_TYPE_1, I guess) be achievable at all with bitfields, even in 
combination with the packed attribute?

Paolo