From: Philip Craig <philipc@xxxxxxxxxxxx>
Date: Tue, 29 Apr 2008 16:00:35 +1000
> David Miller wrote:
> > Now these elements cannot be assumed to have any particular
> > alignment, so gcc will emit byte-at-a-time accesses to these
> > elements.
>
> Ah, so the problem is the __attribute__(packed) removes align
> restrictions for the whole structure, in addition to removing
> padding.
>
> Is there any way to tell gcc to pack but still align the whole
> thing?
Is there any way to tell ARM not to layout structures in such a silly
way by default? :-)))
> eg what does it do for this:
> struct t {
> struct s {
> u16 a;
> u16 b;
> } __attribute__(packed);
> };
'a' and 'b' cannot be assumed to be aligned in any particular way,
and thus byte-at-a-time accesses will be made to them.
This program:
--------------------
typedef unsigned short u16;
struct t {
struct s {
u16 a;
u16 b;
} __attribute__((packed)) x;
};
u16 a(struct t *p)
{
return p->x.a;
}
u16 b(struct t *p)
{
return p->x.b;
}
--------------------
results in the following on sparc64:
a:
ldub [%o0], %g1
ldub [%o0+1], %o0
sllx %g1, 8, %g1
jmp %o7+8
or %o0, %g1, %o0
.size a, .-a
.align 4
.align 32
.global b
.type b, #function
.proc 04
b:
ldub [%o0+2], %g1
ldub [%o0+3], %o0
sllx %g1, 8, %g1
jmp %o7+8
or %o0, %g1, %o0
.size b, .-b
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