Hi,
I understand that unions inside structs have to be aligned in case they
are referenced by a pointer or someone creates an array of them but when
the union is completely anonymous and thus none of this is possible,
shouldn't the alignment rules be relaxed?
Case in point, here I'm using pahole to show the layout and padding of the
struct:
struct cb3_item {
union {
struct {
void * ptr1; /* 0 8 */
void * ptr2; /* 8 8 */
}; /* 0 16 */
uint8_t raw[60]; /* 0 60 */
}; /* 0 64 */
/* --- cacheline 1 boundary (64 bytes) --- */
unsigned int _avail; /* 64 4 */
/* size: 72, cachelines: 2, members: 2 */
/* padding: 4 */
/* last cacheline: 8 bytes */
};
So my issue here is that IMHO since the union inside the struct is
anonymous there should be no need for the 4 byte padding between the union
and the _avail member. The _avail being a 32-bit integer is perfectly fine
on a 4 byte alignment so fits perfectly at position 60 which would make the
full struct 64 bytes in size and thus perfectly aligned for 64-bit
alignment.
But instead GCC (v13.2.0 in this case) aligns the union in case I would
make a pointer to it or an array of them but this cannot be done when
defined the way it is (I mean if I had done it as "union { } *p;" or
similar then I would fully understand that gcc would align it this way).
And yes I know that I can "fix" this with __attribute__((packed,aligned(1))
but that is a very broad hammer here since now one have to perform manual
alignment of each union member since alignment inside the union is ok here
but the issue is the alignment of the union as a whole.
Unless ofc if there are some other nifty solution to this that is not as
broad as using forced alignment?
/HH
