Re: attribute aligned(4) ignored

[corey taylor <corey dot taylor at gmail dot com>]

Does the placement of the aligned and packed structure member affect the result?

What happens when the double D is at the end of the structure?

corey

On 7/25/05, Chris Lattner <sabre@nondot.org> wrote:
> On Mon, 25 Jul 2005, Ian Lance Taylor wrote:
> > Chris Lattner <sabre@nondot.org> writes:
> >> Hi All, I'm trying to reduce the alignment of the double in this
> >> structure to 4 bytes (from 8) on Darwin.  The goal of doing this is to
> >> reduce the structure to 12 bytes in size.  Here is my testcase:
> >>
> >> struct Test {
> >>    double D __attribute__((aligned(4)));    // only 4 bytes, not 8!
> >>    int X;
> >> };
> >>
> >> int X() {
> >>    return sizeof(struct Test);    // Should return 12, not 16.
> >> }
> >>
> >> Despite my attempt above, the structure is still 16 bytes in size, due
> >> to the tail padding required to pad the structure to ensure 8-byte
> >> alignment.
> >>
> >> Does anyone know how to reduce the alignment of the double,
> >> eliminating the tail padding?
> >
> > How about
> >
> >   double D __attribute__((aligned(4), packed));
> 
> This is exactly the sort of thing I want to do.  My structs can be
> arbitrarily complex, and can have a bunch of stuff in them that should not
> be packed.  This means that I don't want to use attribute packed on the
> structure itself, but using it on the member would be fine.
> 
> However, when I try this:
> 
> struct Test {
>    double D __attribute__((packed,aligned(4)));
>    short X;
> };
> 
> ... the struct maintains its 8-byte alignment even though nothing inside
> of it requires 8-byte alignment any more.  In this particular case, for
> example, I want the struct to be 12-bytes, not 10: just reducing the
> alignment requirement of the double, without eliminating all intra-struct
> padding.  This is why I don't think I can use attribute packed on the
> struct itself.
> 
> Unfortunately, I'm not sure if this is possible, at least without using
> 'packed' on the struct, then inserting a bunch of dummy members to
> explicitly insert the intra-field packing.  I hope there is a better way
> though, as this is really nasty and fragile.
> 
> Thanks,
> 
> -Chris
> 
> --
> http://nondot.org/sabre/
> http://llvm.org/
>