Re: [cxx-abi-dev] Re: C++ ABI: Reuse of tail padding

[Kerch Holt <kerch at cup dot hp dot com>]

Mark Mitchell wrote:
> --On Wednesday, August 14, 2002 10:15:07 PM +0100 Nathan Sidwell 
> <nathan@codesourcery.com> wrote:
>
> > Mark Mitchell wrote:
> >
> > > We have a problem here.
> >
> > > Intel's compiler does not pack things into the tail padding.
> > > Neither, I expect, does HP's.
> > I would like to get hard data from more than the two compilers
> > you've verified.
Looks like the HP compiler does make use of the tail padding.
For example it reports "sizeof(S2)" to be 12. I think c2
is placed in the "tail padding".

So this would favor g++'s implementation.

Kerch

> There aren't many more to find.  HP, Intel, and GCC are the compilers
> that have implemented the ABI to date.
>
> Others are in the process, which is part of why this is important.
>
> > > I think I would prefer to change G++, and drop this idea from the spec,
> > > even though it is an optimization.
> >
> > Although using tail padding is neat, how many bytes would it
> > save in real programs? You only get it with structs of the form
> >     int i;
> >     char c;
> > or (on some non-i86 machines)
> >     double d;
> >     int i;
>
> Exactly.
>
> Well, that sounds like two votes for changing G++ (and clarifying the
> spec to drop any hint of using tail padding).