On Tue, Jun 26, 2018 at 8:21 AM, Eric Covener <cove...@gmail.com> wrote:
> On Tue, Jun 26, 2018 at 9:02 AM Yann Ylavic <ylavic....@gmail.com> wrote:
> >
> > On Tue, Jun 26, 2018 at 2:48 PM, Eric Covener <cove...@gmail.com> wrote:
> > > On Tue, Jun 26, 2018 at 8:37 AM Yann Ylavic <ylavic....@gmail.com>
> wrote:
> > >>
> > >> On Mon, Jun 25, 2018 at 10:49 PM, Eric Covener <cove...@gmail.com>
> wrote:
> > >> >
> > >> > Bill or Yann, do you remember the specific gotcha with setting aside
> > >> > addl bits and re-using them later?
> > >>
> > >> I've never thought it was an issue (re compat) to add some bit(s) in a
> > >> bitfield if there is a hole, wherever this field is in the struct.
> > >> It doesn't change the size and there is no way for the user to have
> > >> used the address of any bit in the first place (it can't break
> > >> anything IMHO).
> > >> Bill objected to this, I can't remember why (and he is in better
> > >> position to explain it), status quo so far...
> > >>
> > >
> > > Just to be clear, I am talking about claiming the lost ones _before_
> > > the struct is further extended with a reserved :31 (or whatever) as
> > > opposed to going back and claiming gaps from the past. Maybe the
> > > former is OK.
> >
> > I think "reserved" or not doesn't change anything, name it or not the
> > hole is there in any case.
> > Why extending firstbit:1;reserved:31 to e.g.
> > firstbit:1;secondbit:1;reserved:30 would be more compatible than the
> > implicit firstbit:1;secondbit:1 ?
> > Both should be allowed IMHO.
>
> Speculating only, it seems like there is a gradient of risk:
>
> - Old unused bits we try to repurpose that someone is squatting on
> - New unused bits we try to claim now (this is the case for the
> recent single bit field)
> - Unused bits that are explicitly marked as reserved when the first
> bit field is defined.
>
My sole concern was whether the addition of c below...
struct {
int a:2;
int b:2;
int c:2; /* Added */
}
...ever assured that the two bits of c will not displace a or b, and this
has never been a concern on Intel implementatins. Actually Eric you
have more background on big endian and odd word size architectures
than I do :) I've never found a reference that assures us of this, maybe
things have changed in very recent C specs?
