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.
