[PATCH v7 1/2] libstdc++/regex: Make DFS executor non-recursive [PR86164]

[Jonathan Wakely]

On Fri, 30 Jan 2026 at 15:25, Patrick Palka <ppalka@redhat.com> wrote:
>
> On Thu, 29 Jan 2026, Jonathan Wakely wrote:
> >
> > I wondered about replacing _M_bytes[7] with a union like:
> >
> >   union {
> >     unsigned char _M_bytes[7];
> >     struct { // Used by restore_rep_count frame
> >       unsigned char _M_rep_count : 2;
> >     };
> >     struct { // Used by restore_cur_results frame
> >       unsigned char _M_matched : 1;
> >       unsigned char _M_restore_first : 1;
> >     };
> >   };
> >
> > So that we use more precise names than just _M_bytes[0] (and could
> > still extend it later with other alternatives if needed).
> >
> > This might be over-complicating things, but if we *did* want to do it,
> > I think we should decide now because the difference between setting
> > _M_bytes[0] = 0xff and setting _M_restore_first = true would not be
> > compatible if we changed it later (old TUs might expect to find 0xff
> > there and new TUs would only set one bit).
>
> I think slightly more flexible would be:
>
>    union {
>      unsigned char _M_byte0;
>      struct { // Used by restore_rep_count frame
>        unsigned char _M_rep_count : 2;
>      };
>      struct { // Used by restore_cur_results frame
>        unsigned char _M_matched : 1;
>        unsigned char _M_restore_first : 1;
>      };
>    };
>    unsigned char _M_bytes[6];
>
> So that we could decide how to use each byte independently of the other
> (e.g. maybe one byte could be for universal flags that apply to every
> code).

Do we want to value-init the _M_byte0 member in the constructor, so
that we know all the bit-fields are initially zero?

I think we can use a NSDMI for union members even in C++11, so this should work:

unsigned char _M_byte0 = 0;