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

[Patrick Palka]

On Sat, 31 Jan 2026, Jonathan Wakely wrote:

> 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;

Zero-initializing the flags makes sense but I think we have to manually
init each bit-field (according to the state id) so that the appropriate
union member gets activated?  Or can we pretend union type punning is
well defined (in non-constexpr code)?