Re: [patch 3/N] std::regex refactoring - _Executor DFS / BFS

[Jonathan Wakely <jwakely at redhat dot com>]

On 28/04/14 11:40 -0400, Tim Shen wrote:
> On Mon, Apr 28, 2014 at 10:46 AM, Jonathan Wakely <jwakely@redhat.com> wrote:
> > This change splits _Executor::_M_main() into two overloaded
> > _M_main_dispatch() functions, choosing which to run based on the
> > __dfs_mode template parameter.
> >
> > I think this gives a (very) small improvement in compilation time when
> > using regexes.
> >
> > Splitting _M_main() allows the _M_match_queue and _M_visited members
> > (which are only used in BFS mode) to be replaced with an instantiation
> > of the new _States class template. _States<__dfs> only contains the
> > start state (so that it's not empty and doesn't waste space) but
> > _States<__bfs> contains the start state and also the match queue and
> > visited list.
>
> This is great! I keep worrying about the ugly all-in-one _Executor
> class; I've tried to specialize two versions of it, but that
> introduced much duplicated code. Your abstract is nice!

Thanks. I'll clean up that patch and commit it soon too.

The next thing I plan to look at, which I haven't done yet, is to see
if passing the __match_mode template parameter as a runtime function
parameter makes any difference to the way the code is structuted. Do
you have any thoughts in that, before I waste time doing something
that won't work?

> > As the visited list never changes size after construction I changed
> > its type from unique_ptr<vector<bool>> to unique_ptr<bool[]>, which
> > avoids an indirection, although now that that member is only present
> > when actually required it could just be vector<bool>. An array of bool
> > is simpler to access, but takes more heap memory. vector<bool> uses
> > less space but the compiler has to do all the masking to address
> > individual bits.
>
> What about bitset? Anyway we should get rid of vector<bool>.

The size of a bitset is fixed at compile-time. If we could use
dynarray<bool> that might be nice, but we can't :-)