[patch v2 1/4][libstdc++]: Continue with regex DFS traversals without next frames [PR126274]
Tamar Christina
Tamar.Christina@arm.com
Tue Aug 18 07:34:43 GMT 2026
> -----Original Message-----
> From: Patrick Palka <ppalka@redhat.com>
> Sent: 30 July 2026 05:28
> To: Tamar Christina <Tamar.Christina@arm.com>
> Cc: gcc-patches@gcc.gnu.org; nd <nd@arm.com>; libstdc++@gcc.gnu.org;
> jwakely@redhat.com; tkaminsk@redhat.com; ppalka@redhat.com
> Subject: Re: [patch v2 1/4][libstdc++]: Continue with regex DFS traversals
> without next frames [PR126274]
>
> On Wed, 29 Jul 2026, Tamar Christina wrote:
>
> > The change in r16-7193-g158ad5f96954da5fa24d5c2a91ae92417fb62e20
> changed
> > the recursive implementation with an iterative one using an explicit heap.
> >
> > However one benefit of the previous implementation is that the frame did
> > not have to be saved and popped when the match is supposed to continue.
> >
> > This means that on hot paths we now have additional memory accesses and
> > need additional instructions to calculate the memref addresses.
> >
> > For DFS matching this is clearly suboptimal since when _M_rep_once_more
> > then we push and pop the same state but there is enough other acceses
> > in between the push and pop that we get a lot of cache misses.
> >
> > This makes all the private _m_handle_* methods return a _StateIdT which
> allows
> > the caller to deal with the value, so that for DFS we can avoid pushing the
> > frame if needed.
> >
> > For DFS we try to consume the state immediately until we're told to
> > stop.
> >
> > For this to work the methods have to me marked always inline, because a
> key part
> > of the optimization is to keep the values in registers rather than passing
> > through stack and the function call overheads and AAPCS requirements
> would
> > negate the benefits.
> >
> > The patch also reserves some frames in the initial vector to avoid having
> > resizes on the hot path. To avoid large RSS before matching even starts
> > we provide a cap to the initial reservations. However I have not yet
> addressed
> >
> > Jakub's comment that the cap at 255 is likely to big. I need to do more
> > experiments here to figure out if it's even needed. For now I left it since I
> > am expecting another respin here.
> >
> > The __dfs_mode changes are because the constexpr patch still gave a big
> boost so
> > it prepares to apply it.
> >
> > There is still a regression until the end of the series and each patch
> > will chip away at it.
> >
> > Also note that with none of these changes do I see an increase heap or stack
> > usage that the original fix fixed. RSS stays about the same.
> >
> > PS. thanks for the link to the algorithm in the source, it was useful to
> > understand how the machinery works!
> >
> > Benchmark improvements vs GCC 16:
> >
> > at -O2:
> >
> > email: +36.1%
> > URI: +36.5%
> > IPv4 +33.0%
> >
> > at -O3:
> >
> > email: +45.5%,
> > URI: +44.9%
> > IPv4: +43.0%
> >
> > On Neoverse-V1
> >
> > Bootstrapped Regtested on aarch64-none-linux-gnu,
> > arm-none-linux-gnueabihf, x86_64-pc-linux-gnu
> > -m32, -m64 and no issues.
> >
> > Ok for master?
> >
> > Thanks,
> > Tamar
> >
> > libstdc++-v3/ChangeLog:
> >
> > PR libstdc++/126274
> > * include/bits/regex_executor.h (_Executor): Reserve frame space.
> > (_M_rep_once_more, _M_handle_repeat, _M_handle_subexpr_begin,
> > _M_handle_subexpr_end, _M_handle_line_begin_assertion,
> > _M_handle_line_end_assertion, _M_handle_word_boundary,
> > _M_handle_subexpr_lookahead, _M_handle_match,
> _M_handle_backref,
> > _M_node): return StateIdT.
> > (_M_visited): Mark inline.
> > * include/bits/regex_executor.tcc (_M_rep_once_more,
> _M_handle_repeat,
> > _M_handle_subexpr_begin, _M_handle_subexpr_end,
> > _M_handle_line_begin_assertion, _M_handle_line_end_assertion,
> > _M_handle_word_boundary, _M_handle_subexpr_lookahead,
> _M_handle_match,
> > _M_handle_backref): Return state, mark always inline.
> > (_M_node): Return StateIdT and also decide what to do with the value
> > after return.
> > (_M_dfs): Traverse states iteratively for _S_fopcode_next,
> > _S_fopcode_fallback_next, _S_fopcode_fallback_rep_once_more
> > and _S_fopcode_rep_once_more.
> >
> > ---
> > diff --git a/libstdc++-v3/include/bits/regex_executor.h b/libstdc++-
> v3/include/bits/regex_executor.h
> > index
> 797ad702784b6d1bf07734d91683946d989630f5..23fe828078a32194d99
> 6aeb3bfc37e2c46513c41 100644
> > --- a/libstdc++-v3/include/bits/regex_executor.h
> > +++ b/libstdc++-v3/include/bits/regex_executor.h
> > @@ -86,6 +86,11 @@ namespace __detail
> > using namespace regex_constants;
> > if (__flags & match_prev_avail) // ignore not_bol and not_bow
> > _M_flags &= ~(match_not_bol | match_not_bow);
> > + // Reserve NFA sized frames up front to prevent having to constantly
> > + // reallocate frames. To avoid an explosion in state with large regexp
> > + // before any matching is ever done limit the reservation to 256.
> > + // This should cover a large class of regexp.
> > + _M_frames.reserve(std::min<size_t>(_M_nfa.size(), 256));
> > if (_M_search_mode == _Search_mode::_BFS)
> > _M_visited_states = new bool[_M_nfa.size()];
> > }
> > @@ -113,43 +118,43 @@ namespace __detail
> > _M_search();
> >
> > private:
> > - void
> > + _StateIdT
> > _M_rep_once_more(_Match_mode __match_mode, _StateIdT);
> >
> > - void
> > + _StateIdT
> > _M_handle_repeat(_Match_mode, _StateIdT);
> >
> > - void
> > + _StateIdT
> > _M_handle_subexpr_begin(_Match_mode, _StateIdT);
> >
> > - void
> > + _StateIdT
> > _M_handle_subexpr_end(_Match_mode, _StateIdT);
> >
> > - void
> > + _StateIdT
> > _M_handle_line_begin_assertion(_Match_mode, _StateIdT);
> >
> > - void
> > + _StateIdT
> > _M_handle_line_end_assertion(_Match_mode, _StateIdT);
> >
> > - void
> > + _StateIdT
> > _M_handle_word_boundary(_Match_mode, _StateIdT);
> >
> > - void
> > + _StateIdT
> > _M_handle_subexpr_lookahead(_Match_mode, _StateIdT);
> >
> > - void
> > + _StateIdT
> > _M_handle_match(_Match_mode, _StateIdT);
> >
> > - void
> > + _StateIdT
> > _M_handle_backref(_Match_mode, _StateIdT);
> >
> > - void
> > + _StateIdT
> > _M_handle_accept(_Match_mode, _StateIdT);
> >
> > - void
> > + _StateIdT
> > _M_handle_alternative(_Match_mode, _StateIdT);
> >
> > - void
> > + _StateIdT
> > _M_node(_Match_mode, _StateIdT);
> >
> > void
> > @@ -247,7 +252,7 @@ namespace __detail
> > return (_M_re._M_automaton->_M_options() & __m) == __m;
> > }
> >
> > - bool
> > + inline bool
> > _M_visited(_StateIdT __i)
> > {
> > if (_M_visited_states)
> > diff --git a/libstdc++-v3/include/bits/regex_executor.tcc b/libstdc++-
> v3/include/bits/regex_executor.tcc
> > index
> 167a7a345300868ed5e0852e328569aec47e3d0d..ed53df63a5a304f1db04
> c3519b986f0e5750e3f5 100644
> > --- a/libstdc++-v3/include/bits/regex_executor.tcc
> > +++ b/libstdc++-v3/include/bits/regex_executor.tcc
> > @@ -250,8 +250,14 @@ namespace __detail
> > // infinite loop by refusing to continue when it's already been
> > // visited more than twice. It's `twice` instead of `once` because
> > // we need to spare one more time for potential group capture.
> > + //
> > + // If the node cannot be re-entered anymore from the current state then
> return
> > + // _S_invalid_state_id otherwise return the current state without going
> > + // through a vector, allowing the caller to decide what to do with the state
> > + // This is beneficial for DFS since DFS can continue with the next state
> > + // immediately
> > template<typename _BiIter, typename _Alloc, typename _TraitsT>
> > - void _Executor<_BiIter, _Alloc, _TraitsT>::
> > + _StateIdT _Executor<_BiIter, _Alloc, _TraitsT>::
> > _M_rep_once_more(_Match_mode, _StateIdT __i)
> > {
> > const auto& __state = _M_nfa[__i];
> > @@ -263,7 +269,7 @@ namespace __detail
> > _M_frames.back()._M_count = __rep_count.second;
> > __rep_count.first = _M_current;
> > __rep_count.second = 1;
> > - _M_frames.emplace_back(_S_fopcode_next, __state._M_alt);
> > + return __state._M_alt;
> > }
> > else
> > {
> > @@ -271,9 +277,10 @@ namespace __detail
> > {
> > __rep_count.second++;
> > _M_frames.emplace_back(_S_fopcode_decrement_rep_count,
> __i);
> > - _M_frames.emplace_back(_S_fopcode_next, __state._M_alt);
> > + return __state._M_alt;
> > }
> > }
> > + return _S_invalid_state_id;
> > }
> >
> > // _M_alt branch is "match once more", while _M_next is "get me out
> > @@ -281,8 +288,11 @@ namespace __detail
> > // mean the same thing, and we need to choose the correct order under
> > // given greedy mode.
> > template<typename _BiIter, typename _Alloc, typename _TraitsT>
> > - void _Executor<_BiIter, _Alloc, _TraitsT>::
> > - _M_handle_repeat(_Match_mode, _StateIdT __i)
> > +#ifdef __OPTIMIZE__
> > + [[__gnu__::__always_inline__]]
> > +#endif
> > + inline _StateIdT _Executor<_BiIter, _Alloc, _TraitsT>::
> > + _M_handle_repeat(_Match_mode __match_mode, _StateIdT __i)
> > {
> > const auto& __state = _M_nfa[__i];
> > // Greedy.
> > @@ -294,7 +304,7 @@ namespace __detail
> > _M_current);
> > else
> > _M_frames.emplace_back(_S_fopcode_next, __state._M_next);
> > - _M_frames.emplace_back(_S_fopcode_rep_once_more, __i);
> > + return _M_rep_once_more(__match_mode, __i);
> > }
> > else // Non-greedy mode
> > {
> > @@ -303,7 +313,7 @@ namespace __detail
> > // vice-versa.
> > _M_frames.emplace_back(_S_fopcode_fallback_rep_once_more,
> __i,
> > _M_current);
> > - _M_frames.emplace_back(_S_fopcode_next, __state._M_next);
> > + return __state._M_next;
> > }
> > else
> > {
> > @@ -316,97 +326,122 @@ namespace __detail
> > // accepted state *must* be better than a solution that
> > // matches a non-greedy quantifier one more time.
> >
> _M_frames.emplace_back(_S_fopcode_fallback_rep_once_more, __i);
> > - _M_frames.emplace_back(_S_fopcode_next,
> __state._M_next);
> > + return __state._M_next;
> > }
> > }
> > }
> > + return _S_invalid_state_id;
> > }
> >
> > template<typename _BiIter, typename _Alloc, typename _TraitsT>
> > - void _Executor<_BiIter, _Alloc, _TraitsT>::
> > +#ifdef __OPTIMIZE__
> > + [[__gnu__::__always_inline__]]
> > +#endif
> > + inline _StateIdT _Executor<_BiIter, _Alloc, _TraitsT>::
> > _M_handle_subexpr_begin(_Match_mode, _StateIdT __i)
> > {
> > const auto& __state = _M_nfa[__i];
> > auto& __res = _M_cur_results[__state._M_subexpr];
> > - _M_frames.emplace_back(_S_fopcode_restore_cur_results,
> > - static_cast<_StateIdT>(__state._M_subexpr),
> > - __res.first);
> > + if (_M_nfa._M_has_backref
> > + || __state._M_subexpr != 0
> > + || _M_search_mode != _Search_mode::_DFS)
> > + _M_frames.emplace_back(_S_fopcode_restore_cur_results,
> > + static_cast<_StateIdT>(__state._M_subexpr),
> > + __res.first);
>
> On second thought is this certainly a win? I worry this optimization is
> not worth the overhead, it avoids just two restore_cur_results frames
> (for the implicit outermost capture group) at the expense of additional
> memory accesses and instructions every time we enter any capture group.
>
It looks like it's giving 5% on both the email and URI test at both -O2 and -O3.
So It does seem worth having.
> > __res.first = _M_current;
> > - _M_frames.emplace_back(_S_fopcode_next, __state._M_next);
> > + return __state._M_next;
> > }
> >
> > template<typename _BiIter, typename _Alloc, typename _TraitsT>
> > - void _Executor<_BiIter, _Alloc, _TraitsT>::
> > +#ifdef __OPTIMIZE__
> > + [[__gnu__::__always_inline__]]
> > +#endif
> > + inline _StateIdT _Executor<_BiIter, _Alloc, _TraitsT>::
> > _M_handle_subexpr_end(_Match_mode, _StateIdT __i)
> > {
> > const auto& __state = _M_nfa[__i];
> > auto& __res = _M_cur_results[__state._M_subexpr];
> > - _M_frames.emplace_back(_S_fopcode_restore_cur_results,
> > - static_cast<_StateIdT>(__state._M_subexpr),
> > - __res.second);
> > - _M_frames.back()._M_subexpr_end = true;
> > - _M_frames.back()._M_matched = __res.matched;
> > + if (_M_nfa._M_has_backref
> > + || __state._M_subexpr != 0
> > + || _M_search_mode != _Search_mode::_DFS)
> > + {
> > + _M_frames.emplace_back(_S_fopcode_restore_cur_results,
> > + static_cast<_StateIdT>(__state._M_subexpr),
> > + __res.second);
> > + _M_frames.back()._M_subexpr_end = true;
> > + _M_frames.back()._M_matched = __res.matched;
> > + }
> > +
> > __res.second = _M_current;
> > __res.matched = true;
> > - _M_frames.emplace_back(_S_fopcode_next, __state._M_next);
> > + return __state._M_next;
> > }
> >
> > template<typename _BiIter, typename _Alloc, typename _TraitsT>
> > - inline void _Executor<_BiIter, _Alloc, _TraitsT>::
> > + inline _StateIdT _Executor<_BiIter, _Alloc, _TraitsT>::
> > _M_handle_line_begin_assertion(_Match_mode, _StateIdT __i)
> > {
> > const auto& __state = _M_nfa[__i];
> > if (_M_at_begin())
> > - _M_frames.emplace_back(_S_fopcode_next, __state._M_next);
> > + return __state._M_next;
> > + return _S_invalid_state_id;
> > }
> >
> > template<typename _BiIter, typename _Alloc, typename _TraitsT>
> > - inline void _Executor<_BiIter, _Alloc, _TraitsT>::
> > + inline _StateIdT _Executor<_BiIter, _Alloc, _TraitsT>::
> > _M_handle_line_end_assertion(_Match_mode, _StateIdT __i)
> > {
> > const auto& __state = _M_nfa[__i];
> > if (_M_at_end())
> > - _M_frames.emplace_back(_S_fopcode_next, __state._M_next);
> > + return __state._M_next;
> > + return _S_invalid_state_id;
> > }
> >
> > template<typename _BiIter, typename _Alloc, typename _TraitsT>
> > - inline void _Executor<_BiIter, _Alloc, _TraitsT>::
> > + inline _StateIdT _Executor<_BiIter, _Alloc, _TraitsT>::
> > _M_handle_word_boundary(_Match_mode, _StateIdT __i)
> > {
> > const auto& __state = _M_nfa[__i];
> > if (_M_word_boundary() == !__state._M_neg)
> > - _M_frames.emplace_back(_S_fopcode_next, __state._M_next);
> > + return __state._M_next;
> > + return _S_invalid_state_id;
> > }
> >
> > // Here __state._M_alt offers a single start node for a sub-NFA.
> > // We recursively invoke our algorithm to match the sub-NFA.
> > template<typename _BiIter, typename _Alloc, typename _TraitsT>
> > - void _Executor<_BiIter, _Alloc, _TraitsT>::
> > + _StateIdT _Executor<_BiIter, _Alloc, _TraitsT>::
> > _M_handle_subexpr_lookahead(_Match_mode, _StateIdT __i)
> > {
> > const auto& __state = _M_nfa[__i];
> > if (_M_lookahead(__state._M_alt) == !__state._M_neg)
> > - _M_frames.emplace_back(_S_fopcode_next, __state._M_next);
> > + return __state._M_next;
> > + return _S_invalid_state_id;
> > }
> >
> > template<typename _BiIter, typename _Alloc, typename _TraitsT>
> > - void _Executor<_BiIter, _Alloc, _TraitsT>::
> > +#ifdef __OPTIMIZE__
> > + [[__gnu__::__always_inline__]]
> > +#endif
> > + inline _StateIdT _Executor<_BiIter, _Alloc, _TraitsT>::
> > _M_handle_match(_Match_mode, _StateIdT __i)
> > {
> > const auto& __state = _M_nfa[__i];
> > if (_M_current == _M_end)
> > - return;
> > + return _S_invalid_state_id;
> > if (_M_search_mode == _Search_mode::_DFS)
> > {
> > if (__state._M_matches(*_M_current))
> > {
> > ++_M_current;
> > - _M_frames.emplace_back(_S_fopcode_next, __state._M_next);
> > + return __state._M_next;
> > }
> > }
> > else
> > if (__state._M_matches(*_M_current))
> > _M_match_queue.emplace_back(__state._M_next, _M_cur_results);
> > +
> > + return _S_invalid_state_id;
> > }
> >
> > template<typename _BiIter, typename _TraitsT>
> > @@ -462,7 +497,7 @@ namespace __detail
> > // (_M_current, _M_current + (__submatch.second - __submatch.first)).
> > // If matched, keep going; else just return and try another state.
> > template<typename _BiIter, typename _Alloc, typename _TraitsT>
> > - void _Executor<_BiIter, _Alloc, _TraitsT>::
> > + _StateIdT _Executor<_BiIter, _Alloc, _TraitsT>::
> > _M_handle_backref(_Match_mode, _StateIdT __i)
> > {
> > __glibcxx_assert(_M_search_mode == _Search_mode::_DFS);
> > @@ -470,7 +505,7 @@ namespace __detail
> > const auto& __state = _M_nfa[__i];
> > auto& __submatch = _M_cur_results[__state._M_backref_index];
> > if (!__submatch.matched)
> > - return;
> > + return _S_invalid_state_id;
> > auto __last = _M_current;
> > for (auto __tmp = __submatch.first;
> > __last != _M_end && __tmp != __submatch.second;
> > @@ -482,12 +517,17 @@ namespace __detail
> > __submatch.first, __submatch.second, _M_current, __last))
> > {
> > _M_current = __last;
> > - _M_frames.emplace_back(_S_fopcode_next, __state._M_next);
> > + return __state._M_next;
> > }
> > +
> > + return _S_invalid_state_id;
> > }
> >
> > template<typename _BiIter, typename _Alloc, typename _TraitsT>
> > - void _Executor<_BiIter, _Alloc, _TraitsT>::
> > +#ifdef __OPTIMIZE__
> > + [[__gnu__::__always_inline__]]
> > +#endif
> > + inline _StateIdT _Executor<_BiIter, _Alloc, _TraitsT>::
> > _M_handle_accept(_Match_mode __match_mode, _StateIdT)
> > {
> > if (_M_search_mode == _Search_mode::_DFS)
> > @@ -528,7 +568,7 @@ namespace __detail
> > {
> > if (_M_current == _M_begin
> > && (_M_flags & regex_constants::match_not_null))
> > - return;
> > + return _S_invalid_state_id;
> > if (__match_mode == _Match_mode::_Prefix || _M_current ==
> _M_end)
> > if (!_M_has_sol)
> > {
> > @@ -536,10 +576,14 @@ namespace __detail
> > _M_results = _M_cur_results;
> > }
> > }
> > + return _S_invalid_state_id;
> > }
> >
> > template<typename _BiIter, typename _Alloc, typename _TraitsT>
> > - void _Executor<_BiIter, _Alloc, _TraitsT>::
> > +#ifdef __OPTIMIZE__
> > + [[__gnu__::__always_inline__]]
> > +#endif
> > + inline _StateIdT _Executor<_BiIter, _Alloc, _TraitsT>::
> > _M_handle_alternative(_Match_mode, _StateIdT __i)
> > {
> > const auto& __state = _M_nfa[__i];
> > @@ -549,7 +593,7 @@ namespace __detail
> > // Pick lhs if it matches. Only try rhs if it doesn't.
> > _M_frames.emplace_back(_S_fopcode_fallback_next,
> __state._M_next,
> > _M_current);
> > - _M_frames.emplace_back(_S_fopcode_next, __state._M_alt);
> > + return __state._M_alt;
> > }
> > else
> > {
> > @@ -557,7 +601,7 @@ namespace __detail
> > // See "case _S_opcode_accept:" handling above.
> > _M_frames.emplace_back(_S_fopcode_posix_alternative,
> __state._M_next,
> > _M_current);
> > - _M_frames.emplace_back(_S_fopcode_next, __state._M_alt);
> > + return __state._M_alt;
> > }
> > }
> >
> > @@ -565,50 +609,63 @@ namespace __detail
> > #ifdef __OPTIMIZE__
> > [[__gnu__::__always_inline__]]
> > #endif
> > - inline void _Executor<_BiIter, _Alloc, _TraitsT>::
> > + inline _StateIdT _Executor<_BiIter, _Alloc, _TraitsT>::
> > _M_node(_Match_mode __match_mode, _StateIdT __i)
> > {
> > - if (_M_visited(__i))
> > - return;
> > + // DFS has no _M_visited implementation as such don't even have the
> branch
> > + // or the check in the call graph.
> > + if (_M_search_mode == _Search_mode::_BFS)
> > + if (_M_visited(__i))
> > + return _S_invalid_state_id;
> >
> > + _StateIdT __next = _S_invalid_state_id;
> > switch (_M_nfa[__i]._M_opcode())
> > {
> > case _S_opcode_repeat:
> > - _M_handle_repeat(__match_mode, __i); break;
> > + __next = _M_handle_repeat(__match_mode, __i); break;
> > case _S_opcode_subexpr_begin:
> > - _M_handle_subexpr_begin(__match_mode, __i); break;
> > + __next = _M_handle_subexpr_begin(__match_mode, __i);
> > + break;
> > case _S_opcode_subexpr_end:
> > - _M_handle_subexpr_end(__match_mode, __i); break;
> > + __next = _M_handle_subexpr_end(__match_mode, __i);
> > + break;
> > case _S_opcode_line_begin_assertion:
> > - _M_handle_line_begin_assertion(__match_mode, __i); break;
> > + __next = _M_handle_line_begin_assertion(__match_mode, __i);
> break;
> > case _S_opcode_line_end_assertion:
> > - _M_handle_line_end_assertion(__match_mode, __i); break;
> > + __next = _M_handle_line_end_assertion(__match_mode, __i); break;
> > case _S_opcode_word_boundary:
> > - _M_handle_word_boundary(__match_mode, __i); break;
> > + __next = _M_handle_word_boundary(__match_mode, __i); break;
> > case _S_opcode_subexpr_lookahead:
> > - _M_handle_subexpr_lookahead(__match_mode, __i); break;
> > + __next = _M_handle_subexpr_lookahead(__match_mode, __i);
> break;
> > case _S_opcode_match:
> > - _M_handle_match(__match_mode, __i); break;
> > + __next = _M_handle_match(__match_mode, __i); break;
> > case _S_opcode_backref:
> > if (_M_search_mode == _Search_mode::_DFS)
> > - _M_handle_backref(__match_mode, __i);
> > + __next = _M_handle_backref(__match_mode, __i);
> > else
> > __builtin_unreachable();
> > break;
> > case _S_opcode_accept:
> > - _M_handle_accept(__match_mode, __i); break;
> > + __next = _M_handle_accept(__match_mode, __i); break;
> > case _S_opcode_alternative:
> > - _M_handle_alternative(__match_mode, __i); break;
> > + __next = _M_handle_alternative(__match_mode, __i); break;
> > default:
> > __glibcxx_assert(false);
> > }
> > + if (_M_search_mode == _Search_mode::_BFS)
> > + {
> > + if (__next != _S_invalid_state_id)
> > + _M_frames.emplace_back(_S_fopcode_next, __next);
> > + return _S_invalid_state_id;
>
> I don't see why we can't also return the next state when in BFS mode?
> As long as _M_dfs continues to handle a returned state id as if a next
> frame was pushed to the top of the stack it shouldn't matter whether
> we're in BFS or DFS mode.
That's fair. I've refactored this now.
>
> > + }
> > + else
> > + return __next;
> > }
> >
> > template<typename _BiIter, typename _Alloc, typename _TraitsT>
> > void _Executor<_BiIter, _Alloc, _TraitsT>::
> > _M_dfs(_Match_mode __match_mode, _StateIdT __start)
> > {
> > - const bool __dfs_mode = (_M_search_mode == _Search_mode::_DFS);
> > _M_frames.emplace_back(_S_fopcode_next, __start);
> >
> > while (!_M_frames.empty())
> > @@ -621,27 +678,49 @@ namespace __detail
> > case _S_fopcode_fallback_next:
> > if (_M_has_sol)
> > break;
> > - if (__dfs_mode)
> > + if (_M_search_mode == _Search_mode::_DFS)
> > _M_current = __frame._M_pos;
> > [[__fallthrough__]];
> > case _S_fopcode_next:
> > - _M_node(__match_mode, __frame._M_state_id);
> > + if (_M_search_mode == _Search_mode::_DFS)
> > + // Follow immediate successors without re-entering the frame
> > + // loop until we fail. This avoids the needless state save and
> > + // restore through memory.
> > + for (_StateIdT __next = __frame._M_state_id;
> > + __next != _S_invalid_state_id;)
> > + __next = _M_node(__match_mode, __next);
> > + else
> > + _M_node(__match_mode, __frame._M_state_id);
>
> I don't see why we can't do the loop in BFS mode. BFS mode handles a
> next frame on the top of the stack the same as DFS does -- it
> immediately pops it and evaluates it via _M_node. We should be able to
> do the fast path in BFS mode unless I'm missing something.
>
> > break;
> >
> > case _S_fopcode_fallback_rep_once_more:
> > if (_M_has_sol)
> > break;
> > - if (__dfs_mode)
> > + if (_M_search_mode == _Search_mode::_DFS)
> > _M_current = __frame._M_pos;
> > [[__fallthrough__]];
> > case _S_fopcode_rep_once_more:
> > - _M_rep_once_more(__match_mode, __frame._M_state_id);
> > + {
> > + _StateIdT __next
> > + = _M_rep_once_more(__match_mode,
> __frame._M_state_id);
> > + if (_M_search_mode == _Search_mode::_DFS)
> > + // _M_rep_once_more returned the repeated body's start
> state.
> > + // Continue directly in DFS; BFS must materialize the state as
> > + // a queue/frame item because it advances by input position
> > + // rather than by backtracking order. Splitting this in a
> > + // specialized path preserves the behavior for both but for
> > + // DFS it avoids the intermediate allocations.
> > + for (; __next != _S_invalid_state_id;)
> > + __next = _M_node(__match_mode, __next);
> > + else if (__next != _S_invalid_state_id)
> > + _M_frames.emplace_back(_S_fopcode_next, __next);
>
> Ditto.
>
> Rather than having two such fast paths, we could just have a single one
> at the top of _M_dfs so that _M_dfs looks like:
Ack.
Done, sending v3.
Thanks for the reviews!
Tamar
>
> _M_dfs(_Match_mode __match_mode, _StateIdT __start)
> {
> _StateIdT __next = __start;
>
> while (true)
> {
> while (__next != _S_invalid_state_id)
> __next = _M_node(__match_mode, __next);
>
> if (_M_frames.empty())
> break;
>
> _ExecutorFrame<_BiIter> __frame = std::move(_M_frames.back());
> _M_frames.pop_back();
>
> switch (__frame._M_op)
> {
> case _S_fopcode_rep_once_more:
> ...
> __next = _M_rep_once_more(...);
> ...
> break;
>
> case _S_fopcode_posix_alternative:
> ...
> __next = __frame._M_state_id;
> ...
> break;
>
> ...
> }
> }
> }
>
> This way, we also avoid needing to push a frame for the start state.
>
> For the ABI issue we probably should just wrap the entirety of _Executor
> in regex.h, regex_executor.{h,tcc} with
>
> _GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE(_V2)
> ...
> _GLIBCXX_END_INLINE_ABI_NAMESPACE(_V2)
>
> to give all its member functions different manglings (even if not all of
> them need it).
>
> > + }
> > break;
> >
> > case _S_fopcode_posix_alternative:
> > _M_frames.emplace_back(_S_fopcode_merge_sol, 0,
> _M_has_sol);
> > _M_frames.emplace_back(_S_fopcode_next,
> __frame._M_state_id);
> > - if (__dfs_mode)
> > + if (_M_search_mode == _Search_mode::_DFS)
> > _M_current = __frame._M_pos;
> > _M_has_sol = false;
> > break;
> >
> >
> > --
> >
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