[PATCH v8 2/2] libstdc++/regex: Defer _M_current restoration to backtracking nodes [PR86164]

[Patrick Palka]

Changes in v8:
  - Rebased

Changes in v7:
  - Rebased

Changes in v6:
  - Rebased

Changes in v5:
  - Restrict _M_current restoration to DFS mode, it's unnecessary
    (and a harmless no-op) in BFS mode

Changes in v4:
  - Rebased

Changes in v3:
  - Rebased against latest version of previous patch

-- >8 --

The actual incrementing of the current input string position is done by
_M_handle_match which also makes sure to restore it afterwards, via a
restore_current frame.  But restoring _M_current is naturally only
necessary when backtracking is involved, not after every single match.

So this patch moves the responsibility of saving/restoring _M_current
from _M_handle_match to the branching nodes _M_handle_alternative and
_M_handle_repeat.  This is done by storing _M_current within the
fallback_next, fallback_rep_once_more and posix_alternative frames.
And we can get rid of the restore_current frame.

This reduces the maximum size of the _M_frames stack by 15% for

  regex_match(string(200000, 'a'), "(a|b|c)*")

	PR libstdc++/86164

libstdc++-v3/ChangeLog:

	* include/bits/regex_executor.tcc (__detail::_ExecutorFrameOpcode):
	Remove _S_fopcode_restore_current.
	(__detail::_Executor::_M_handle_repeat): Pass _M_current when
	pushing a fallback_next or fallback_rep_once_more frame.
	(__detail::_Executor::_M_handle_match): Don't push a
	restore_current frame.
	(__detail::_Executor::_M_handle_backref): Likewise and simplify
	accordingly.
	(__detail::_Executor::_M_handle_alternative): Pass _M_current when
	pushing a fallback_next or posix_alternative frame.
	(__detail::_Executor::_M_dfs) <case _S_fopcode_fallback_next>:
	Restore _M_current.
	<case _S_fopcode_fallback_rep_once_more>: Likewise.
	<case _S_fopcode_posix_alternative>: Likewise.
	<case _S_fopcode_restore_current>: Remove.
---
 libstdc++-v3/include/bits/regex_executor.tcc | 34 +++++++++-----------
 1 file changed, 16 insertions(+), 18 deletions(-)

diff --git a/libstdc++-v3/include/bits/regex_executor.tcc b/libstdc++-v3/include/bits/regex_executor.tcc
index 36c7e05dcdfb..5d8ce24d47f2 100644
--- a/libstdc++-v3/include/bits/regex_executor.tcc
+++ b/libstdc++-v3/include/bits/regex_executor.tcc
@@ -64,7 +64,6 @@ _GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE(_V2)
     _S_fopcode_fallback_rep_once_more,
     _S_fopcode_posix_alternative,
     _S_fopcode_merge_sol,
-    _S_fopcode_restore_current,
     _S_fopcode_restore_cur_results,
     _S_fopcode_restore_rep_count,
     _S_fopcode_decrement_rep_count,
@@ -294,7 +293,8 @@ _GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE(_V2)
 	{
 	  if constexpr (__dfs_mode)
 	    // If it's DFS executor and already accepted, we're done.
-	    _M_frames.emplace_back(_S_fopcode_fallback_next, __state._M_next);
+	    _M_frames.emplace_back(_S_fopcode_fallback_next, __state._M_next,
+				   _M_current);
 	  else
 	    _M_frames.emplace_back(_S_fopcode_next, __state._M_next);
 	  _M_frames.emplace_back(_S_fopcode_rep_once_more, __i);
@@ -304,7 +304,8 @@ _GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE(_V2)
 	  if constexpr (__dfs_mode)
 	    {
 	      // vice-versa.
-	      _M_frames.emplace_back(_S_fopcode_fallback_rep_once_more, __i);
+	      _M_frames.emplace_back(_S_fopcode_fallback_rep_once_more, __i,
+				     _M_current);
 	      _M_frames.emplace_back(_S_fopcode_next, __state._M_next);
 	    }
 	  else
@@ -410,7 +411,6 @@ _GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE(_V2)
 	{
 	  if (__state._M_matches(*_M_current))
 	    {
-	      _M_frames.emplace_back(_S_fopcode_restore_current, 0, _M_current);
 	      ++_M_current;
 	      _M_frames.emplace_back(_S_fopcode_next, __state._M_next);
 	    }
@@ -493,14 +493,8 @@ _GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE(_V2)
 	      _M_re._M_automaton->_M_traits)._M_apply(
 		  __submatch.first, __submatch.second, _M_current, __last))
 	{
-	  if (__last != _M_current)
-	    {
-	      _M_frames.emplace_back(_S_fopcode_restore_current, 0, _M_current);
-	      _M_current = __last;
-	      _M_frames.emplace_back(_S_fopcode_next, __state._M_next);
-	    }
-	  else
-	    _M_frames.emplace_back(_S_fopcode_next, __state._M_next);
+	  _M_current = __last;
+	  _M_frames.emplace_back(_S_fopcode_next, __state._M_next);
 	}
     }
 
@@ -568,14 +562,16 @@ _GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE(_V2)
 	{
 	  // TODO: Fix BFS support. It is wrong.
 	  // Pick lhs if it matches. Only try rhs if it doesn't.
-	  _M_frames.emplace_back(_S_fopcode_fallback_next, __state._M_next);
+	  _M_frames.emplace_back(_S_fopcode_fallback_next, __state._M_next,
+				 _M_current);
 	  _M_frames.emplace_back(_S_fopcode_next, __state._M_alt);
 	}
       else
 	{
 	  // Try both and compare the result.
 	  // See "case _S_opcode_accept:" handling above.
-	  _M_frames.emplace_back(_S_fopcode_posix_alternative, __state._M_next);
+	  _M_frames.emplace_back(_S_fopcode_posix_alternative, __state._M_next,
+				 _M_current);
 	  _M_frames.emplace_back(_S_fopcode_next, __state._M_alt);
 	}
     }
@@ -638,6 +634,8 @@ _GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE(_V2)
 	    case _S_fopcode_fallback_next:
 	      if (_M_has_sol)
 		break;
+	      if constexpr (__dfs_mode)
+		_M_current = __frame._M_pos;
 	      [[__fallthrough__]];
 	    case _S_fopcode_next:
 	      _M_node(__match_mode, __frame._M_state_id);
@@ -646,6 +644,8 @@ _GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE(_V2)
 	    case _S_fopcode_fallback_rep_once_more:
 	      if (_M_has_sol)
 		break;
+	      if constexpr (__dfs_mode)
+		_M_current = __frame._M_pos;
 	      [[__fallthrough__]];
 	    case _S_fopcode_rep_once_more:
 	      _M_rep_once_more(__match_mode, __frame._M_state_id);
@@ -654,6 +654,8 @@ _GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE(_V2)
 	    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 constexpr (__dfs_mode)
+		_M_current = __frame._M_pos;
 	      _M_has_sol = false;
 	      break;
 
@@ -661,10 +663,6 @@ _GLIBCXX_BEGIN_INLINE_ABI_NAMESPACE(_V2)
 	      _M_has_sol |= __frame._M_val;
 	      break;
 
-	    case _S_fopcode_restore_current:
-	      _M_current = __frame._M_pos;
-	      break;
-
 	    case _S_fopcode_restore_cur_results:
 	      if (!__frame._M_end)
 		_M_cur_results[__frame._M_state_id].first = __frame._M_pos;
-- 
2.53.0.rc1.65.gea24e2c554