[gcc r16-7194] libstdc++/regex: Defer _M_current restoration to backtracking nodes [PR86164]
Patrick Palka
ppalka@gcc.gnu.org
Fri Jan 30 17:31:17 GMT 2026
https://gcc.gnu.org/g:ead66d5f229edda244d1a91125524eca9abdb9b9
commit r16-7194-gead66d5f229edda244d1a91125524eca9abdb9b9
Author: Patrick Palka <ppalka@redhat.com>
Date: Fri Jan 30 12:29:17 2026 -0500
libstdc++/regex: Defer _M_current restoration to backtracking nodes [PR86164]
The incrementing of the current input string position (_M_current) 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.
In turn we can get rid of the now unused restore_current frame kind.
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.
Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Diff:
---
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 b76f1dce952c..ccdec934b49c 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;
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