On Wed, 21 Jan 2026, Patrick Palka wrote:
> If __rep_count.second is already 1 we don't need to add a cleanup frame
> to restore it to 1.
FWIW it seems unnecessary to restore __rep_count.first and
__rep_count.second at all here. The __rep_count stuff is for infinite
loop detection, and I don't see why such backtracking is necessary for
that..
>
> This reduces the maximum size of the __frames stack by 12% for
> regex_match(string(200000, 'a'), "(a|b|c)*").
>
> PR libstdc++/86164
>
> libstdc++-v3/ChangeLog:
>
> * include/bits/regex_executor.tcc: Only add a
> restore_rep_count_val frame when necessary.
> ---
> libstdc++-v3/include/bits/regex_executor.tcc | 9 ++++++---
> 1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/libstdc++-v3/include/bits/regex_executor.tcc
> b/libstdc++-v3/include/bits/regex_executor.tcc
> index 0e78cb2e2c87..ca631fb77ce9 100644
> --- a/libstdc++-v3/include/bits/regex_executor.tcc
> +++ b/libstdc++-v3/include/bits/regex_executor.tcc
> @@ -181,12 +181,15 @@ namespace __detail
> auto& __rep_count = _M_rep_count[__i];
> if (__rep_count.second == 0 || __rep_count.first != _M_current)
> {
> - __frames.emplace_back(_S_fopcode_restore_rep_count_val,
> - __i, __rep_count.second);
> + if (__rep_count.second != 1)
> + {
> + __frames.emplace_back(_S_fopcode_restore_rep_count_val,
> + __i, __rep_count.second);
> + __rep_count.second = 1;
> + }
> __frames.emplace_back(_S_fopcode_restore_rep_count_pos,
> __i, __rep_count.first);
> __rep_count.first = _M_current;
> - __rep_count.second = 1;
> __frames.emplace_back(_S_fopcode_next, __state._M_alt);
> }
> else
> --
> 2.53.0.rc0.25.gb5c409c40f
>
>
