PATCH] libstdc++: skip regex_search positions that cannot start a match

[Andrea Pinski]

On Wed, Jul 29, 2026 at 6:19 PM Sebastian Pop <spop@nvidia.com> wrote:
>
> std::regex_search advances one position at a time and runs a full NFA match
> attempt at every position, which is O(n*m) on a text with few matches.  For most
> patterns a match can only begin with a byte from a small, statically known set:
> the "first-character set", i.e. the bytes accepted by the _S_opcode_match states
> in the epsilon-closure of the NFA start state.  This patch computes that set once
> per regex and, before each match attempt, skips over input positions whose byte
> is not in it:
>
>   * a single required byte  -> memchr (the platform libc scans it vectorized);
>   * a small byte set        -> a 256-bit byte-set bitmap scan.
>
> The set is computed at the end of NFA construction, while the automaton is still
> single-threaded, and cached in _NFA_base; it is then immutable, so a const
> std::regex stays safe to share across threads, and repeated searches
> (regex_iterator / regex_token_iterator) pay no per-call setup.
>
> The optimization is disabled -- falling back to the exact stock scan -- whenever
> it could change behaviour: when the start-closure can reach an empty match
> (_S_opcode_accept), an anchor (^, $, \b, \B), a lookahead, or a backref; for
> non-1-byte character types; for non-contiguous iterators; and under
> match_continuous.  It is guarded on __cpp_if_constexpr, so pre-C++17 translation
> units keep the scalar behaviour byte-for-byte.  Because a non-empty,
> non-anchored match must begin with a byte from the set, skipping the others
> finds exactly the same matches: the 28_regex conformance suite is unchanged, and
> a dedicated harness confirms byte-identical match positions, prefixes and
> regex_replace output across anchors, empty/optional matches, alternation,
> character classes, case-insensitivity, multiline, backrefs, lookahead, embedded
> NUL bytes and the match flags.
>
> Because the scan is delivered by the platform libc (memchr) or a portable
> bitmap, this uplifts every architecture.  Measured on NVIDIA Vera (aarch64)
> with a stock glibc, via testsuite/performance/28_regex/first_char_skip.cc (a
> 32 MiB corpus with sparse matches): a literal-prefix pattern (Z[a-z]+, single
> required byte) improves ~88x, and a digit-class IPv4 pattern (bitmap skip)
> ~37x.  On the mariomka regex benchmark corpus the IPv4 pattern improves ~35x and
> dense first-character sets (URI/email word characters) ~1.1x, with match counts
> unchanged.  A libc tuned for the target compounds the memchr case further.


Note libstdc++ patches should be CC'ed to libstdc++@ also. Some of the
maintainers of libstdc++ are not subscribed to the main gcc-patches@
list.

Thanks,
Andrea

>
> libstdc++-v3/ChangeLog:
>
>         * include/bits/regex_automaton.h (_NFA_base): Add first-character-set
>         cache fields _M_first_char_state, _M_first_char_count, _M_first_char
>         and _M_first_char_set.
>         (_NFA::_M_compute_first_char_set): New member function; compute the set
>         of bytes a match may begin with as the epsilon-closure of the start
>         state, stopping at match states, and cache it; mark it unusable on empty
>         match, anchor, lookahead, backref or non-1-byte character type.
>         * include/bits/regex_compiler.tcc (_Compiler::_Compiler): Call
>         _M_compute_first_char_set once the NFA is fully built.
>         * include/bits/regex_executor.tcc (_Executor::_M_search): When the
>         cached set is usable and the input is contiguous 1-byte, skip positions
>         that cannot start a match with memchr (single byte) or a byte-set
>         bitmap before each match attempt.
>         * testsuite/performance/28_regex/first_char_skip.cc: New test.
>
> Signed-off-by: Sebastian Pop <spop@nvidia.com>
>