On Wed, Jul 29, 2026 at 6:19 PM Sebastian Pop <[email protected]> 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 <[email protected]> >
