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.
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]>
0001-libstdc-skip-regex_search-positions-that-cannot-star.patch
Description: 0001-libstdc-skip-regex_search-positions-that-cannot-star.patch
