mruby-regexp: stop looking up MatchData, $~ and the Regexp ivars by string on every match - #7271

Every successful search in mruby-regexp goes through create_matchdata(), and that function looked its names up by string on every call: mrb_class_get(mrb, "MatchData"), which interns the name and then reads the constant; mrb_intern_lit() of source and regexp for the two ivars that keep the snapshot and the pattern reachable; and mrb_intern_lit() of $~ in set_match_globals() and clear_match_globals(). Each of those is a binary search over the presym table with a memcmp per probe. The thirteen names derived from the match were already cached in nth_syms[] and last_match_syms[] by ensure_match_syms(); $~ was the one left out. Outside the match, regexp_init() read the Regexp class by mrb_class_get(mrb, "Regexp") and @source and @flags by mrb_intern_lit(), and since a regexp literal is compiled every time it is evaluated, that ran once per turn of a loop over /o/ as well.

Under callgrind, Regexp.__byte_search with /o/ on a 480 byte subject cost 8,254 Ir per search, 7,166 of them in create_matchdata(); find_symbol() alone was 17% of the whole program and memcmp under it another 6%. The match itself (mrb_re_exec) was below 1%.

Fix

Two commits, both mechanical, neither changing behaviour.

The first takes the four lookups on the match path: mrb_class_get_id(mrb, MRB_SYM(MatchData)), MRB_SYM(source) and MRB_SYM(regexp) for the ivar names, and $~ cached in ensure_match_syms() next to the other match symbols (MRB_GVSYM() takes a word after the $, which ~ is not, so it cannot name it directly). The ivar names stay the same symbols, so source and regexp remain hidden from instance_variables as before.

The second takes the rest of the file: mrb_class_get_id(mrb, MRB_SYM(Regexp)), which regexp.c already used in four other places, and MRB_IVSYM(source), MRB_IVSYM(flags) and MRB_IVSYM(named_captures) in regexp_init(), the accessors, to_s, inspect, eql?, hash and casefold?. Again the same symbols, so the mrblib side that reads @named_captures sees what it did.

Cost

Callgrind, default configuration (-O3), 1000.times { pos = 0; 72.times { md = Regexp.__byte_search(/o/, s480, pos); pos += 6 } } with s480 = "hello world foo bar " * 24; per-search figures are inclusive Ir over 108,000 searches, the loop turn is Ir(1500 iterations) - Ir(500 iterations) over 72,000 turns so that start-up cancels.

Wall clock, the cases of the review on #7267, minimum of 5 alternating runs, -O3, default configuration:

s480 = "hello world foo bar " * 24
30000.times { s480.gsub(/o/) { } }        # gsub480_blk
100000.times { "abc".gsub(/b/) { } }      # gsub_abc_blk
100000.times { "abc".scan(/b/) { } }      # scan_abc_blk
100000.times { "abc".sub!(/b/) { } }      # sub!_abc_blk
30000.times { s480.gsub(/o/, "0") }       # gsub480_str
100000.times { "abc".scan(/b/) }          # scan_abc
100000.times { "abc".sub!(/b/, "0") }     # sub!_abc_str
30000.times { s480.gsub(/z/) { } }        # gsub480_none

The block forms publish a MatchData per match, and that is where the first commit lands. The string and blockless forms run in C (__gsub_str, __scan, __sub_str) and publish one MatchData at the end of the call, so the first commit reaches them once per call; sub! searches once to decide its return value and once more in sub, so both of its rows publish twice. The literal is compiled on every turn in all eight cases, so the second commit reaches each of them, and on the three byte subject a call is mostly the literal, which is why scan_abc and sub!_abc_str move as much as the block forms. gsub480_none matches nothing, publishes nothing, and shows the literal alone.

.text of bin/mruby is 1196814 on master, 1198190 after the first commit and 1197774 at the tip (+960). None of it is the change: regexp.o shows clear_match_globals() (130 bytes on master) at 0 and each of its twelve call sites about 100 bytes larger, so gcc inlined it once it lost the $~ intern; create_matchdata(), regexp_init() and the accessors are all smaller.

Testing

No test is added: nothing observable changes. Full suite green at both commits; MRUBY_CONFIG=ci/gcc-clang rake -m test at the tip and the default configuration (rake -m test, no MRUBY_CONFIG, fresh build directory) at each commit.

Environment

# full-debug
gcc -std=gnu99 -g -O3 -Wall -Wundef -Werror-implicit-function-declaration -Wwrite-strings -g3 -O0 -DMRB_GC_STRESS -DMRB_USE_DEBUG_HOOK -DMRBGEM_MRUBY_REGEXP_VERSION=0.0.0 -DMRB_DEBUG -DMRB_USE_BIGINT -DMRB_USE_COMPLEX -DHAVE_MRUBY_ENCODING_GEM -DMRB_UTF8_STRING -DHAVE_MRUBY_IO_GEM -DMRB_USE_RATIONAL -DHAVE_MRUBY_REGEXP_GEM -DMRB_USE_SET -DMRB_USE_TASK_SCHEDULER mrbgems/mruby-regexp/src/regexp.c
# bintest
gcc -std=gnu99 -g -O3 -Wall -Wundef -Werror-implicit-function-declaration -Wwrite-strings -DMRB_GC_FIXED_ARENA -DMRBGEM_MRUBY_REGEXP_VERSION=0.0.0 -DMRB_USE_BIGINT -DMRB_USE_COMPLEX -DHAVE_MRUBY_ENCODING_GEM -DMRB_UTF8_STRING -DHAVE_MRUBY_IO_GEM -DMRB_USE_RATIONAL -DHAVE_MRUBY_REGEXP_GEM -DMRB_USE_SET -DMRB_USE_TASK_SCHEDULER -DMRB_USE_DEBUG_HOOK mrbgems/mruby-regexp/src/regexp.c
# cxx_abi
gcc -g -O3 -Wall -Wundef -Wwrite-strings -x c++ -std=gnu++03 -DMRB_GC_FIXED_ARENA -DMRBGEM_MRUBY_REGEXP_VERSION=0.0.0 -DMRB_USE_CXX_EXCEPTION -DMRB_USE_CXX_ABI -DMRB_USE_BIGINT -DMRB_USE_COMPLEX -DHAVE_MRUBY_ENCODING_GEM -DMRB_UTF8_STRING -DHAVE_MRUBY_IO_GEM -DMRB_USE_RATIONAL -DHAVE_MRUBY_REGEXP_GEM -DMRB_USE_SET -DMRB_USE_TASK_SCHEDULER mrbgems/mruby-regexp/src/regexp.c
# byte-string
gcc -std=gnu99 -g -O3 -Wall -Wundef -Werror-implicit-function-declaration -Wwrite-strings -DMRBGEM_MRUBY_REGEXP_VERSION=0.0.0 -DMRB_USE_BIGINT -DMRB_USE_COMPLEX -DHAVE_MRUBY_IO_GEM -DMRB_USE_RATIONAL -DHAVE_MRUBY_REGEXP_GEM -DMRB_USE_SET -DMRB_USE_TASK_SCHEDULER mrbgems/mruby-regexp/src/regexp.c
# ascii-case
gcc -std=gnu99 -g -O3 -Wall -Wundef -Werror-implicit-function-declaration -Wwrite-strings -DMRB_USE_ASCII_CASE -DMRBGEM_MRUBY_REGEXP_VERSION=0.0.0 -DMRB_USE_BIGINT -DMRB_USE_COMPLEX -DHAVE_MRUBY_ENCODING_GEM -DMRB_UTF8_STRING -DHAVE_MRUBY_IO_GEM -DMRB_USE_RATIONAL -DHAVE_MRUBY_REGEXP_GEM -DMRB_USE_SET -DMRB_USE_TASK_SCHEDULER mrbgems/mruby-regexp/src/regexp.c
# default (no MRUBY_CONFIG), the build every figure above was measured on
gcc -std=gnu99 -g -O3 -Wall -Wundef -Werror-implicit-function-declaration -Wwrite-strings -DMRBGEM_MRUBY_REGEXP_VERSION=0.0.0 -DHAVE_MRUBY_REGEXP_GEM -DMRB_USE_SET -DHAVE_MRUBY_IO_GEM -DMRB_USE_RATIONAL -DMRB_USE_COMPLEX -DMRB_USE_BIGINT -DMRB_USE_DEBUG_HOOK mrbgems/mruby-regexp/src/regexp.c

Summary by CodeRabbit

• Performance Improvements
  • Improved regular expression processing by reusing cached identifiers and class references.
  • Reduced overhead when handling match results and named captures.
• Reliability
  • Improved management of regular-expression match data during cleanup and garbage collection.