From @jimav

This is a bug report for perl from jim.avera@​gmail.com,
generated with the help of perlbug 1.39 running under perl 5.18.2.

Regexs like /\G^/msgc seem abnormally slow.

The run-time appears to be quadratic in the total length of the string
being searched, as if a failed match causes the *entire* string to be
re-processed, even though the /gc should mean just back up to where \G was.

The problem is specific to using ^ i.e. the exact same regex without ^
runs quadratically faster.

The demo script below scans a large string using a sequence of stanzas
beginning with either /\G^x/msgc or /\Gx/msgc
(neither of which ever match).

Here are my results​:
  Scan 5k bytes using \G^x​: 8.43/s
  Scan 10k bytes using \G^x​: 2.11/s
  Scan 20k bytes using \G^x​: .53/s
  Scan 40k bytes using \G^x​: .13/s
  Scan 50k bytes using \Gx : 168.07/s

#!/usr/bin/perl
use strict; use warnings;
use Benchmark;
sub test($$) {
  my ($re, $datasize) = @​_;
  my $data = "foo\n" x ($datasize/5);
  for (;;) {
  if ($data =~ /$re/msgc) { die "unexpected match" }
  elsif ($data =~ /\G(\w+)([^\n]*\n)/sgc) { }
  elsif ($data =~ /\G\z/sgc) { last; }
  else { die "bug" }
  }
  $data =~ /\G./msg && die "bug"; # reset pos
}
timethis (700, sub{ test('\Gx', 50000) }, "Scan 50k bytes using \\Gx ");
timethis (30, sub{ test('\G^x', 5000) }, "Scan 5k bytes using \\G^x");
timethis (15, sub{ test('\G^x', 10000) }, "Scan 10k bytes using \\G^x");
timethis (5, sub{ test('\G^x', 20000) }, "Scan 20k bytes using \\G^x");
timethis (3, sub{ test('\G^x', 40000) }, "Scan 40k bytes using \\G^x");

Flags​:
  category=core
  severity=medium

Site configuration information for perl 5.18.2​:

Configured by Debian Project at Thu Mar 27 18​:28​:21 UTC 2014.

Summary of my perl5 (revision 5 version 18 subversion 2) configuration​:
 
  Platform​:
  osname=linux, osvers=3.2.0-58-generic, archname=x86_64-linux-gnu-thread-multi
  uname='linux brownie 3.2.0-58-generic #88-ubuntu smp tue dec 3 17​:37​:58 utc 2013 x86_64 x86_64 x86_64 gnulinux '
  config_args='-Dusethreads -Duselargefiles -Dccflags=-DDEBIAN -D_FORTIFY_SOURCE=2 -g -O2 -fstack-protector --param=ssp-buffer-size=4 -Wformat -Werror=format-security -Dldflags= -Wl,-Bsymbolic-functions -Wl,-z,relro -Dlddlflags=-shared -Wl,-Bsymbolic-functions -Wl,-z,relro -Dcccdlflags=-fPIC -Darchname=x86_64-linux-gnu -Dprefix=/usr -Dprivlib=/usr/share/perl/5.18 -Darchlib=/usr/lib/perl/5.18 -Dvendorprefix=/usr -Dvendorlib=/usr/share/perl5 -Dvendorarch=/usr/lib/perl5 -Dsiteprefix=/usr/local -Dsitelib=/usr/local/share/perl/5.18.2 -Dsitearch=/usr/local/lib/perl/5.18.2 -Dman1dir=/usr/share/man/man1 -Dman3dir=/usr/share/man/man3 -Dsiteman1dir=/usr/local/man/man1 -Dsiteman3dir=/usr/local/man/man3 -Duse64bitint -Dman1ext=1 -Dman3ext=3perl -Dpager=/usr/bin/sensible-pager -Uafs -Ud_csh -Ud_ualarm -Uusesfio -Uusenm -Ui_libutil -Uversiononly -DDEBUGGING=-g -Doptimize=-O2 -Duseshrplib -Dlibperl=libperl.so.5.18.2 -des'
  hint=recommended, useposix=true, d_sigaction=define
  useithreads=define, usemultiplicity=define
  useperlio=define, d_sfio=undef, uselargefiles=define, usesocks=undef
  use64bitint=define, use64bitall=define, uselongdouble=undef
  usemymalloc=n, bincompat5005=undef
  Compiler​:
  cc='cc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fstack-protector -fno-strict-aliasing -pipe -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64',
  optimize='-O2 -g',
  cppflags='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fstack-protector -fno-strict-aliasing -pipe -I/usr/local/include'
  ccversion='', gccversion='4.8.2', gccosandvers=''
  intsize=4, longsize=8, ptrsize=8, doublesize=8, byteorder=12345678
  d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16
  ivtype='long', ivsize=8, nvtype='double', nvsize=8, Off_t='off_t', lseeksize=8
  alignbytes=8, prototype=define
  Linker and Libraries​:
  ld='cc', ldflags =' -fstack-protector -L/usr/local/lib'
  libpth=/usr/local/lib /lib/x86_64-linux-gnu /lib/../lib /usr/lib/x86_64-linux-gnu /usr/lib/../lib /lib /usr/lib
  libs=-lgdbm -lgdbm_compat -ldb -ldl -lm -lpthread -lc -lcrypt
  perllibs=-ldl -lm -lpthread -lc -lcrypt
  libc=, so=so, useshrplib=true, libperl=libperl.so.5.18.2
  gnulibc_version='2.19'
  Dynamic Linking​:
  dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E'
  cccdlflags='-fPIC', lddlflags='-shared -L/usr/local/lib -fstack-protector'

Locally applied patches​:
  DEBPKG​:debian/cpan_definstalldirs - Provide a sensible INSTALLDIRS default for modules installed from CPAN.
  DEBPKG​:debian/db_file_ver - http​://bugs.debian.org/340047 Remove overly restrictive DB_File version check.
  DEBPKG​:debian/doc_info - Replace generic man(1) instructions with Debian-specific information.
  DEBPKG​:debian/enc2xs_inc - http​://bugs.debian.org/290336 Tweak enc2xs to follow symlinks and ignore missing @​INC directories.
  DEBPKG​:debian/errno_ver - http​://bugs.debian.org/343351 Remove Errno version check due to upgrade problems with long-running processes.
  DEBPKG​:debian/libperl_embed_doc - http​://bugs.debian.org/186778 Note that libperl-dev package is required for embedded linking
  DEBPKG​:fixes/respect_umask - Respect umask during installation
  DEBPKG​:debian/writable_site_dirs - Set umask approproately for site install directories
  DEBPKG​:debian/extutils_set_libperl_path - EU​:MM​: Set location of libperl.a to /usr/lib
  DEBPKG​:debian/no_packlist_perllocal - Don't install .packlist or perllocal.pod for perl or vendor
  DEBPKG​:debian/prefix_changes - Fiddle with *PREFIX and variables written to the makefile
  DEBPKG​:debian/fakeroot - Postpone LD_LIBRARY_PATH evaluation to the binary targets.
  DEBPKG​:debian/instmodsh_doc - Debian policy doesn't install .packlist files for core or vendor.
  DEBPKG​:debian/ld_run_path - Remove standard libs from LD_RUN_PATH as per Debian policy.
  DEBPKG​:debian/libnet_config_path - Set location of libnet.cfg to /etc/perl/Net as /usr may not be writable.
  DEBPKG​:debian/mod_paths - Tweak @​INC ordering for Debian
  DEBPKG​:debian/module_build_man_extensions - http​://bugs.debian.org/479460 Adjust Module​::Build manual page extensions for the Debian Perl policy
  DEBPKG​:debian/prune_libs - http​://bugs.debian.org/128355 Prune the list of libraries wanted to what we actually need.
  DEBPKG​:fixes/net_smtp_docs - [rt.cpan.org #36038] http​://bugs.debian.org/100195 Document the Net​::SMTP 'Port' option
  DEBPKG​:debian/perlivp - http​://bugs.debian.org/510895 Make perlivp skip include directories in /usr/local
  DEBPKG​:debian/cpanplus_definstalldirs - http​://bugs.debian.org/533707 Configure CPANPLUS to use the site directories by default.
  DEBPKG​:debian/cpanplus_config_path - Save local versions of CPANPLUS​::Config​::System into /etc/perl.
  DEBPKG​:debian/deprecate-with-apt - http​://bugs.debian.org/702096 Point users to Debian packages of deprecated core modules
  DEBPKG​:debian/squelch-locale-warnings - http​://bugs.debian.org/508764 Squelch locale warnings in Debian package maintainer scripts
  DEBPKG​:debian/skip-upstream-git-tests - Skip tests specific to the upstream Git repository
  DEBPKG​:debian/patchlevel - http​://bugs.debian.org/567489 List packaged patches for 5.18.2-2ubuntu1 in patchlevel.h
  DEBPKG​:debian/skip-kfreebsd-crash - http​://bugs.debian.org/628493 [perl #96272] Skip a crashing test case in t/op/threads.t on GNU/kFreeBSD
  DEBPKG​:fixes/document_makemaker_ccflags - http​://bugs.debian.org/628522 [rt.cpan.org #68613] Document that CCFLAGS should include $Config{ccflags}
  DEBPKG​:debian/find_html2text - http​://bugs.debian.org/640479 Configure CPAN​::Distribution with correct name of html2text
  DEBPKG​:debian/hurd_test_skip_stack - http​://bugs.debian.org/650175 Disable failing GNU/Hurd tests dist/threads/t/stack.t
  DEBPKG​:fixes/manpage_name_Test-Harness - http​://bugs.debian.org/650451 [rt.cpan.org #73399] cpan/Test-Harness​: add NAME headings in modules with POD
  DEBPKG​:debian/makemaker-pasthru - http​://bugs.debian.org/660195 [rt.cpan.org #28632] Make EU​::MM pass LD through to recursive Makefile.PL invocations
  DEBPKG​:debian/perl5db-x-terminal-emulator.patch - http​://bugs.debian.org/668490 Invoke x-terminal-emulator rather than xterm in perl5db.pl
  DEBPKG​:debian/cpan-missing-site-dirs - http​://bugs.debian.org/688842 Fix CPAN​::FirstTime defaults with nonexisting site dirs if a parent is writable
  DEBPKG​:fixes/memoize_storable_nstore - [rt.cpan.org #77790] http​://bugs.debian.org/587650 Memoize​::Storable​: respect 'nstore' option not respected
  DEBPKG​:fixes/net_ftp_failed_command - [rt.cpan.org #37700] http​://bugs.debian.org/491062 Net​::FTP​: cope gracefully with a failed command
  DEBPKG​:fixes/perlbug-patchlist - [3541c11] http​://bugs.debian.org/710842 [perl #118433] Make perlbug look up the list of local patches at run time
  DEBPKG​:fixes/module_metadata_security_doc - [68cdd4b] CVE-2013-1437 documentation fix
  DEBPKG​:fixes/module_metadata_taint_fix - [bff978f] http​://bugs.debian.org/722210 [rt.cpan.org #88576] untaint version, if needed, in Module​::Metadata
  DEBPKG​:fixes/IPC-SysV-spelling - http​://bugs.debian.org/730558 [rt.cpan.org #86736] Fix spelling of IPC_CREAT in IPC-SysV documentation
  DEBPKG​:fixes/fix-undef-source -

@​INC for perl 5.18.2​:
  /home/jima/lib/perl
  /home/jima/perl5/lib/perl5/x86_64-linux-gnu-thread-multi
  /home/jima/perl5/lib/perl5/x86_64-linux-gnu-thread-multi
  /home/jima/perl5/lib/perl5
  /etc/perl
  /usr/local/lib/perl/5.18.2
  /usr/local/share/perl/5.18.2
  /usr/lib/perl5
  /usr/share/perl5
  /usr/lib/perl/5.18
  /usr/share/perl/5.18
  /usr/local/lib/site_perl
  .

Environment for perl 5.18.2​:
  HOME=/home/jima
  LANG=en_US.UTF-8
  LANGUAGE=en_US
  LD_LIBRARY_PATH=/home/jima/local/lib
  LOGDIR (unset)
  PATH=/home/jima/perl5/bin​:/home/jima/bin​:/home/jima/local/bin​:/home/jima/jima_tools/x86_64/bin​:/home/jima/jima_tools/bin​:/opt/Adobe/Reader9/bin​:/usr/bin​:/bin​:/usr/sbin​:/sbin​:/usr/bin/X11​:/usr/local/bin​:/usr/local/sbin​:/usr/games​:/usr/local/games​:/usr/lib/jvm/java-7-oracle/bin​:/usr/lib/jvm/java-7-oracle/db/bin​:/usr/lib/jvm/java-7-oracle/jre/bin​:.
  PERL5LIB=/home/jima/lib/perl​:/home/jima/perl5/lib/perl5/x86_64-linux-gnu-thread-multi​:/home/jima/perl5/lib/perl5
  PERL_BADLANG (unset)
  PERL_LOCAL_LIB_ROOT=/home/jima/perl5
  PERL_MB_OPT=--install_base /home/jima/perl5
  PERL_MM_OPT=INSTALL_BASE=/home/jima/perl5
  SHELL=/bin/bash

From @iabyn

On Fri, Nov 07, 2014 at 11​:19​:14PM -0800, via RT wrote​:

Regexs like /\G^/msgc seem abnormally slow.

The run-time appears to be quadratic in the total length of the string
being searched, as if a failed match causes the *entire* string to be
re-processed, even though the /gc should mean just back up to where \G was.

The problem is specific to using ^ i.e. the exact same regex without ^
runs quadratically faster.

The demo script below scans a large string using a sequence of stanzas
beginning with either /\G^x/msgc or /\Gx/msgc
(neither of which ever match).

Here are my results​:
Scan 5k bytes using \G^x​: 8.43/s
Scan 10k bytes using \G^x​: 2.11/s
Scan 20k bytes using \G^x​: .53/s
Scan 40k bytes using \G^x​: .13/s
Scan 50k bytes using \Gx : 168.07/s

#!/usr/bin/perl
use strict; use warnings;
use Benchmark;
sub test($$) {
my ($re, $datasize) = @​_;
my $data = "foo\n" x ($datasize/5);
for (;;) {
if ($data =~ /$re/msgc) { die "unexpected match" }
elsif ($data =~ /\G(\w+)([^\n]*\n)/sgc) { }
elsif ($data =~ /\G\z/sgc) { last; }
else { die "bug" }
}
$data =~ /\G./msg && die "bug"; # reset pos
}
timethis (700, sub{ test('\Gx', 50000) }, "Scan 50k bytes using \\Gx ");
timethis (30, sub{ test('\G^x', 5000) }, "Scan 5k bytes using \\G^x");
timethis (15, sub{ test('\G^x', 10000) }, "Scan 10k bytes using \\G^x");
timethis (5, sub{ test('\G^x', 20000) }, "Scan 20k bytes using \\G^x");
timethis (3, sub{ test('\G^x', 40000) }, "Scan 40k bytes using \\G^x");

Turns out the optimiser (re_intuit_start()) doesn't cope too well
with multiple anchors (both \G and /^/m), and currently prioritises
the second one; so it goes hunting for /^x/m across most of the string
length (from pos()..end), rather than only looking in the one position
based on the \G restriction.

I'm halfway though refactoring re_intuit_start() (that work went into
5.20); I'll add this to my big list of things to do once I get round to
finishing it off.

--
Please note that ash-trays are provided for the use of smokers,
whereas the floor is provided for the use of all patrons.
  -- Bill Royston

The RT System itself - Status changed from 'new' to 'open'