Here's some stuff that anybody who wants to work on regex optimization in perl6 will want to know:
You can get a print-out of rakudo's internal AST that is generated for the regex by passing --target=ast or --target=optimize to the perl6 commandline. I recommend grep -C5 Regex to skip some of the boilerplate that's always there, but not important for regex stuff. Here's an example of the output for the optimized p6 regex /foo<?before bar>/: > - QAST::Regex(:rxtype(concat) :subtype()) > - QAST::Regex(:rxtype(scan) :subtype()) > - foo > - QAST::Regex(:rxtype(concat) :subtype()) foo<?before bar> > - QAST::Regex(:rxtype(literal) :subtype()) f > - foo > - QAST::Regex(:rxtype(literal) :subtype(zerowidth)) b > - bar > - QAST::Regex(:rxtype(pass) :subtype()) And here it is before optimization: > - QAST::Regex(:rxtype(concat) :subtype()) > - QAST::Regex(:rxtype(scan) :subtype()) > - foo > - QAST::Regex(:rxtype(concat) :subtype()) foo<?before bar> > - QAST::Regex(:rxtype(literal) :subtype()) f > - foo > - QAST::Regex(:rxtype(subrule) :subtype(zerowidth) :name(before)) > <?before bar> > - QAST::NodeList > - QAST::SVal(before) > - QAST::Block(:cuid(1)) :orig_qast<?> :code_object<?> > - QAST::Var(local self :decl(param)) > - QAST::Var(lexical $¢ :decl(var)) > - QAST::Regex(:rxtype(concat) :subtype()) > - QAST::Regex(:rxtype(scan) :subtype()) > - bar > - QAST::Regex(:rxtype(concat) :subtype()) bar > - QAST::Regex(:rxtype(literal) :subtype()) b > - bar > - QAST::Regex(:rxtype(pass) :subtype()) > - QAST::Regex(:rxtype(pass) :subtype()) As you can see, it shrunk quite a bit; Before optimization, the "bar" inside of <?before bar> was put into the regex as a subrule with its own complete block with lexical variables and everything. On top of that, there was a scan operation there, which is normal for regexes in general, but not useful for a lookahead or lookbehind (since they are anchored implicitly). The optimizer simply turns all of that into a "literal" "zerowidth" for "bar". If you want to see how all that is currently implemented, you can look at nqp's QRegex::Optimizer. The method "simplify_assertion" is probably what does the optimization i outlined in the previous paragraph: https://github.com/perl6/nqp/blob/master/src/QRegex/P6Regex/Optimizer.nqp#L88 Here's a perl6 one-liner that lets you eval a piece of perl6 code and get the optimized QAST result as a tree of QAST nodes: > perl6 -e 'use Perl6::Compiler:from<NQP>; use nqp; say > nqp::getcomp("perl6").eval(Q[/foo/], :target<optimize>).dump' Perhaps prototyping optimizations would be easiest in perl6 and later translated into nqp. If you want to hack on the "actual" regex optimizer, I recommend writing your test code as nqp scripts; nqp is quickly recompiled to have the new optimizer in it, and it applies the same way to perl6 regexes as it does to nqp regexes. Running a rakudo with the changed optimizer requires a full rebuild of rakudo, which of course includes the core setting. The main optimization opportunities I can see for perl6 regexes are exactly what nick points out in his mail: If you can find out that a regex can only match if the target string contains a certain substring (or even a short list of substrings that can be or'd together would be interesting), you can generate code that does a quick first check. If you can prove that a specific part of the regex has a certain minimum length, any scan through the target string can be aborted further from the end. If the regex starts with a lookbehind that has a known minimum length, the regex match attempt doesn't have to start at the very beginning of the string. I'm not entirely sure how we should go about skipping the regex engine when a "simple match" can quickly be created, though. On 13/08/2019 11:36, Nicholas Clark wrote: > I'm cheating here - I'm using an e-mail message as a way to publish some notes > for Jonathan (or *anyone else interested*): > > Jonathan did a talk in Riga, Perl 6 performance update, > https://perlcon.eu/talk/80 > The (re-uploaded) live stream is at > https://www.youtube.com/watch?v=S5iVBlk7pdg#t=4h39m > > One thing that this talk revealed is that (currently) Perl 5 beats > Rakudo on various regular expressions. What *I* know is that this is because > Perl 5 cheats - it has an "optimiser", which happens to automatically do > what jnthn then showed manually implemented in some of his benchmarks. > > You can see what the Perl 5 regex optimiser is doing by using the re pragma. > I'm no expert on this thing, but playing with it, I can see that it can do > various things > > 0) Know that it can't do anything useful (it is skipped) > 1) Know that a fixed string must be present > Look for it (which is fast), not find it, and immediately return "no match" > 2) Know that a fixed string must be present > Look for it find it, carry on into the real engine > (which arguably is slower than no optimiser) > 3) Know that a fixed string must be present, *and* is the entire match > Look for it (which is fast), find it, and immediately return "match". > > Clearly cases (1) and (3) are the big wins here. Case (3) was the one I had > forgotten about - the cheating is so good that often the engine is never > entered. > > > So, if you run this: > > $ perl -wle 'use re "debug"; print "Perl rules" =~ /^Perl/ ? "Y" : "n"' > Compiling REx "^Perl" > Final program: > 1: SBOL /^/ (2) > 2: EXACT <Perl> (4) > 4: END (0) > anchored "Perl" at 0 (checking anchored noscan) anchored(SBOL) minlen 4 > Matching REx "^Perl" against "Perl rules" > Intuit: trying to determine minimum start position... > Looking for check substr at fixed offset 0... > Intuit: Successfully guessed: match at offset 0 > 0 <> <Perl rules> | 0| 1:SBOL /^/(2) > 0 <> <Perl rules> | 0| 2:EXACT <Perl>(4) > 4 <Perl> < rules> | 0| 4:END(0) > Match successful! > Y > Freeing REx: "^Perl" > > > "Final program" describes how the regex ended up being compiled. This isn't > really of interest here. > > What is of interest is the parts between the two lines "Intuit". That's the > optimiser. For the case I specifically chose here, what we have is that > > 1) the optimiser knows that the fixed string "Perl" must be in target string > 2) which it searches for and finds > 3) at which point it calls the main engine (these lines): > > 0 <> <Perl rules> | 0| 1:SBOL /^/(2) > 0 <> <Perl rules> | 0| 2:EXACT <Perl>(4) > 4 <Perl> < rules> | 0| 4:END(0) > > (that part isn't actually of interest here. I'm just noting it as "this is > the main engine, and comparable to what Rakudo does). > > > The interesting cases are things like: > > $ perl -wle 'use re "debug"; print "Perl rules" =~ /er./ ? "Y" : "n"' > Compiling REx "er." > Final program: > 1: EXACT <er> (3) > 3: REG_ANY (4) > 4: END (0) > anchored "er" at 0 (checking anchored) minlen 3 > Matching REx "er." against "Perl rules" > Intuit: trying to determine minimum start position... > doing 'check' fbm scan, [0..9] gave 1 > Found anchored substr "er" at offset 1 (rx_origin now 1)... > (multiline anchor test skipped) > try at offset... > Intuit: Successfully guessed: match at offset 1 > 1 <P> <erl rules> | 0| 1:EXACT <er>(3) > 3 <Per> <l rules> | 0| 3:REG_ANY(4) > 4 <Perl> < rules> | 0| 4:END(0) > Match successful! > Y > Freeing REx: "er." > > > You can see that > 1) the compiler records that the longest fixed string is 'er' > 2) the optimiser looks for this before even hitting the real engine > 3) the optimiser tells the real engine that it doesn't even need to consider > trying to match at string offset 0 > > > and this: > > $ perl -wle 'use re "debug"; print "Perl rules" =~ /er/ ? "Y" : "n"' > Compiling REx "er" > Final program: > 1: EXACT <er> (3) > 3: END (0) > anchored "er" at 0 (checking anchored isall) minlen 2 > Matching REx "er" against "Perl rules" > Intuit: trying to determine minimum start position... > doing 'check' fbm scan, [0..10] gave 1 > Found anchored substr "er" at offset 1 (rx_origin now 1)... > (multiline anchor test skipped) > try at offset... > Intuit: Successfully guessed: match at offset 1 > Y > Freeing REx: "er" > > > The optimiser matches, and knows that if its crib[1] matches, the entire > regex must match, so it doesn't even call into the engine. > > > and this: > > $ perl -wle 'use re "debug"; print "Perl rules" =~ /Good *, #MoarVM/ ? "Y" : > "n"' > Compiling REx "Good *, #MoarVM" > Final program: > 1: EXACT <Good> (3) > 3: STAR (6) > 4: EXACT < > (0) > 6: EXACT <, #MoarVM> (10) > 10: END (0) > anchored "Good" at 0 floating ", #MoarVM" at 4..9223372036854775807 (checking > floating) minlen 13 > n > Freeing REx: "Good *, #MoarVM" > > The optimiser knows that the pattern can't match less than 13 characters, but > the offered string is too short. > > The optimiser also knows at what offsets the within the target string the > fixed string must lie, if there is also some variable length stuff, but this > is getting beyond what I know the plan for: > > $ perl -wle 'use re "debug"; print "Perl rules" =~ /Perl.*s/ ? "Y" : "n"' > Compiling REx "Perl.*s" > Final program: > 1: EXACT <Perl> (3) > 3: STAR (5) > 4: REG_ANY (0) > 5: EXACT <s> (7) > 7: END (0) > anchored "Perl" at 0 floating "s" at 4..9223372036854775807 (checking > anchored) minlen 5 > Matching REx "Perl.*s" against "Perl rules" > Intuit: trying to determine minimum start position... > doing 'check' fbm scan, [0..9] gave 0 > Found anchored substr "Perl" at offset 0 (rx_origin now 0)... > doing 'other' fbm scan, [4..10] gave 9 > Found floating substr "s" at offset 9 (rx_origin now 0)... > (multiline anchor test skipped) > Intuit: Successfully guessed: match at offset 0 > 0 <> <Perl rules> | 0| 1:EXACT <Perl>(3) > 4 <Perl> < rules> | 0| 3:STAR(5) > | 0| REG_ANY can match 6 times out of > 2147483647... > 9 <Perl rule> <s> | 1| 5:EXACT <s>(7) > 10 <Perl rules> <> | 1| 7:END(0) > Match successful! > Y > Freeing REx: "Perl.*s" > > > If you're wearing Peril Sensitive Sunglasses, then the code that does this is > the self-recursive Perl_re_intuit_start() in > https://perl5.git.perl.org/perl.git/blob/HEAD:/regexec.c > > As a guide to reading that, for things like > > DEBUG_EXECUTE_r(Perl_re_printf( aTHX_ > " String too short...\n")); > > > DEBUG_EXECUTE_r() only generates that debugging output if the command line > flag -Dr is set, *and* the `use re "debug";` de facto sets -Dr, even on a > perl binary compiled without all the other -D flags enabled. > > > I know that PCRE also exposes some information it derives from the pattern > which can be used to pre-match -- see PCRE_INFO_FIRSTCHARACTER and > PCRE_INFO_REQUIREDCHAR in > https://www.pcre.org/original/doc/html/pcre_fullinfo.html > > I *thought* that Google's re2 library (which came out of code search) could > return the longest fixed string (if any) as part of its API, but I can't find > that now. In https://swtch.com/~rsc/regexp/regexp4.html it mentions building > trigrams (directly) from parsing the regex (with ANDs and ORs) which is what > some $ork code is doing (but then using PCRE, not re2). > > Anyway, I hope that this helps someone, and helps make Rakudo's parser faster > > I assume that the useful strategy is roughly > > 0) Do the most simple thing - no alternations, anchored starts. Reject match > if not present > 1) Then 1+ character fixed substrings - Reject match if not present > 2) Then the big cheat of "string is actually fixed" - compile to a sub that > build a match result immediately, instead of the more general engine > 3) Then the even more general "fixed string as part of larger regex", where > failure is a fast-path, but otherwise the regular regex engine follows > 4) Minimum match length > 5) The stuff where one starts to tell the main engine how > > > but of course no plan survives contact with the enemy. > > Nicholas Clark > > 1: https://en.wiktionary.org/wiki/crib -- definitions 14 and 23: > (usually in the plural) A collection of quotes or references for > use in speaking, for assembling a written document, or as an aid to > a project of some sort; a crib sheet. > [and] > A cheat sheet or past test used by students; crib sheet.
