This Week on perl5-porters - 20-26 February 2006

This Week on perl5-porters - 20-26 February 2006

Nicholas Clark awarded a TPF grant to work on perl5 -- a new maint snapshot available -- work on saving memory continues.

Topics of Interest

DBI fails to build on blead

Andreas Koenig noticed that blead@27244 breaks DBI, and thought that it might be a good idea to have the smoke testers smoke test a good handful of CPAN modules on a regular basis.

Nicholas Clark pointed out that DBI does lots of naughty things under the covers, and was loathe to back out a change that enabled a non-trivial amount of memory savings, and so he was hoping that DBI could be patched instead. And suggested that if anyone could see to smoking a good heap o' CPAN, it would be most appreciated.

Tim Bunce cheerfully admitted his guilt and patched DBI to Do The Right Thing. Version 1.51 should be out soon. As it says in the comments:

 /* If we are calling an XSUB we jump directly to its C code and
  * bypass perl_call_sv(), pp_entersub() etc. This is fast.

  27244 breaks DBI, DBI breaks 27244

How to save 3Kb

Through an extraordinary piece of luck, Nicholas Clark happened to chance upon a simple technique to save 2720 bytes on the final perl binary with no changes to any C files, simply by rearranging the SV flag bits. He wondered if this was because the x86 architecture has shorter instructions for 16-bit immediate constants (as opposed to longer instructions for 32-bit constants).

Andy Dougherty recalled that Larry Wall developed 5.000 mainly on sparc, and that this issue has probably never appeared on anybody's radar. He noted a significant reduction as well on current sparc hardware.

Nick Ing-Simmons suggested that the bits were probably allocated on a least significant bit/first come, first served basis.

  The joys of CISC

study and tied variables

Nicholas Clark noticed that study will study any old thing you care to feed it, and realised that if you gave it a tied variable, it might be possible to pull the rug out from underneath it, simply because what study saw and what the regular expression saw were not the same things.

If this were the case, then

  study $tied_var;
  $tied_var =~ /$pattern/;

might fail, but remove the study line and it starts to work. Nicholas was having difficulty, however, in coming up with a test case to prove or disprove this theory. Rick Delaney responded with a nice short snippet of code to show that Nicholas was right to be suspicious. Andy Lester picked it up to transform it into a unit test.

Rick a added a bit more code to show that not only is study's behaviour is only stranger than we imagine, it is stranger than we can imagine.

  Studying ties

  Andy's test. applied by Rafael as #27271

TPF awards a grant to Nicholas Clark

You may have heard the news, but The Perl Foundation has awarded a grant to Nicholas Clark to work on various parts of Perl 5 for the next three months.

Nicholas divided the matter into three parts

  • Items for blead
  • Items for blead that could make it back to maint
  • Items for blead that should make it back to maint

The third item above is a large list of goodies, so let's hope that Nicholas is successful.

I know it's in the boilerplate of this summary, but it's worth saying again: if you find these summaries useful, consider making a donation to the Perl Foundation. If it helps people like Nicholas, Rafael or Robin Houston work on Perl full-time for a while then only good things can come of it.

  The details from the foundation

  Contribute! Contribute!

Note, the original message to p5p had an excessive number of repeated punctuation characters in the subject line, that led some poor soul's brain-dead mail reader to classify it as spam. If you missed this item on the mailing list, you may have to fish it out of your Junk folder.

  Make perl Fast

Part of the thread went sideways and started talking about spam. Nonetheless, it was still an interesting discussion.

  Off-topic discussion about spam

Detecting unrealised constant subroutines

Last year, Nicholas Clark rewrote the way constant subroutines are implemented, which led to significant memory savings. Adam Kennedy realised that this may be interfering with an idiom used in his code to whether a function (and thus a constant is defined):

  if (defined *{$glob}{CODE}) { ... } # it is defined

Nicholas showed that that code fragment may have problems even as far back as 5.005. Rafael suggested that a much better technique was to use exists instead, such as

  if (exists &{ref($obj).'::'.$method}) { ... } # no, it exists

  Constantly learning new things

Unwanted vivification

Steve Peters committed a new test as change #27287, to note that the following program should not produce any output:

  my %h;
  foreach (@h{a, b}) {
    # do nothing
  print "$_\n" for keys %h;

Except that it does (as noted in bug #2166). Nicholas observed that hash slices also showed the same behaviour when used as subroutine arguments. Nick Ing-Simmons admitted to having used that feature.

  It's not a bug!

Shrinking the body_details table

Nicholas committed change #27290 to replace a field by U8 instead of size_t. Jan Dubois wanted to know whether the cost of an unaligned U8 fetch would outweigh the slowdown incurred for what would appear to be a very modest reduction in size.

Nicholas wasn't sure what the impact would be on x86 but noted that there was no impact on ARM, and wondered whether (and how) he should attempt to instrument cache use.

  Do not cross the (cache) line

A maint snapshot (perl@27284)

With a couple of minor improvements to 5.8.8, Nicholas decided it was time for a maintenance snapshot.

  Smaller *and* faster

Dealing with op_next and op_sibling

A couple of years ago, Jim Cromie made the observation that in the process of turn Perl code into opcodes, there are two fields in BASEOP, op_next and op_sibling and yet only one is in use at any given time, and a back of the envelope calculation showed Jim that it might be possible to reduce the amount of memory required to store opcodes by 20 percent.

Jim took another look at the idea this week, this time with a patch to implement the idea as a proof of concept, and requested comments.

This time, rather than doing away with op_sibling, he just wanted to push it away somewhere else, so that the result blob of opcodes would be "denser" and thus more of them would fit in a cache line, which could lead to a measurable increase in run-time performance.

A run-time option may be made available to drop it altogether, which would reduce the memory footprint of the op-coded program (on the understanding that one shouldn't expect the B:: modules and the like to continue to work afterwards).

Nicholas Clark did some arithmetic on pointers eliminated and needed, and concluded that the result would in most cases would end up as a net loss.

All the same, the conversation turned to the question of optimisations, and the issue was of trying to gauge how much work it would take to make the peep-hole optimiser truly pluggable. One could then switch over to a more powerful optimiser that spent more time in rewriting the op-code tree, in order to improve the run-time performance of long-running programs.

  A hare-brained scheme

Patches of Interest

Sloppy stat() on Windows

Following on from the thread kicked off last week concerning trailing slashes on directory names and their effects of the result of stat operations in Win32, Jan Dubois delivered a patch to improve the performance of stating, and the cost of potential problems with hard linked files on NTFS partitions. Applied by Rafael as change #27283.

Jan's preliminary benchmarks showed a significant increase in speed.

It turns out that Perl has had the ability to create hard links under Win32 for quite some time, the proviso being that the account from which the process is run must have Backup/Restore privileges.

  From last week

  Fast and sloppy

B, CGI and ExtUtils::MM_Unix

Joshua ben Jore rolled up a composite patch to tidy some warnings that were cropping up in the test suite under blead. Steve Peters tried the patch but received a "you planned 55 tests but ran 2 extra" warning. After Joshua delivered an updated patch, Steve applied it with a couple of further tweaks.

Joshua had another patch for and realised to his horror that he had perltidied the source, so had to go back to fresh copy and redo the patch. He asked if people minded if he tidied the code in the B namespace as he worked on it.

Rafael said that he didn't mind, so long as formatting changes and code changes were not mixed up in the same patch (in other words, send in a patch that does a whitespace reformatting, and then send in a second patch that changes the code).

Making SDBM_File work with -Duse64bitall on Darwin

Dominic Dunlop supplied a patch to clear up the test failures relating to SDBM_File when compiling with -Duse64bitall on Darwin 8.x (Mac OS X 10.4.x). Patch applied without fuss by Rafael as change #27250.

Insecure Dependency in t/op/utftaint

Dominic was also having trouble with $ENV{CDPATH} after change #27236; the test was dying with a taint error, so he rewrote the code to resolve the failure but was worried that it might have broken something that the code was actually supposed to check. In any event, Rafael applied the patch as change #27248.

Fixing op/magic failures on cygwin after 1.5.19-4

Yitzchak Scott-Thoennes sent in a patch to deal with changes in $^X in recent snapshots of cygwin. Steve Peters applied the patch, but H.Merijn Brand wondered whether the s/\.exe// in the patch should be anchored with a $.

Unused context warnings, revisited

Andy Lester sent in a new and improved version of his patch to get rid of warnings of unused context. Better still, in unthreaded perls, the PERL_UNUSED_CONTEXT vanishes to... nothing. Steve Peters liked the patch so much, he applied it as #27300.

Avoiding a valgrind error

Jarkko posted another patch that applied equally well to both maint and blead. The problem was this:

  $_ = 'a';
  print eval '$&';

And he finally has a patch to solve the leak. Applied by Rafael as #27270. Nicholas managed to cook up a snippet that snuck past Jarkko's patch. Jarkko was pretty disgusted.

Basically, a lot of work has gone into trying to optimise unnecessary copying of $&, but perhaps it's time to throw it all away and just copy the string, all the time, everywhere.

  The root of all evil

Andy makes SvREFCNT_inc go faster

Andy Lester rewrote the SvREFCNT_inc macro that it used whenever the reference count of an sv needs to be incremented. He noticed a tiny improvement as a result, but was a bit surprised that the difference wasn't greater. Andy's looking for people to take blead for a spin, before and after change #27334 to see if anyone else can measure an improvement.

  Faster! Faster!

He also consted gv.c and gv.h, most of which was applied.

And replaced uses of constructs such as (char*)NULL by plain old NULL. Mark Jason Dominus warned that NULL needs to be cast when passed as an argument to a variadic (varying number of arguments) or unprototyped function. Andy cooked up a fresh patch.

Zsban Ambrus noted a few discrepancies in perlutil.pod and provided a patch to bring reality in-line with the documentation (or possibly the other way around). Not (yet) applied.

Watching smoke signals

Smoke [5.8.8] 27170 FAIL(F) MSWin32 WinXP/.Net SP2 (x86/2 cpu)

One of Steve Hay's smoke machine starting emitting black smoke. He tried to track the problem down with Nicholas Clark but nothing conclusive was found. Finally things seemed to settle down after the machine was rebooted. Nicholas wondered if that step could be added automatically between make and make test.

Smoke [5.9.4] 27265 FAIL(F) MSWin32 WinXP/.Net SP2 (x86/2 cpu)

Steve Hay had another run produce copious gouts of black smoke, and leak lots of disgusting fluids on the floor. Nicholas wondered what he had done to merit that. Steve analysed the problem and pin-pointed change #27249 as the reason. Nicholas thought it was some kind of alignment problem.

Worse, for all the tests that failed, they ran just fine under the debugger. After recompiling everything with /Zp4, Steve noticed that the culprit was in fact #27248.

New and old bugs from RT

Unusual tell results with :crlf layer and multibyte input (#38587)

Alex Davies posted a short test script that demonstrated what he thought was a bug, but has apparently been Warnocked.

Slow Unicode regular expressions (#38595)

This bug report received a few comments this week. Sadahiro Tomoyuki explained what was eating all the cycles and offered a 4 character patch (&& i, if you're curious) to be applied somewhere in the bowels of mg.c.

Rafael was a bit worried by the implications of what Tomoyuki was saying and countered with one-line patch to utf8.c, and Tomoyuki came back with two ways in which Rafael's patch could be broken.

Nick Ing-Simmons consulted his store of Perl arcana and added another data point to the picture. In some places of the code, it's easier to indicate to the called function that it has to determine the length of a buffer itself, rather than determining the length in the calling function. Tcl and C++'s STL both use this trick.

But with all that said, I'm not that Unicode regexps are back up to speed.

Storable 2.15 freeze/thaw corrupts qr// on 5.8.8 (#38605)

David James noticed that if you take a qr// regexp, freeze and then thaw it, you get garbage, or at the very least, something that certainly won't match what the initial regexp did.

He noted that Regexp::Storable on CPAN fixes the problem, but until you have a problem, you may not realise that Regexp::Storable is the solution, and thus it would be better if that functionality were available directly in Storable. Yves Orton suggested that using Data::Dump::Streamer might be more successful (which I incorrectly named ...Dumper... instead of ...Dump... in last week's summary. Shame on me).

Data::Dumper dump core in 5.8.6, fixed by 5.8.7 (#38612)

Jarkko Hietaniemi noted that a shortish snippet of code used to cause Data::Dumper to dump core, is now fixed in 5.8.7. I think this latter point was sufficient for people to not bother to find out what fixed it.

  It works now

lc, uc and substr misbehave with UTF-8 (#38619)

"benizi" filed a remarkably horrible bug using simple substr(lc($var),0) constructs when _utf8_on (from Encode) was used. Andreas Koenig traced it back to a change (#18353) integrated by Jarkko in December 2002 to perform cache the differences between lengths and bytes (since UTF-8 is a method of storing variable-width characters -- some take 1 byte, some take 2 or more).

Functions like substr and index stand to benefit from this information, since it means they don't have to scan from the beginning of the string to find the offset that interests them, they can dip into the cache and find a much closer start position from which to start counting. (Summariser's note: at least that's the way I understand it. Corrections welcome if in fact I am propagating an incorrect meme).

And as the check-in comment from Jarkko said: "Code this hairy is bound to have hairy trolls hiding under it." It took more than three years to shake the troll out into the open. Sadahiro Tomoyuki, Unicode bug squasher extraordinaire immediately saw what was wrong, and proposed a patch that was gratefully applied (since it meant one less thing to tackle during his TPF grant) by Nicholas as change #27329.

The true semantics of /$pat/o (#38625)

Ulrich Windl want to know what happens with /$pat/o when $pat has two different values in two different lexical scopes. Dave Mitchell set him straight and Hugo van der Sanden filled in the missing blanks.

tied variables don't work with .= <> (#38631)

Nicholas discovered a bug with tied variables in 5.8: you cannot do $tied .= <FH>. Well, nothing is stopping you, but then again it won't work as expected. Nicholas reasoned that it due to a change in 5.8 using the rcatline op, and some sort of deficit of appropriate magic.

Andreas Koenig tracked the problem down to change #11634.

Brendan O'Dea forwarded a bug from the Debian tracking system. It turns out that something like *a= $a = *b; $a = 42; causes spectacular fireworks to occur, all the way to 5.005. Abigail discovered that it segfaults as far back as 5.000.

John E. Malmberg noted that this is now caught semi-gracefully by an assertion in blead. Nicholas started to understand what was going wrong as the summary went to press.

  Don't Do That Then

Perl5 Bug Summary

  1547 open items


In Brief

Help Michael tear himself away from his Worlds of Warcraft addiction, and make blead better at the same time. It's a win-win situation.

  Schwern must pay

Nicholas thought about merging hasargs and lval within the struct block_sub structure that takes care of the business of subroutine contexts. He then realised that savings would only kick in after nesting 50 or so calls, and decided it was not worth the effort

Nicholas lined up lvalue functions in the cross-hairs. There's an ugly bit of code in op.c to deal with returning temporaries from lvalue subroutines. If things were done differently, it would allow PVGVs to be laid out differently, which would in turn resolve a certain number of special cases in the code.

Joshua ben Jore wanted to know if there was a way of distinguishing between methods and function calls under the debugger. Nick Ing-Simmons said there was no difference between the two. On Perlmonks, Adrian Howard had noted that Devel::Caller used to be able to do this, but its test suite fails on perls more recent than 5.8.4.

Dave Mitchell made it back safe and sound from India. When asked by Nicholas whether he enjoyed himself, he replied that it was a bit of curate's egg.

Long time readers of World Wide Words may recall the meaning, but if not:

(WWW is a weekly mailing list devoted to, well, words. I recommend it highly).

Adam Kennedy started to wonder about the implications of the new constant subroutine optimisations in terms of mod_perl. Specifically, he wanted to know whether it was worthwhile to flesh them out forcibly in the parent process, to allow a greater sharing of VM pages between the kids. According to Nicholas, it would only be a problem with mod_perl children that eval or require a lot of code on the fly, which will kill page sharing in any event.

Seungho Han had a couple of questions about using the B:: modules, and Joshua ben Jore provided all the necessary information

Linda W spotted an inconsistency in the output of "-V" and wondered if it was a bug or a feature. No takers.

Nicholas Clark showed a short snippet of code involving typeglobs and some really weird behaviour you don't want to know about. It has come about as a result of fairly aggressive refactorings of the codebase aimed at reducing the amount of memory required to store a scalar. I think he also managed to explain why.

  Undesirable emergent C<typeglob> behaviour

Salvador Fandino discovered that the debugger chokes if you try to dump a tied glob that lacks a FILENO method, and patched it to handle things more gracefully. Applied by Rafael as change #27342.

John L. Allen was trying to get IO::Tty to build on AIX, and was having trouble with the constant TIOCSCTTY, which, it turns out, was being created in XS C code, rather than Perl, which may have something to do with the problem.

Anno Siegel added a couple of checks to the test suite to exercise the stringification of hash keys in t/op/hashassign.t.

About this summary

This summary was written by David Landgren.

Last fortnight's summary gathered a few replies, mainly about the ActiveState/NTFS hard links issue, and also Joshua Juran's work on Mac OS/Lamp.

Stephen McCamant also provided some useful information concerning the historical divergence of the code behind &= versus |= and ^=.

Information concerning bugs referenced in this summary (as #nnnnn) may be viewed at

Information concerning patches to maint or blead referenced in this summary (as #nnnnn) may be viewed at

If you want a bookmarklet approach to viewing bugs and change reports, there are a couple of bookmarklets that you might find useful on my page of Perl stuff:

Weekly summaries are published on and posted on a mailing list, (subscription: The archive is at Corrections and comments are welcome.

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