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

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

Dave Mitchell converts the regular expression engine from recursive to iterative.

Topics of Interest

More on Module::Build on VMS

Ken Williams got back to Craig A. Berry's patch from last week for Module::Build on VMS, and implemented a new approach to deal with backtick captures. John E. Malmberg and Craig batted it around for a while until it looked ready. John wrapped it up as a new version of ExtUtils::CBuilder. John noted that there might be issues with older VMS versions that limit command lines to 255 characters, but decided to punt the issue for the time being.

  Looking good

Building a threads-friendly debugger.

Dean Arnold wrote to say that he was in the process of hacking ptkdb to make it easier to deal with debugging multi-threaded programs. He had reached the point where it seemed that the most promising way forward was to change the $DB::single variable to be globally shared across all the threads.

After the usual admonishments ("You're mad!", "No-one who has ventured there has ever come back alive!"), Dave Mitchell said that he thought that it couldn't do much harm, except that it was likely to bring about a significant loss in performance, as the threads fought amongst themselves to acquire a lock on $DB::single to read it.

Dean ran a couple of benchmarks and saw that Dave was right, the resulting performance curve was pretty atrocious (about two orders of magnitude).

  Where hackers fear to tread

Dynamic libraries on AIX 5.1

Last time we heard from John L. Allen, he had been busy doing battle with 32/64 bit builds with Oracle on AIX. This week he was having trouble with Math::Pari, and he and Ilya Zakharevich, Math::Pari's author, were stuck.

The problem revolved around which libraries were being linked, which meant that the wrong version of the C language pow function being used. John wanted to understand what was happening and why. H.Merijn Brand guided him through the twisty mazes of AIX linker techniques.

By the end of the thread John had managed to concoct a method for making it work, and H.Merijn made a plea for an AIX maven to step in and take over (and revise) the README.aix file.

  Fear and loathing

New Time::Local failure

Rafael Garcia-Suarez attempted to upgrade blead with Time::Local version 1.12, and saw that the test suite failed. Steve Hay recalled that this was the result of a bug that he had encountered in LWP's test suite. Gisle Aas isolated the problem with Time::Local, and Dave Mitchell came up with the patch.

Steve wondered whether that patch should be applied only to the Win32 platform. Dave Rolsky, author of the module, responded saying that there were some problems with integer overflow that gets triggered only in certain time zones. He said that it was all a bit of a mess, but that he was going to get it sorted out and release 1.13.

  It's about time

Revamped UTF-8 caching code

Nicholas Clark checked in some code to rework how UTF-8 caching is performed.

First, some background: finding the offset of an arbitrary character in a UTF-8 string can be a difficult proposition, depending on the number of wide characters encountered in the string. The brute force method consists of starting from the beginning, and then counting characters until the desired offset is reached. Depending on the length of the string, this can be very time-consuming.

To lessen this cost, perl maintains a cache of where wide characters appear in a string, to minimise the amount of linear scanning required. A few weeks ago, a bug report revealed that there were some problems with the existing cache management code.

So Nicholas reworked it a fair bit, adding a ${^UTF8CACHE} variable to allow the caching code to be enabled and disabled at will, as well as a PERL_UTF8_CACHE_ASSERT build-time switch to force extra checking (verifying that the cached and uncached results agree). He also discovered that the code wasn't taking full benefit of the gathered information, and tweaked the code to minimise the amount of linear scanning required.

  And accessible from the command-line too

  see also

The regexp engine no longer uses recursion

Dave Mitchell announced that he had reworked the regular expression engine to use an iterative technique rather than recursive. He achieved this feat by making S_regmatch() save its match context on the heap and restart the main loop, rather than on the stack by calling itself.

Dave measured that the heap allocation induced a 3% slowdown, but that this should be avoided by switching to an arena-based allocation scheme or similar, further down the track.

Before you ask, yes, /(??{$re}/) still causes recursion. And Hugo van der Sanden thinks undoing that would be hard.

  No more nasty stack overflow bugs

Patches of Interest

Upgrading to threads version 1.12

Jerry Hedden had delivered a patch to sync blead with CPAN. Dave Mitchell declined the patch, saying that a patch must never mix functionality and whitespace formatting changes. If the whitespace is to be changed (and in general the rule is: never), then that should be delivered in a separate patch.

Dave also thought that the approach was back to front. The changes should be applied to blead first, and then after the changes have had time to settle, the blead version can be released to CPAN.

Jan Dubois agreed that he too would prefer it this way around, since each change is tracked in Perforce, the perl5-changes mailing list gets to hear about it, and e-mail Message-IDs from the latter list make it easier to cross-reference the changes with traffic on perl5-porters.

Jerry also asked about the definition of THREAD_RET_TYPE, in the process of coming to grips with the threads code base but received no answers.

and finally got a patch accepted to sync blead with CPAN.

Serialising closures via Storable

David Wheeler wanted to know whether Storable could be used to dump out a closure, bring it back again, and have it work. For instance, to be able to say

  my $var = 1;
  my $code = sub { $var };
  print $code->();
  $code = thaw(freeze($code));
  print $code->();

And have it print out "1" twice, rather than once and a warning about uninitialised values in print. Yuval Kogman explained how it was more or less possible, and the pitfalls one would encounter if one were brave enough to insist on the approach.

Yves Orton, author of Data::Dump::Streamer. showed how using that module could probably provide something closer to what David was after. Joshua realised that one only had to teach Storable to use DDS instead of B::Deparse and it would Just Work.

Rafael noted that Storable is in the core, but DDS is not, although it should be possible to teach Storable to use it if it were available locally.

Watching the smoke signals

Compress/IO/Zlib/t/050interop-gzip.t failure on OpenBSD

Steve Peters tracked down the smoke failures occurring on OpenBSD. It turns out that OpenBSD's gzip behaves differently when gzipping a zero-byte file:

  # Cygwin, FreeBSD, Linux, NetBSD, Solaris, ...
  touch /tmp/foo; gzip -c /tmp/foo > /tmp/foo.gz; echo $?
  # OpenBSD
  touch /tmp/foo; gzip -c /tmp/foo > /tmp/foo.gz; echo $?

Paul took that into account, but wondered all the same why the smoke results mentioned "Inconsistent test results (between TEST and harness)", when one should expect that both TEST and harness should fail in exactly the same way.

Steve had a hunch that the problem on OpenBSD arose when the file to be compressed is less than 10 bytes long. Which seems odd, to say the least. Joshua ben Jore mentioned that he had seen similar problems on a Ubuntu Linux but hadn't been paying close attention. He promised to go back and look more closely to see if it was the same error, or something else again.

  One more reason...

  And the patch to fix it

Smoke [5.9.4] 27593 FAIL(F) MSWin32 WinXP/.Net SP2 (x86/2 cpu)

Steve Hay had a Windows build fail due to a problem with ExtUtils::MakeMaker (that Rafael had recently integrated), and asked Michael to integrate the patch he made into the EU::MM repository.

  Earth to Schwern, do you read me?

New and old bugs from RT

print (...) interpreted as function occasionally (#4346)

Many moons ago, Abigail reported that the message "print (...) interpreted as function" appears inconsistently, depending on a peculiar combination of closing braces, whitespace and/or semicolons. Steve Peters said that say has picked up a similar habit.

  The more things change...

More on overloading and reblessing (#34925)

The thread about overloading and reblessing objects continued this week. Nicholas Clark proposed a solution to scan all the references to an object and fix them up. Yitzchak Scott-Thoennes pointed out that such an approach would break the following code:

  $a = $b = {};
  bless $b, OverloadedClass;
  # $a is not overloaded here

Yitzchak admitted that such a construct would probably be quite rare, and wondered whether it wouldn't be better simply to document the fact that the initial example doesn't work, with suggested work-arounds. Nicholas implemented the scan approach in maint as change #27512.

B::Lint chokes on simple script (#38771)

Bart Lateur filed a bug report against B::Lint (on perl 5.8.7). The interesting thing is that the program in question was

  print for 1 .. 10

Joshua ben Jore, who has recently put a fair amount of work into the B:: namespace observed that the problem has been fixed in blead, but that it probably still exists in 5.8.8.

NaNs on Win32 (#38779)

Rob a.k.a Sisyphus posted a bug report concerning NaNs (Not a Number) on Win32. It seems that there is a compiler issue, which is that code compiled with VC7 is correct, but VC6 is not.

Dominic Dunlop noted sadly that the best way to fix this bug would be to add a note to the README.win32 documentation to say that perl should not be built with VC6. There's an article on the MSDN site that goes into more detail about floating point comparison issues.

Yves Orton thought that that was hardly ideal, since VC6 has always been the standard compiler that ActiveState uses for their builds. Except that Dominic was talking about Microsoft's freely downloadable compiler, which, is apparently a slightly different beast.

Jan Dubois came up with the best patch, one that works around compilers that have brain-damaged NaN comparison routines. Looking more closely at the code, Jan realised that perl's handing NaN handling is somewhat uneven. grok_number() will set the IS_NUMBER_NAN and IS_NUMBER_INFINITY bits as appropriate, but sv_2nv() doesn't bother to check them; it ducks the issue and lets atof() deal with it. He also saw that the cmp.t test that tests how <=> deals with NaNs is probably not doing anything meaningful.

In a thread-split elsewhere on the same topic, Jan provided keen insight into the subject of C run-time libraries on Windows.

Constants with undef value deliver arbitrary value at first call (#38783)

Markus Herber posted a bug report dealing with the XS code of IO-Tty that creates constant subroutine with undef as a value. Nicholas Clark understood what was going wrong and promptly supplied a patch which solved the problem. The patch is a bit of a stop-gap measure, but it will do for now.

Deep hash of hashes breaks garbage collector (#38786)

Reto Stamm uncovered a lovely bug in the garbage collector. He posted a program (paraphrased for succinctness here):

  my $root = {};
  my $h = $root;
  $h->{kid} = {} and $h = $h->{kid} for 1..250000

This runs just fine, until the program exits, the garbage collector is run, the garbage collector exhausts the C stack due to recursion and the program goes belly up with a segmentation fault.

chromatic thought that simply rewriting S_hfreeentries, Perl_hv_undef, Perl_sv_clear, Perl_sv_free2, and Perl_hv_free_ent for good measure to use iteration instead of recursion would probably do the trick.

  *crickets chirping*

Fatal doesn't like readdir() (#38790)

Tom Hukins filed a report that showed that readdir breaks when Fatal is used. (Fatal upgrades warnings to to fatal errors).

The trouble is that Fatal gets mixed up between scalar and list context (doesn't everyone?) and throws all the results away. Rafael thought that a judiciously placed wantarray would solve that, but that in turn would alter the behaviour of something as admittedly bizarre as

  my @useless = open my $fh, 'does.not.exist';

Yitzchak suggested hunting down the exceptions (select also seemed to be a likely candidate) and document their limitations in conjunction with Fatal. Joshua thought that this was less than ideal. If someone was going to go to the effort of hunting down all of weird special-context builtins to document them (and there aren't a whole lot), it would take about as much effort to code Fatal to make it do The Right Thing all the time.

Rafael agreed, and kept looking at his inbox for the patch. Joshua mumbled something about some patches to B::Lint he was working on, and promised to do something about this first.

Joshua went looking at Fatal, and stumbled across some AUTOLOAD code, and wondered if and how it was used. Mark Jason Dominus suggested that its purpose was to allow the construct

  use Fatal;

to work in the same manner as

  use Fatal 'open';

Which is either pretty slick, or pretty sick.

  Nice to know

Perl5 Bug Summary

  1560 open tickets

  Right here

In Brief

Dave Mitchell reminded us that our variables and package variables are compiled to the same code internally and as such have identical performance characteristics.

Philip M. Gollucci reported a bug that manifests itself using mod_perl on FreeBSD. Apparently another one of those "this is the second time it's broken" bugs. Robin Barker and Gisle Aas committed a couple of patches, including adding a check in the test suite, so hopefully we won't see the likes of it again.

  Perl_croak and nullch

Jim Cromie reported that bleadperl was uncompilable, due to problems with Dynaloader failing. Rafael traced it to the fact that he was integrating CPAN's ExtUtils::MakeMaker 6.30_01 into blead, and its handling of MAN3PODS was broken. So he fixed that, and bleadperl started compiling again.

  Safe to go back in the water

Dan Kogai found an anomaly whilst playing with YAML::Syck and developed an detailed hypothesis as to what was going wrong. As of summary publishing time, no comments had been made.

  How to mangle the SvTYPEs on arrays and hashes

Someone asked how to use Perl to run Visual Basic code and was directed to Perlmonks.

About this summary

This summary was written by David Landgren.

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

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