This Week on p5p 2001/04/08

Apr 8, 2001 by Simon Cozens

Notes

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Apologies for the delay in this week’s summary; I was travelling this weekend, and didn’t get back until Tuesday.

Perl 5.6.1 and Perl 5.7.1 Released!

The big news this week is that, as you probably already know by now, Perl 5.6.1 and Perl 5.7.1 are available. Get them from CPAN. Well done to Sarathy and Jarkko for getting these out of the door.

Robin Houston Left On ALL WEEK

Someone has just far too much free time on their hands. Robin Houston started the week by adding a warning to

    push @array;

This was slightly controversial, as it might introduce new warnings to old code, (albeit old, broken code) and some people - myself included - wanted to see push default to $_ instead. That really would change the semantics of old code, however. Mark-Jason Dominus wondered whether anyone could write push @array by accident, or even have a program write it legimitately, but both Ronald and Jarkko confessed to have written it when they meant push @array, $_.

Then he moved over to hacking on B::Concise; he found an interesting optimisation where you have code like

@a = split(/pat/, str);

Perl actually places the symbol table entry for @a inside the opcode for the regular expression, casting it into an OP as it goes. B::Concise wasn’t aware of this peculiarity and thought that the op was, well, an op. Boom.

Next, he fixed up something with lexicals; the way lexicals are stored is quite complex, as he explains:

perl stores the names of lexical variables in SV structures. But of course it doesn’t stop there. Oh no! Not only are the IVX and NVX slots used to hold scope information, but SvCUR is used for some nefarious purpose which I don’t understand. (look in op.c/Perl_newASSIGNOP for (at least some of) this SvCUR chicanery. It seems to be connected with the mysterious variable PL_generation.)

B::Concise is making naive assumptions about perl’s sanity again. Try this, for example:

perl -MO=Concise -e ‘my @x; @x=2;’ |less

The variable name (“@x”) is followed by all kinds of weird binary data, because B::Concise believed perl when it gave the length of the string, not suspecting for a moment that the slot which ordinarily holds the length happens to be used for devil-worship in this situation.

This is, as Stephen McCamant explained, related to a clever optimization to allow both

    ($a, $b) = ($c, $d);

and

    ($a, $b) = ($b, $a);

to both work - in the second case, Perl needs to use a temporary variable to swap the values over. It determines whether or not the left-hand and right-hand sides have elements in common by “marking” each element, and that’s what the devil-worship was for.

Robin realised that this meant that variables with very long names used in a list assignment would be truncated, so he fixed them up as well.

By now, however, Robin is unstoppable. He made B::Deparse give sensible output for variables which start with control characters such as $^WIDE_SYSTEM_CALLS. Realising this was a common problem, he made a B::GV::SAFENAME method to ensure the name was printable and converted all the B::* modules to use that. Good work!

But no, it doesn’t end here. He fixed up B::* to cope with IVs that were actually UVs, B::Deparse handling of regular expressions, "${foo}bar" and binmode, and made it warning-clean.

By the end of the week, he was pleased to report that you can now run t/base/lex.t through B::Deparse and back through Perl, and all tests pass. Wow.

More on glob

Benjamin Sugars tried to speed up glob. As you may or may not now, core’s glob automagically loads up File::Glob and uses the glob function that that provides. Unfortunately, this also pulls in Exporter and all sorts of other modules. Benjamin rewrote File::Glob to avoid the compile-time dependencies on these modules, and fiddled the bit in the core (after a few false starts) which loads up the module to make it equivalent to use File::Glob (). He also documented load_module, which is the core function for magical module using.

Changes files in core

John Allen asked “does anyone else think it may be time to grant the voluminous ChangesX.XXX files in the standard perl distribution their own separately downloadable gzip file?” Well, as usual, some did and some didn’t. The motivation for not doing so would be that it would not be necessary to download anything other than the Perl source tarball to understand the sources. Further, Jarkko pointed out that the Changes files describe patches that affect more than one file at once and wouldn’t be sensibly documented in any single one of them.

Benjamin Sugars suggested moving them to a separate directory rather than getting rid of them altogether; Jarkko hinted that he wasn’t going to go through the Perforce hell of moving files just for the sake of moving them.

John’s eventual compromise was to keep them in the development sources for those people who want to hack on them, and remove them from the stable sources. This seems to suit everyone, but nobody said anything further.

How Magic and Ties Work

I provided a sample few sections from my forthcoming Perl Internals training course about how magic and ties work. Enjoy.

Various

Olaf Flebbe turned in EPOC fixes before 5.6.1 went out, and Craig Berry updated README.vms while Peter Prymmer updated all sorts of other VMS things; Gisle came up with a neat patch to let

    @foo = do { ... };

propagate array context to the do block.

Paul Schinder came up with some OS X fixes for 5.6.1; apparently OS X’s gcc isn’t very gcc, and this caused the preprocessor to do weird things which broke [Errno].

John Peacock fixed up some bugs in Math::BigFloat found by Tom Roche but the only thanks he got was a discussion of the indentation style of the Perl sources. 8-wide tabs, 4 spaces, people. ( Russ says we should use spaces instead of tabs, but it’s a bit late now.)

Jonathan Stowe tried to change something about $#; Sarathy suggested that this might break some ten year old code, even though the bug’s been in there since forever…

The TIESLICE/ STORESLICE issue came back. The discussion produced more light than heat.

And finally… Dan Brian explained what happens if you cross Black and White with Perl5-Porters.

Until next week I remain, your humble and obedient servant,

Simon Cozens - Notes - Perl 5.6.1 and Perl 5.7.1 Released! - Robin Houston Left On ALL WEEK - More on glob - Changes files in core - How Magic and Ties Work - Various