This Week on p5p 2000/10/08

Oct 8, 2000 by Simon Cozens

• Notes
• Self-tying is broken
• Virtual values
• Why is unshift slow?
• More Perl hacking guidelines
• Integer and floating-point handling
• printf %v format bug fixed
• Jarkko impersonates me
• Various

Notes

First, the meta-news this week is that you have a new author. I’ve taken over producing the perl5-porters summary from Mark-Jason Dominus, so from now on, please send your corrections and additions to simon@brecon.co.uk.

This week was relatively quiet, with just a couple of hundred mails.

Self-tying is broken

Alan Burlison noted that you can’t create a self-tie anymore without Perl going into nasty recursion. What’s a self-tie? As you may guess, tieing a variable to itself goes like this:

    package MyTie;
    sub TIEARRAY { bless $_[1], $_[0] }

    package main;
    my (@self);
    tie(@self, "MyTie", \@self);

The problem comes with how tying is implemented. Perl uses a structure called “magic” attached to each tied variable to contain subs like FETCH. When you fetch a variable’s value, Perl checks to see whether there’s a FETCH sub stored in the variable’s magic. If there is, Perl calls it to determine the variable’s value. If this sub tries to look at the variable itself, Perl then calls the FETCH sub stored in the variable’s magic to try and get its value, which, in turn, tries to look at the variable, and things end up in a mess. Alan then investigatedand tentatively blamed Sarathy, but this turns out to have been a bad call; the problem’s much deeper than that and nobody could think of a way of fixing it without breaking nested ties or threading. Part of the problem appears to be that nobody really knows what self-ties are supposed to do anyway. No patch was forthcoming, and the discussion fizzled out.

Elsewhere, Daniel Chetlin fixed some bugs with tied filehandles. He makes some other interesting points about what happened while he was working on it. Read about it.

Virtual values

Jarkko came up with a new idea for copy-on-write sharing of scalars, but said that it may have to wait until Perl 6 before anyone implements it. Ilya popped up and characteristically said that he’d already done it.

Why is unshift slow?

Ben Tilly started a discussion on why unshift is so slow,which didn’t get much attention but contains a lot of juicy goodies for people thinking about how to make array handling faster. No patches or benchmarks came out of the discussion, but there’s an opportunity for someone to try some of the ideas there and see whether there’s any improvement.

More Perl hacking guidelines

Mark Fisher contributed a helpful patch for perlhack on using Purify to debug Perl, prompting people to promise to explain how to use their favourite code checkers. None of the other patches has materialized, and the discussion moved on to the fine print of PERL_DESTRUCT_LEVEL.

Integer and floating-point handling.

Currently, putting together two variables which contain integers is actually a floating-point operation, meaning that variables have to get upgraded to hold floating-point values when they don’t necessarily need to. It was hard to see a good way around this, but Sarathy came up with a suggestion. Someone with a few hours to spare should consider applying Sarathy’s idea and coming back with some benchmarks.

In other floating-point news, Nick Clark found some 64-bit bugs to do with the conversion between UVs and NVs. There was some discussion, but no patch.

printf %v format bug fixed

[This section of the report was written by Dominus.]

In an earlier report, I mentioned a bug in the %v format specifier for printf and suggested that this was a good candidate for fixing by someone who was just starting out fixing Perl bugs. Avi F. stepped up to the challenge and provided a patch, which was accepted. Unfortunately, Avi did this right after I stopped doing p5p summaries, so never received proper credit. Thank you very much, Avi!

Read about it.

[We now return you to Simon Cozens.]

Jarkko impersonates me

I noticed a bug in split which caused Unicode values to be denatured; Jarkko’s worsening influenza produced a delirium that caused him to not only babble to himself but also to impersonate me.

But at least he fixed the bug. If I hack Unicode too long, will this happen to me too? Find out next week!

Various

The usual collection of bug reports, test results, bug fixes, test results, esoterica and more test results. The clown who was sending duplicate copies of CCs to people has been ejected from the premises.

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

Simon Cozens