This Week on perl5-porters - 30 March-5 April 2008
The extent that map/grep go to to keep the calling overhead of the
block is horrendous and getting that to work for reduce in
"List::Util" was difficult. Doing it with multiple blocks is going to
be potentially very difficult. -- Graham Barr, not exaggerating how
hard it is to work on the parser and optree generator.
Topics of Interest
Dual-lifing "Pod::Html"
Steffen Müller gave David Landgren a commit bit last week to take over
the maintenance of "Pod::Html". After looking around the blead
directory tree, David wondered where the tests were. Jan Dubois
pointed out their hiding place.
mmm, hand-rolled test harnesses
http://xrl.us/bi956
Lack of 5.10.x smoking
Dave Mitchell looked through the smoke results from March and saw less
than half a dozen smokes for 5.10.1-tobe. This led him to ask if some
of the regular smokers could schedule a smoke or two on a more regular
frequency (especially after 5.8.9 is released).
Bram asked for some help on how to start smoking, such as what the
most desirable combinations are for smoking. One important point to
come out of the discussion was how useful "ccache" can be to cut down
smoking time.
chained smoking
http://ccache.samba.org/
http://xrl.us/bi958
Make built-in list functions continuous
Nicholas Clark noticed that one of the Google Summer of Code projects
was to improve the performance of built-in functions, by getting them
to skip the construction of intermediate lists. Nicholas wsa curious
as to what was meant by this, since it isn't part of the current TODO
list.
Wren Argetlahm replied that it is an optimisation known as
"deforestation" in Haskell parlance, and comes into play when you have
a series of chained maps or greps, and a pipeline of SVs between each
step. The answer to this is to use continuous functions, which is just
a fancy way of saying that they operate on input and output streams.
Wren offered some rewriting strategies that he thought would speed
things up. It all began to fall apart when Nicholas explained that
during compilation there was never at any point a usable abstract
syntax tree (or AST) that could be used as a basis for such
manipulations, since the tokeniser and lexer emit what is more or less
the final optree directly. Some additional obligatory fixups are then
performed on the tree, as well as some peep-hole optimisations, but
both of these operations are hopelessly intertwined. A distinct,
pluggable optimiser for Perl 5 remains an elusive dream.
It gets worse. Nicholas said it took him a full time week's worth of
work, just to create opcode optimisations for "reverse sort @pig_pen"
and "foreach (reverse @recusandae)". It took him a day or so to remove
the "srefgen" and "ex-list" ops from the creation of arrayrefs (like
"[1, 3, 7]") and hashrefs.
He wasn't sure how long it took for Dave Mitchell to teach the
optimiser to perform in-place sorts for "@schlip = sort @schlip", or
for Yves Orton to achieve a faster "if (%hash) {...}", but these are
the only known examples of optree optimisations in the past three
years. Dave admitted that it was "quite hard".
Dave explained that the naive approach of "look for a long string of
ops and replace them by a shorter string" are hard to do and very
fragile: they are either easily broken, or they break other things.
And Rafael chipped in to say that it is difficult to write regression
tests for them to boot.
Nicholas thought a better approach would be to get "B::Generate" and
co. into the state where one could write optree rewriters in perl Perl
and start to explore where the real wins lie. And it just so happens
that Steffen Müller has been playing around with "B" and "B::Utils" to
manipulate the optree and was beginning to make progress towards doing
just that.
The other alternative that Nicholas came up with was to investigate
Larry Wall's MAD work, which purportedly allows one to recover the
original source after compilation (although I believe no-one has
actually managed to achieve this in the general case).
deforestation
http://www.cse.unsw.edu.au/~dons/papers/CSL06.html
for de trees
http://xrl.us/bi96a
Expose "ptr-table" funcs, add "ptr-table-delete", and benchmark them
Jim Cromie wrote a patch to expose the underlying hashing mechanisms
used by the internals, so that XS code could use it directly. He
wasn't entirely convinced that it was wise to do so, but a factor of 5
speed-up was nothing to sneeze at. The fact that it might help
"Devel::Size" caught Tels's attention, but he wasn't sure he
understood what the patch offered.
the street finds its own use for things
http://xrl.us/bi96c
Stupid Transaction Idea?
Curtis "Ovid" Poe wanted to know if anyone had ever thought about
using forks or threads to create a poor man's transactional memory.
Robin Barker pointed to a talk made by Simon Wistow on the subject.
Mark-Jason Dominus made the connection between this question and a
thread from June 2006 regarding reversible debugging. This revived the
discussion about reversible debuggers and missile launches, until
Abigail dragged things back on track, pointing out that rolling back
transactions is a much simpler proposition than rolling the universe.
For instance, a "fire_missile()" appears really to fire a missile,
except that in reality it doesn't, not until the "commit()" is issued.
Paul Fenwick thought that if anyone was brave enough to pursue the
idea, they could do worse than use a "Safe" compartment to ensure that
no operations that could not be rolled back were performed.
Simon says
http://london.pm.org/lpw-2004/talks/simon_wistow-perl_voodoo.ppt
the p5p thread
http://xrl.us/bi96e
TodoTracker - get money for fixing TODO tests
Thomas Klausner and the Vienna.pm crew announced the grand opening of
their TODO bounty hunter scheme, whereby people who write patches to
solve TODO problems earn real money (that is, Euros).
What exactly is a TODO, and what it is worth is a work in progress,
and you can find out more about it on their wiki:
http://socialtext.useperl.at/woc/index.cgi?todo_test_bounties
make money fast
http://xrl.us/bi96g
Leopard has more standard "/etc/passwd" files than previous
Back in October 2007, Rafael Garcia-Suarez committed change #32200 to
resolve a problem on an older OS/X. In newer OS/X versions, a file
crucial to the test suite, "nidump", is not longer available, and thus
the test suite fails. Jan Dubois suggested that scraping the output of
"dscl" might do the job instead. Unfortunately he lacked the tuits to
do so.
Nicholas Clark said that Jan should refile it as a bug report so that
it isn't left behind.
http://xrl.us/bi96i
Unicode 5.1.0
The latest Unicode specification was released by UCD. Of particular
interest was the inclusion of uppercase Uppercase ß (eszet). Tels made
a cogent argument for the gradual disappearance of such characters:
they are really fiddly to text via SMS. In any event, Perl now does
5.1.0, which is going to simplify the task of people who wish to write
domino servers (the game, not the Lotus kind).
http://xrl.us/bi96k
TODO of the week
(here, this should be an easy one).
"perlmodlib.PL" rewrite
Currently perlmodlib.PL needs to be run from a source directory where
perl has been built, or some modules won't be found, and others will
be skipped. Make it run from a clean perl source tree (so it's
reproducible).
Patches of Interest
Double magic with "substr"
Vincent Pit had been sufficiently annoyed by magic in "substr" being
triggered twice, when once was enough, that he sat down and crafted an
elegant patch to fix it up. He had a couple of doubts about how to
deal with the API change.
Nicholas explained how to resolve that by having the old
implementation shuffle off to "mathoms.c", and writing a macro that
exposes the old name in terms of the new.
old functions never die
http://xrl.us/bi96n
they just mathom
http://xrl.us/bi96p
Double magic with '\&$x'
In his continuing quest to rid the core of twice-invoked magic,
Vincent also delivered a patch to fix up the magic associated with
"\&$x". He knew there was another possibility of magic being
triggered, but questioned the wisdom of invoking magic for something
as tedious as creating an error message.
a surfeit of magic
http://xrl.us/bi96r
Make "PL_AMG_names" and "PL_AMG_namelens" static
Jan Dubois noticed that a couple of new symbols were being exported
for 5.8.9-tobe. Since they really should be private, he made them
static in blead. Steve Hay applied the patch, and tweaked regen.pl to
get it to keep track of overload.c and overload.h.
Nicholas Clark thought that since 5.10 was out in the wild, it would
not be possible for to hide them, since someone might already have
discovered a way of using them, and thus removing their public
visibility would cause such code to break (or at least, become
unlinkable).
http://xrl.us/bi96t
perlfunc.pod: "atan2(0,0)" returns 0, not "undef"
Paul Fenwick noticed a small error in the documentation concerning
"atan2(0,0)", as the result of those arguments is undefined. Paul felt
that perl should return "undef", but in fact it returns 0.
Mark-Jason Dominus wondered if it would be better to have it throw an
exception, like the logarithm of a negative number, or dividing by
zero. Unfortunately that would be almost certain to break a lot of
code in the wild. Paul felt that a warning would be sufficient, since
people would be free to "use Fatal" and thus obtain an exception in
due form.
Rafael Garcia-Suarez invited interested parties to look at the "atan2"
manpage on FreeBSD, which put forward some reasons why returning 0 can
make sense.
Dave Mitchell then looked at the source and discovered that perl just
returns whatever the underlying C library does. Andy Dougherty
investigated further and determined that some platforms do indeed
return 0 (as dictated by the C89 standard) and some will also set
"errno" to EDOM.
Nicholas Clark was of the opinion that "CORE::atan2" should return 0,
and that leaves "POSIX::atan2" free to call the underlying library.
getting atan
http://xrl.us/bi96v
New and old bugs from RT
possible fd bug in "PerlIOStdio_close" (#46173)
Last last year, Steve Peters outlined a scenario where "dup"ing a file
descriptor during a "close" could cause a file descriptor to be
leaked.
Nicholas Clark admitted this week that since Nick Ing-Simmons's
passing, probably no-one understood how "PerlIO" works deep down. In
any event, he thought the code as it stood appeared to be sufficiently
wrong to merit a fix.
This it turn reminded Craig Berry to ask why "PerlIOUnix_open"
hard-wires the opened file to 0666 wide-open permissions, and wondered
why the code didn't honour the current "umask" setting. Dave Mitchell
explained that the kernel took care of that.
http://xrl.us/bi96x
"[[:print:]]" *versus* "\p{Print}" (#49302)
Given that no-one had been able to reconcile the differences between
these two syntaxes (for example, that the former fails to match some
things that the latter does), Robin Barker chose to document the
differences.
if you can't beat 'em
http://xrl.us/bi96z
"utf8::valid" rejects characters in "\x14_FFFF - \x1F_FFFF" (#51710)
Steve Peters wondered whether the patch included in bug #43294 would
fix this problem. Which it didn't, but that left him asking why
"\x14ffff" was considered to be a valid character.
Chris Hall thought that it was but "utf::valid" was also happy with
0x000000 through 0x13ffff and 0x150000 through 0x7fffffff, which left
him puzzled as to why "utf::valid" was singling out the "0x14xxxx"
range.
Chris wondered if the patch Steve was looking at was causing
"utf::valid" to reject both 'ill-formed' byte sequences as well as
'non-characters'. Either way, it seemed to be sitting on the fence and
not have a clear purpose.
After that I lost it a bit.
we need a unicode-porters list
http://xrl.us/bi963
http://xrl.us/bi965
Segfault in "B::SVOP::sv" (#52284)
"Inferno" filed a bug which actually works correctly on a threaded
perl, only non-threaded perls have problems. Reini Urban thought that
the best solution was for "B::Size" to die a quick, painless death,
and to use "Devel::Size" instead, as it is so much nicer.
bug in march, answer in april
http://xrl.us/bi967
http://xrl.us/bi969
Attempt to free temp prematurely (perl 5.8.8) (#52386)
Frank v Waveren reported a bug in 5.8.8 that Nicholas Clark determined
had been fixed in 5.8.9 to be, although he didn't know off-hand what
change was responsible for the fix. Frank tracked it down via the git
repository, and identified change #30166 as being the fix.
http://xrl.us/bi97b
"lc"/"uc" have unexpected side effects inside for loop (#52412)
Mike Wver discovered that the following snippet
my $foo = 'A';
for my $bar (uc($foo)) {
my $lower_bar = lc $bar;
print "$foo $bar\n"; # $bar should still be 'A'
}
prints "A a" instead of "A A". No-one knew why, but Abigail pointed
out that it was fixed in 5.10.0.
http://xrl.us/bi97d
"map" isn't context aware in some cases (#52452)
Stefan Wehinger wondered why slightly different nested map constructs
use some, a lot, or all available memory. David Nicol made a decent
stab at explaining it in terms of lists being reclaimed sufficiently
early or not.
Nicholas Clark suggested that the desired behaviour described in the
report can be achieved, along with a sane level of memory consumption,
by rewriting the loops with "foreach" instead of "map".
http://xrl.us/bi97f
Perl5 Bug Summary
1807 (+7 -3)
http://xrl.us/bi97h
http://rt.perl.org/rt3/NoAuth/perl5/Overview.html
New Core Modules
Math::BigInt 1.88
Tels announced the release of a brand new Math::BigInt, along with
an updated "bignum" pragma, "Math::BigInt::FastCalc" and
"Math::BigRat". This release closes out nearly all the existing
bugs, only two remain, at the bottom of the barrel. In the
meantime, Tels is sitting back and waiting to see what the CPAN
Testers make of them.
http://xrl.us/bi97j
In Brief
Tels wondered if Reini Urban had had time to check out his patch for
"Devel::Size" and bleadperl, but Reini was moving house this week.
http://xrl.us/bi97m
Robin Barker's verbosity tweaks to regen.pl and friends made it in.
http://xrl.us/bi97o
Jan Dubois felt that "PL_bincompat_opt" should be exported on AIX and
Windows. Steve Hay thought so too, but realised that Jan was really
talking about "PL_bincompat_options". Applied.
http://xrl.us/bi97q
Jarkko Hietaniemi got H.Merijn Brand to tweak Configure in order to
align floating point policies of gcc and cc on Tru64.
http://xrl.us/bi97s
Jan Dubois thought that change #23984 should be integrated into 5.8.x,
as it gets "corelist" installed on Win32. Nicholas Clark said that it
was already in, the reason being that it help "perlbug" go about its
business.
http://xrl.us/bi97u
Andreas König warned that lib/CGI/t/upload_post_text.txt was checked
in as binary and wanted to know if it be changed. Rafael said that it
was binary for a reason; it was in fact a GIF file.
and patent-free
http://xrl.us/bi97w
Jerry D. Hedden ran into trouble with the above file, and Nicholas
Clark straightened things out.
all packed up
http://xrl.us/bi97y
Paul Fenwick issued an RFC for "Fatal"/"autodie" exception handling
naming and structures.
http://xrl.us/bi972
Last week's summary
Tels clarified a point regarding the use of POD for wiki markup,
explaining that his MediaWiki-Pod distribution on CPAN was a subclass
of "Pod::Simple::HTML" that fixes up a lot of the problems that people
encounter when using "Pod::Simple::HTML".
This Week on perl5-porters - 23-29 March 2008
http://xrl.us/bi974
About this summary
This summary was written by David Landgren.
Weekly summaries are published on http://use.perl.org/ and posted on a
mailing list, (subscription: [EMAIL PROTECTED]). The
archive is at http://dev.perl.org/perl5/list-summaries/. Corrections
and comments are welcome.
If you found this summary useful, please consider contributing to the
Perl Foundation or attending a YAPC to help support the development of
Perl.
