This Week on perl5-porters (11-20 November 2005)

This Week on perl5-porters - 11-20 November 2005

   The crackdown. A number of bugs were fixed this week, possibly
   obsolete code was scrutinised, Hugo patched the regexp engine and Perl
   on Windows was improved.

"h2ph" finds "gcc"'s include directory

   Brendan O'Dea posted a patch to use the "gcc" command line switch
   "--print-libgcc-file-name" to locate the "gcc" include directory. The
   previous technique of parsing "gcc -v" no longer works on version 4.

   H.Merijn Brand wanted to know when "--print-libgcc-file-name" was
   introduced (otherwise things would start failing for older gcc
   compilers). Brendan said that he had reached as far back as 2.95, and
   the switch was supported. Abe Timmerman chipped in with a justifiably
   ancient 2.7.2.2 gcc compiler that also supported the switch.

   Jason Vas Dias pointed to bug #37582 that he had filed on the issue,
   and that the patch had been applied by Steve Peters as change #26074.

     Brendan's patch
     http://xrl.us/iiau

     Jason's patch
     https://rt.perl.org/rt3/Ticket/Display.html?id=37582

Eliminating discrete arenaroots

   Jim Cromie issued a reworked, tested on threaded and unthreaded, patch
   to remove the fifteen different type-specific arenaroots, and asked
   people to have a smoke. Nicholas Clark went over the patch very
   carefully and made a number of suggestions, and Jim proposed a
   revision of the patch, which Nicholas applied as #26139.

   After looking at Nicholas's reworking of the patch, Jim added a number
   of corrections to the comments in the code, and then posted a series
   of benchmark results showing that the patch should have made things go
   faster (with the usual proviso about statistics).

   Later in the week, Jim came back with another patch that gathered the
   arenas into a linked list. This has some upsides and some downsides,
   but notably, it allows the possibility of exposing the arena to allow
   performance tuning.

     The elimination
     http://xrl.us/iiav

     Benchmark results
     http://xrl.us/iiaw

     RFC - linked list of arenas
     http://xrl.us/iiax

A better warning in maint

   Yitzchak Scott-Thoennes asked whether change #22807, which makes "$^O
   !~ /xxx/" emit the warning "Useless use of negative pattern binding
   (!~ in void context)" instead of "Useless use of not in void context"
   should be applied to maint. Nicholas yelped in pain and whispered
   something about perl 5.10. Yitzchak sympathised, and went through his
   own blead patches, evaluating which ones were worthy (safe enough) to
   be applied to maint.

     The question
     http://xrl.us/iiay

     Yitzchak's list
     http://xrl.us/iiaz

"h2xs" and backwards compatibility

   Steven Schubiger voiced his disquiet over the way h2xs ties the Perl
   version of the module to the module of perl installed on the author's
   machine (even when it might run perfectly well on any number of prior
   versions).

   Rafael Garcia-Suarez agreed that the orginal design decisions was
   probably incorrect and Nicholas remembered a previous thread on the
   matter, which Steven tracked down. Sébastien Aperghis-Tramoni was
   annoyed by modules that failed to install on 5.8.5 because they
   required 5.8.7, and wondered if it wouldn't be better to focus on the
   branch version (that is, 5.8), rather than relying on the subversion
   (the x in 5.8.x), and offered a patch. And also pointed out that
   "XSLoader" is backwards-compatible as far back as "5.005_04".

   Rafael declined to pick up the patch, pointing out that there is no
   way of ensuring that code that runs on 5.8.x will run on 5.8.x-1 and
   lower.

   This latter point prompted David Nicol to ask whether any
   documentation existed relating feature to version, and when the
   feature moved from absent, to experimental, to robust.

     Doubts over h2xs
     http://xrl.us/iia2

     2001, a Perl odyssey
     http://xrl.us/iia3

Abigail and the regexp engine

   Abigail posted some code where it was expected that "'A B' =~
   /^(.)\s+.$(?(1))/" should match. In 5.005 it used to work, but from
   5.6.0 and beyond it fails.

   Andreas König tried to track down the patch that caused the change in
   behaviour, but "gcc" got in the way. In fact, depending on whether he
   used version 3.2.3 or 4.0.3 the pattern matched, or didn't,
   respectively.

   Hugo van der Sanden burnt a precious tuit or two and rummaged around
   in the regexp engine internals and made a one-line change that fixed
   the problem, but said that he didn't have time to study the
   ramifications implied by the patch and noted that ""intuit_start" is
   probably the least maintainable piece of code in perl".

     Now why would you want to do a thing like that?
     http://xrl.us/iia4

Tracking down a Win32 smoke failure

   One of Steve Hay's boxes phoned home with a smoke failure. After first
   ignoring it and hoping it would just go away of its own accord, Steve
   had a closer look and noticed that it was a real bug, and only
   occurred when "PERL_IMPLICIT_SYS" was defined. Unfortunately, this
   particular "#define" is needed by ActivePerl, and Steve tracked the
   change in behaviour down to patch #26108. A three line change to tweak
   stacks in "BEGIN" blocks.

   Nicholas shrunk the problem down to a neat

     #!./perl -w

     BEGIN {
       $SIG{INT} = sub {exit(0)};
       kill 'INT', $$;
     }

   which causes a little known routine named "S_unwind_handler_stack" to
   blow up. After a certain amount of detective work, he was able to show
   quite clearly *why* the code was failing, but was at a loss as to
   *how* to fix it.

   Steve Hay pointed to a thread from last year where Jan Dubois posted a
   patch that seemed promising at the time, but for whatever reason was
   never polished and applied to blead.

   Jan spoke up and offered a bit more detail about how the patch worked,
   and what it needed in order to be complete. The next day he came back
   with a much better version. Stever Peters applied it and all the tests
   passed. Yay Jan!

     Smoke issues forth
     http://xrl.us/iia5

     The offending patch
     http://public.activestate.com/cgi-bin/perlbrowse?patch=26108

     "free to wrong pool", circa January 2004
     http://xrl.us/iia6

Is "STRANGE_MALLOC" still useful?

   Nicholas wondered about the different code paths taken depending on
   whether "STRANGE_MALLOC" is defined. He wondered whether it would be
   safe to assume that "malloc"/"realloc"/"free" work correctly these
   days, and that by chainsawing it out we would escape some
   no-longer-needed complexity?

   Andy Dougherty supplied a valuable history lesson, explaining why
   things came to be the way they are. And concurring that at this point
   in time, it could possibly get the axe, but it would be important to
   try out a couple of big applications to see how they fare.

     Compare the simple code path to the complex code path
     http://xrl.us/iia7

     Sarathy's 1995 summary of the problem
     http://xrl.us/iia8

When errors are reported on the wrong line

   David Hillman crafted a detailed bug report concerning errors being
   reported on the wrong line (which of course tends to drive people
   insane when they try debugging it).

   Dave Mitchell, noting the relative frequency with which these sorts of
   bug reports tend to appear, wondering what the cost benefit ratio
   would be of using up more memory per op in order to keep accurate
   track of the current line.

   Jim Cromie came up with what he termed a rather ugly solution to the
   matter. Then again, beauty is in the eye of the beholder. Dave
   Mitchell followed up with some preliminary numbers on the memory cost.
   The concensus is that a certain amount of memory could be traded for
   the peace of mind it would bring.

     The bug report
     http://xrl.us/iia9

     An ugly fix
     http://xrl.us/iiba

Sun Studio compiler for Linux

   Steve Peters tweaked hints/linux.sh to get the Sun Studio compilers
   for Linux up and running. The results are promising, although the
   patch, in Steve's own opinion, needs work.

   He also used it to take a look at Jarkko Hietaniemi's "valgrind"
   finding from last month.

     The tweak
     http://xrl.us/iibb

     Problems with sprintf()
     http://xrl.us/iibc

     Jarkko's valgrind discovery last month
     http://xrl.us/iibd

Perl 5 Bug Summary

   The current number of open issues stands at 1508.

     The bugs at the beginning of the week
     http://xrl.us/iibe

     The Bug Summary
     http://rt.perl.org/rt3/NoAuth/perl5/Overview.html

In brief

   "abs2rel("/foo/bar", "/foo/bar")" should return '.' or something else
   sensible, according to Ken Williams (and not an empty string). After
   asking around if anything other than "File::Spec" relied on the
   behaviour, Ken went ahead and fixed the problem in "Path::Class".

     http://xrl.us/iibf

   "local" and "thread::shared" don't mix. "dean" filed bug #37671
   showing how localising variables bound to shared variables can cause
   the shared variable to become undefined. At least in version 5.8.4.

     http://xrl.us/iibg

   "Compress::Zlib" should now build nicely on Win32 environments, now
   that Paul Marquess has fixed a lurking "binmode" bug.

     http://xrl.us/iibh

   Recent "const"ing changes to the codebase had broken "perlbug"'s
   ability to grok patchlevel.h. Yitzchak filed a patch to make it work
   again and threw some compatibility future-proofing into the bargain.

     http://xrl.us/iibi

   Dynamically appending code to an existing file. Jos I. Boumans wants
   to do this from time to time, and showed several techniques for
   implementing it, however, only two methods work. Looking at
   "pp_require" in pp_ctl.c, it looks as if the code is going straight to
   the file, rather than going through a higher layer. Jos asked if this
   could or should be fixed, or nailed down in the documentation. No
   takers.

     http://xrl.us/iibj

   Another source of segmentation faults killed. Peter Dintelmann
   followed on from last week and continued to nail segfaults. He noted
   that executing the code snippet "PerlIO::get_layers undef" would end
   in grief and patched universal.c to make it work correctly (or rather,
   not crash). Applied by Steve Peters as #26144.

     http://xrl.us/iibk

   Vadim Konovalov supplied a patch make WinCE compile blead and wondered
   whether ./wince shouldn't be made a subdirectory of the ./win32
   directory in the source tree. H.Merijn made a few remarks about the
   patch, basically prodding Vadim to give himself and Rainer a bit more
   credit in the README.

     http://xrl.us/iibm

   When in doubt, use brute force. "fbriere" noted a cute bug: if you
   require a module that dies when it is loaded, it fails the first time.
   But the second time, it succeeds!

     http://xrl.us/iibn

   Andy Lester took a preliminary run through the codebase and axed the
   ancient RCS commit comments in the source. (These days the codebase is
   managed in a Perforce repository).

     http://xrl.us/iibo

About this summary

   This summary was written by David Landgren.

   Information concerning bugs referenced in this summary (as #nnnnn) may
   be viewed at http://rt.perl.org/rt3/Ticket/Display.html?id=nnnnn

   Information concerning patches to maint or blead referenced in this
   summary (as #nnnnn) may be viewed at
   http://public.activestate.com/cgi-bin/perlbrowse?patch=nnnnn

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-- 
"It's overkill of course, but you can never have too much overkill."