[PATCH] Perl TODO related to threads::shared done

[p5sagit/p5-mst-13.2.git] / pod / perltodo.pod

=head1 NAME

perltodo - Perl TO-DO List

=head1 DESCRIPTION

This is a list of wishes for Perl. The tasks we think are smaller or
easier are listed first. Anyone is welcome to work on any of these,
but it's a good idea to first contact I<perl5-porters@perl.org> to
avoid duplication of effort, and to learn from any previous attempts.
By all means contact a pumpking privately first if you prefer.

Whilst patches to make the list shorter are most welcome, ideas to add to
the list are also encouraged. Check the perl5-porters archives for past
ideas, and any discussion about them. One set of archives may be found at:

    http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/

What can we offer you in return? Fame, fortune, and everlasting glory? Maybe
not, but if your patch is incorporated, then we'll add your name to the
F<AUTHORS> file, which ships in the official distribution. How many other
programming languages offer you 1 line of immortality?

=head1 Tasks that only need Perl knowledge

=head2 Remove duplication of test setup.

Schwern notes, that there's duplication of code - lots and lots of tests have
some variation on the big block of C<$Is_Foo> checks.  We can safely put this
into a file, change it to build an C<%Is> hash and require it.  Maybe just put
it into F<test.pl>. Throw in the handy tainting subroutines.

=head2 POD -E<gt> HTML conversion in the core still sucks

Which is crazy given just how simple POD purports to be, and how simple HTML
can be. It's not actually I<as> simple as it sounds, particularly with the
flexibility POD allows for C<=item>, but it would be good to improve the
visual appeal of the HTML generated, and to avoid it having any validation
errors. See also L</make HTML install work>, as the layout of installation tree
is needed to improve the cross-linking.

The addition of C<Pod::Simple> and its related modules may make this task
easier to complete.

=head2 merge checkpods and podchecker

F<pod/checkpods.PL> (and C<make check> in the F<pod/> subdirectory)
implements a very basic check for pod files, but the errors it discovers
aren't found by podchecker. Add this check to podchecker, get rid of
checkpods and have C<make check> use podchecker.

=head2 Parallel testing

(This probably impacts much more than the core: also the Test::Harness
and TAP::* modules on CPAN.)

All of the tests in F<t/> can now be run in parallel, if C<$ENV{TEST_JOBS}>
is set. However, tests within each directory in F<ext> and F<lib> are still
run in series, with directories run in parallel. This is an adequate
heuristic, but it might be possible to relax it further, and get more
throughput. Specifically, it would be good to audit all of F<lib/*.t>, and
make them use C<File::Temp>.

=head2 Make Schwern poorer

We should have tests for everything. When all the core's modules are tested,
Schwern has promised to donate to $500 to TPF. We may need volunteers to
hold him upside down and shake vigorously in order to actually extract the
cash.

=head2 Improve the coverage of the core tests

Use Devel::Cover to ascertain the core modules's test coverage, then add
tests that are currently missing.

=head2 test B

A full test suite for the B module would be nice.

=head2 Deparse inlined constants

Code such as this

    use constant PI => 4;
    warn PI

will currently deparse as

    use constant ('PI', 4);
    warn 4;

because the tokenizer inlines the value of the constant subroutine C<PI>.
This allows various compile time optimisations, such as constant folding
and dead code elimination. Where these haven't happened (such as the example
above) it ought be possible to make B::Deparse work out the name of the
original constant, because just enough information survives in the symbol
table to do this. Specifically, the same scalar is used for the constant in
the optree as is used for the constant subroutine, so by iterating over all
symbol tables and generating a mapping of SV address to constant name, it
would be possible to provide B::Deparse with this functionality.

=head2 A decent benchmark

C<perlbench> seems impervious to any recent changes made to the perl core. It
would be useful to have a reasonable general benchmarking suite that roughly
represented what current perl programs do, and measurably reported whether
tweaks to the core improve, degrade or don't really affect performance, to
guide people attempting to optimise the guts of perl. Gisle would welcome
new tests for perlbench.

=head2 fix tainting bugs

Fix the bugs revealed by running the test suite with the C<-t> switch (via
C<make test.taintwarn>).

=head2 Dual life everything

As part of the "dists" plan, anything that doesn't belong in the smallest perl
distribution needs to be dual lifed. Anything else can be too. Figure out what
changes would be needed to package that module and its tests up for CPAN, and
do so. Test it with older perl releases, and fix the problems you find.

To make a minimal perl distribution, it's useful to look at
F<t/lib/commonsense.t>.

=head2 Bundle dual life modules in ext/

For maintenance (and branch merging) reasons, it would be useful to move
some architecture-independent dual-life modules from lib/ to ext/, if this
has no negative impact on the build of perl itself.

=head2 POSIX memory footprint

Ilya observed that use POSIX; eats memory like there's no tomorrow, and at
various times worked to cut it down. There is probably still fat to cut out -
for example POSIX passes Exporter some very memory hungry data structures.

=head2 embed.pl/makedef.pl

There is a script F<embed.pl> that generates several header files to prefix
all of Perl's symbols in a consistent way, to provide some semblance of
namespace support in C<C>. Functions are declared in F<embed.fnc>, variables
in F<interpvar.h>. Quite a few of the functions and variables
are conditionally declared there, using C<#ifdef>. However, F<embed.pl>
doesn't understand the C macros, so the rules about which symbols are present
when is duplicated in F<makedef.pl>. Writing things twice is bad, m'kay.
It would be good to teach C<embed.pl> to understand the conditional
compilation, and hence remove the duplication, and the mistakes it has caused.

=head2 use strict; and AutoLoad

Currently if you write

    package Whack;
    use AutoLoader 'AUTOLOAD';
    use strict;
    1;
    __END__
    sub bloop {
        print join (' ', No, strict, here), "!\n";
    }

then C<use strict;> isn't in force within the autoloaded subroutines. It would
be more consistent (and less surprising) to arrange for all lexical pragmas
in force at the __END__ block to be in force within each autoloaded subroutine.

There's a similar problem with SelfLoader.

=head2 profile installman

The F<installman> script is slow. All it is doing text processing, which we're
told is something Perl is good at. So it would be nice to know what it is doing
that is taking so much CPU, and where possible address it.


=head1 Tasks that need a little sysadmin-type knowledge

Or if you prefer, tasks that you would learn from, and broaden your skills
base...

=head2 make HTML install work

There is an C<installhtml> target in the Makefile. It's marked as
"experimental". It would be good to get this tested, make it work reliably, and
remove the "experimental" tag. This would include

=over 4

=item 1

Checking that cross linking between various parts of the documentation works.
In particular that links work between the modules (files with POD in F<lib/>)
and the core documentation (files in F<pod/>)

=item 2

Work out how to split C<perlfunc> into chunks, preferably one per function
group, preferably with general case code that could be used elsewhere.
Challenges here are correctly identifying the groups of functions that go
together, and making the right named external cross-links point to the right
page. Things to be aware of are C<-X>, groups such as C<getpwnam> to
C<endservent>, two or more C<=items> giving the different parameter lists, such
as

    =item substr EXPR,OFFSET,LENGTH,REPLACEMENT
    =item substr EXPR,OFFSET,LENGTH
    =item substr EXPR,OFFSET

and different parameter lists having different meanings. (eg C<select>)

=back

=head2 compressed man pages

Be able to install them. This would probably need a configure test to see how
the system does compressed man pages (same directory/different directory?
same filename/different filename), as well as tweaking the F<installman> script
to compress as necessary.

=head2 Add a code coverage target to the Makefile

Make it easy for anyone to run Devel::Cover on the core's tests. The steps
to do this manually are roughly

=over 4

=item *

do a normal C<Configure>, but include Devel::Cover as a module to install
(see F<INSTALL> for how to do this)

=item *

    make perl

=item *

    cd t; HARNESS_PERL_SWITCHES=-MDevel::Cover ./perl -I../lib harness

=item *

Process the resulting Devel::Cover database

=back

This just give you the coverage of the F<.pm>s. To also get the C level
coverage you need to

=over 4

=item *

Additionally tell C<Configure> to use the appropriate C compiler flags for
C<gcov>

=item *

    make perl.gcov

(instead of C<make perl>)

=item *

After running the tests run C<gcov> to generate all the F<.gcov> files.
(Including down in the subdirectories of F<ext/>

=item *

(From the top level perl directory) run C<gcov2perl> on all the C<.gcov> files
to get their stats into the cover_db directory.

=item *

Then process the Devel::Cover database

=back

It would be good to add a single switch to C<Configure> to specify that you
wanted to perform perl level coverage, and another to specify C level
coverage, and have C<Configure> and the F<Makefile> do all the right things
automatically.

=head2 Make Config.pm cope with differences between built and installed perl

Quite often vendors ship a perl binary compiled with their (pay-for)
compilers.  People install a free compiler, such as gcc. To work out how to
build extensions, Perl interrogates C<%Config>, so in this situation
C<%Config> describes compilers that aren't there, and extension building
fails. This forces people into choosing between re-compiling perl themselves
using the compiler they have, or only using modules that the vendor ships.

It would be good to find a way teach C<Config.pm> about the installation setup,
possibly involving probing at install time or later, so that the C<%Config> in
a binary distribution better describes the installed machine, when the
installed machine differs from the build machine in some significant way.

=head2 linker specification files

Some platforms mandate that you provide a list of a shared library's external
symbols to the linker, so the core already has the infrastructure in place to
do this for generating shared perl libraries. My understanding is that the
GNU toolchain can accept an optional linker specification file, and restrict
visibility just to symbols declared in that file. It would be good to extend
F<makedef.pl> to support this format, and to provide a means within
C<Configure> to enable it. This would allow Unix users to test that the
export list is correct, and to build a perl that does not pollute the global
namespace with private symbols.

=head2 Cross-compile support

Currently C<Configure> understands C<-Dusecrosscompile> option. This option
arranges for building C<miniperl> for TARGET machine, so this C<miniperl> is
assumed then to be copied to TARGET machine and used as a replacement of full
C<perl> executable.

This could be done little differently. Namely C<miniperl> should be built for
HOST and then full C<perl> with extensions should be compiled for TARGET.
This, however, might require extra trickery for %Config: we have one config
first for HOST and then another for TARGET.  Tools like MakeMaker will be
mightily confused.  Having around two different types of executables and
libraries (HOST and TARGET) makes life interesting for Makefiles and
shell (and Perl) scripts.  There is $Config{run}, normally empty, which
can be used as an execution wrapper.  Also note that in some
cross-compilation/execution environments the HOST and the TARGET do
not see the same filesystem(s), the $Config{run} may need to do some
file/directory copying back and forth.

=head2 roffitall

Make F<pod/roffitall> be updated by F<pod/buildtoc>.

=head1 Tasks that need a little C knowledge

These tasks would need a little C knowledge, but don't need any specific
background or experience with XS, or how the Perl interpreter works

=head2 Weed out needless PERL_UNUSED_ARG

The C code uses the macro C<PERL_UNUSED_ARG> to stop compilers warning about
unused arguments. Often the arguments can't be removed, as there is an
external constraint that determines the prototype of the function, so this
approach is valid. However, there are some cases where C<PERL_UNUSED_ARG>
could be removed. Specifically

=over 4

=item *

The prototypes of (nearly all) static functions can be changed

=item *

Unused arguments generated by short cut macros are wasteful - the short cut
macro used can be changed.

=back

=head2 Modernize the order of directories in @INC

The way @INC is laid out by default, one cannot upgrade core (dual-life)
modules without overwriting files. This causes problems for binary
package builders.  One possible proposal is laid out in this
message:
L<http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/2002-04/msg02380.html>.

=head2 -Duse32bit*

Natively 64-bit systems need neither -Duse64bitint nor -Duse64bitall.
On these systems, it might be the default compilation mode, and there
is currently no guarantee that passing no use64bitall option to the
Configure process will build a 32bit perl. Implementing -Duse32bit*
options would be nice for perl 5.12.

=head2 Make it clear from -v if this is the exact official release

Currently perl from C<p4>/C<rsync> ships with a F<patchlevel.h> file that
usually defines one local patch, of the form "MAINT12345" or "RC1". The output
of perl -v doesn't report that a perl isn't an official release, and this
information can get lost in bugs reports. Because of this, the minor version
isn't bumped up until RC time, to minimise the possibility of versions of perl
escaping that believe themselves to be newer than they actually are.

It would be useful to find an elegant way to have the "this is an interim
maintenance release" or "this is a release candidate" in the terse -v output,
and have it so that it's easy for the pumpking to remove this just as the
release tarball is rolled up. This way the version pulled out of rsync would
always say "I'm a development release" and it would be safe to bump the
reported minor version as soon as a release ships, which would aid perl
developers.

This task is really about thinking of an elegant way to arrange the C source
such that it's trivial for the Pumpking to flag "this is an official release"
when making a tarball, yet leave the default source saying "I'm not the
official release".

=head2 Profile Perl - am I hot or not?

The Perl source code is stable enough that it makes sense to profile it,
identify and optimise the hotspots. It would be good to measure the
performance of the Perl interpreter using free tools such as cachegrind,
gprof, and dtrace, and work to reduce the bottlenecks they reveal.

As part of this, the idea of F<pp_hot.c> is that it contains the I<hot> ops,
the ops that are most commonly used. The idea is that by grouping them, their
object code will be adjacent in the executable, so they have a greater chance
of already being in the CPU cache (or swapped in) due to being near another op
already in use.

Except that it's not clear if these really are the most commonly used ops. So
as part of exercising your skills with coverage and profiling tools you might
want to determine what ops I<really> are the most commonly used. And in turn
suggest evictions and promotions to achieve a better F<pp_hot.c>.

One piece of Perl code that might make a good testbed is F<installman>.

=head2 Allocate OPs from arenas

Currently all new OP structures are individually malloc()ed and free()d.
All C<malloc> implementations have space overheads, and are now as fast as
custom allocates so it would both use less memory and less CPU to allocate
the various OP structures from arenas. The SV arena code can probably be
re-used for this.

Note that Configuring perl with C<-Accflags=-DPL_OP_SLAB_ALLOC> will use
Perl_Slab_alloc() to pack optrees into a contiguous block, which is
probably superior to the use of OP arenas, esp. from a cache locality
standpoint.  See L<Profile Perl - am I hot or not?>.

=head2 Improve win32/wince.c

Currently, numerous functions look virtually, if not completely,
identical in both C<win32/wince.c> and C<win32/win32.c> files, which can't
be good.

=head2 Use secure CRT functions when building with VC8 on Win32

Visual C++ 2005 (VC++ 8.x) deprecated a number of CRT functions on the basis
that they were "unsafe" and introduced differently named secure versions of
them as replacements, e.g. instead of writing

    FILE* f = fopen(__FILE__, "r");

one should now write

    FILE* f;
    errno_t err = fopen_s(&f, __FILE__, "r"); 

Currently, the warnings about these deprecations have been disabled by adding
-D_CRT_SECURE_NO_DEPRECATE to the CFLAGS. It would be nice to remove that
warning suppressant and actually make use of the new secure CRT functions.

There is also a similar issue with POSIX CRT function names like fileno having
been deprecated in favour of ISO C++ conformant names like _fileno. These
warnings are also currently suppressed by adding -D_CRT_NONSTDC_NO_DEPRECATE. It
might be nice to do as Microsoft suggest here too, although, unlike the secure
functions issue, there is presumably little or no benefit in this case.

=head2 Fix POSIX::access() and chdir() on Win32

These functions currently take no account of DACLs and therefore do not behave
correctly in situations where access is restricted by DACLs (as opposed to the
read-only attribute).

Furthermore, POSIX::access() behaves differently for directories having the
read-only attribute set depending on what CRT library is being used. For
example, the _access() function in the VC6 and VC7 CRTs (wrongly) claim that
such directories are not writable, whereas in fact all directories are writable
unless access is denied by DACLs. (In the case of directories, the read-only
attribute actually only means that the directory cannot be deleted.) This CRT
bug is fixed in the VC8 and VC9 CRTs (but, of course, the directory may still
not actually be writable if access is indeed denied by DACLs).

For the chdir() issue, see ActiveState bug #74552:
http://bugs.activestate.com/show_bug.cgi?id=74552

Therefore, DACLs should be checked both for consistency across CRTs and for
the correct answer.

(Note that perl's -w operator should not be modified to check DACLs. It has
been written so that it reflects the state of the read-only attribute, even
for directories (whatever CRT is being used), for symmetry with chmod().)

=head2 strcat(), strcpy(), strncat(), strncpy(), sprintf(), vsprintf()

Maybe create a utility that checks after each libperl.a creation that
none of the above (nor sprintf(), vsprintf(), or *SHUDDER* gets())
ever creep back to libperl.a.

  nm libperl.a | ./miniperl -alne '$o = $F[0] if /:$/; print "$o $F[1]" if $F[0] eq "U" && $F[1] =~ /^(?:strn?c(?:at|py)|v?sprintf|gets)$/'

Note, of course, that this will only tell whether B<your> platform
is using those naughty interfaces.

=head2 -D_FORTIFY_SOURCE=2, -fstack-protector

Recent glibcs support C<-D_FORTIFY_SOURCE=2> and recent gcc
(4.1 onwards?) supports C<-fstack-protector>, both of which give
protection against various kinds of buffer overflow problems.
These should probably be used for compiling Perl whenever available,
Configure and/or hints files should be adjusted to probe for the
availability of these features and enable them as appropriate.

=head2 Arenas for GPs? For MAGIC?

C<struct gp> and C<struct magic> are both currently allocated by C<malloc>.
It might be a speed or memory saving to change to using arenas. Or it might
not. It would need some suitable benchmarking first. In particular, C<GP>s
can probably be changed with minimal compatibility impact (probably nothing
outside of the core, or even outside of F<gv.c> allocates them), but they
probably aren't allocated/deallocated often enough for a speed saving. Whereas
C<MAGIC> is allocated/deallocated more often, but in turn, is also something
more externally visible, so changing the rules here may bite external code.

=head2 Shared arenas

Several SV body structs are now the same size, notably PVMG and PVGV, PVAV and
PVHV, and PVCV and PVFM. It should be possible to allocate and return same
sized bodies from the same actual arena, rather than maintaining one arena for
each. This could save 4-6K per thread, of memory no longer tied up in the
not-yet-allocated part of an arena.


=head1 Tasks that need a knowledge of XS

These tasks would need C knowledge, and roughly the level of knowledge of
the perl API that comes from writing modules that use XS to interface to
C.

=head2 safely supporting POSIX SA_SIGINFO

Some years ago Jarkko supplied patches to provide support for the POSIX
SA_SIGINFO feature in Perl, passing the extra data to the Perl signal handler.

Unfortunately, it only works with "unsafe" signals, because under safe
signals, by the time Perl gets to run the signal handler, the extra
information has been lost. Moreover, it's not easy to store it somewhere,
as you can't call mutexs, or do anything else fancy, from inside a signal
handler.

So it strikes me that we could provide safe SA_SIGINFO support

=over 4

=item 1

Provide global variables for two file descriptors

=item 2

When the first request is made via C<sigaction> for C<SA_SIGINFO>, create a
pipe, store the reader in one, the writer in the other

=item 3

In the "safe" signal handler (C<Perl_csighandler()>/C<S_raise_signal()>), if
the C<siginfo_t> pointer non-C<NULL>, and the writer file handle is open,

=over 8

=item 1

serialise signal number, C<struct siginfo_t> (or at least the parts we care
about) into a small auto char buff

=item 2

C<write()> that (non-blocking) to the writer fd

=over 12

=item 1

if it writes 100%, flag the signal in a counter of "signals on the pipe" akin
to the current per-signal-number counts

=item 2

if it writes 0%, assume the pipe is full. Flag the data as lost?

=item 3

if it writes partially, croak a panic, as your OS is broken.

=back

=back

=item 4

in the regular C<PERL_ASYNC_CHECK()> processing, if there are "signals on
the pipe", read the data out, deserialise, build the Perl structures on
the stack (code in C<Perl_sighandler()>, the "unsafe" handler), and call as
usual.

=back

I think that this gets us decent C<SA_SIGINFO> support, without the current risk
of running Perl code inside the signal handler context. (With all the dangers
of things like C<malloc> corruption that that currently offers us)

For more information see the thread starting with this message:
http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/2008-03/msg00305.html

=head2 autovivification

Make all autovivification consistent w.r.t LVALUE/RVALUE and strict/no strict;

This task is incremental - even a little bit of work on it will help.

=head2 Unicode in Filenames

chdir, chmod, chown, chroot, exec, glob, link, lstat, mkdir, open,
opendir, qx, readdir, readlink, rename, rmdir, stat, symlink, sysopen,
system, truncate, unlink, utime, -X.  All these could potentially accept
Unicode filenames either as input or output (and in the case of system
and qx Unicode in general, as input or output to/from the shell).
Whether a filesystem - an operating system pair understands Unicode in
filenames varies.

Known combinations that have some level of understanding include
Microsoft NTFS, Apple HFS+ (In Mac OS 9 and X) and Apple UFS (in Mac
OS X), NFS v4 is rumored to be Unicode, and of course Plan 9.  How to
create Unicode filenames, what forms of Unicode are accepted and used
(UCS-2, UTF-16, UTF-8), what (if any) is the normalization form used,
and so on, varies.  Finding the right level of interfacing to Perl
requires some thought.  Remember that an OS does not implicate a
filesystem.

(The Windows -C command flag "wide API support" has been at least
temporarily retired in 5.8.1, and the -C has been repurposed, see
L<perlrun>.)

Most probably the right way to do this would be this:
L</"Virtualize operating system access">.

=head2 Unicode in %ENV

Currently the %ENV entries are always byte strings.
See L</"Virtualize operating system access">.

=head2 Unicode and glob()

Currently glob patterns and filenames returned from File::Glob::glob()
are always byte strings.  See L</"Virtualize operating system access">.

=head2 Unicode and lc/uc operators

Some built-in operators (C<lc>, C<uc>, etc.) behave differently, based on
what the internal encoding of their argument is. That should not be the
case. Maybe add a pragma to switch behaviour.

=head2 use less 'memory'

Investigate trade offs to switch out perl's choices on memory usage.
Particularly perl should be able to give memory back.

This task is incremental - even a little bit of work on it will help.

=head2 Re-implement C<:unique> in a way that is actually thread-safe

The old implementation made bad assumptions on several levels. A good 90%
solution might be just to make C<:unique> work to share the string buffer
of SvPVs. That way large constant strings can be shared between ithreads,
such as the configuration information in F<Config>.

=head2 Make tainting consistent

Tainting would be easier to use if it didn't take documented shortcuts and
allow taint to "leak" everywhere within an expression.

=head2 readpipe(LIST)

system() accepts a LIST syntax (and a PROGRAM LIST syntax) to avoid
running a shell. readpipe() (the function behind qx//) could be similarly
extended.

=head2 Audit the code for destruction ordering assumptions

Change 25773 notes

    /* Need to check SvMAGICAL, as during global destruction it may be that
       AvARYLEN(av) has been freed before av, and hence the SvANY() pointer
       is now part of the linked list of SV heads, rather than pointing to
       the original body.  */
    /* FIXME - audit the code for other bugs like this one.  */

adding the C<SvMAGICAL> check to

    if (AvARYLEN(av) && SvMAGICAL(AvARYLEN(av))) {
        MAGIC *mg = mg_find (AvARYLEN(av), PERL_MAGIC_arylen);

Go through the core and look for similar assumptions that SVs have particular
types, as all bets are off during global destruction.

=head2 Extend PerlIO and PerlIO::Scalar

PerlIO::Scalar doesn't know how to truncate().  Implementing this
would require extending the PerlIO vtable.

Similarly the PerlIO vtable doesn't know about formats (write()), or
about stat(), or chmod()/chown(), utime(), or flock().

(For PerlIO::Scalar it's hard to see what e.g. mode bits or ownership
would mean.)

PerlIO doesn't do directories or symlinks, either: mkdir(), rmdir(),
opendir(), closedir(), seekdir(), rewinddir(), glob(); symlink(),
readlink().

See also L</"Virtualize operating system access">.

=head2 -C on the #! line

It should be possible to make -C work correctly if found on the #! line,
given that all perl command line options are strict ASCII, and -C changes
only the interpretation of non-ASCII characters, and not for the script file
handle. To make it work needs some investigation of the ordering of function
calls during startup, and (by implication) a bit of tweaking of that order.

=head2 Organize error messages

Perl's diagnostics (error messages, see L<perldiag>) could use
reorganizing and formalizing so that each error message has its
stable-for-all-eternity unique id, categorized by severity, type, and
subsystem.  (The error messages would be listed in a datafile outside
of the Perl source code, and the source code would only refer to the
messages by the id.)  This clean-up and regularizing should apply
for all croak() messages.

This would enable all sorts of things: easier translation/localization
of the messages (though please do keep in mind the caveats of
L<Locale::Maketext> about too straightforward approaches to
translation), filtering by severity, and instead of grepping for a
particular error message one could look for a stable error id.  (Of
course, changing the error messages by default would break all the
existing software depending on some particular error message...)

This kind of functionality is known as I<message catalogs>.  Look for
inspiration for example in the catgets() system, possibly even use it
if available-- but B<only> if available, all platforms will B<not>
have catgets().

For the really pure at heart, consider extending this item to cover
also the warning messages (see L<perllexwarn>, C<warnings.pl>).

=head1 Tasks that need a knowledge of the interpreter

These tasks would need C knowledge, and knowledge of how the interpreter works,
or a willingness to learn.

=head2 lexicals used only once

This warns:

    $ perl -we '$pie = 42'
    Name "main::pie" used only once: possible typo at -e line 1.

This does not:

    $ perl -we 'my $pie = 42'

Logically all lexicals used only once should warn, if the user asks for
warnings.  An unworked RT ticket (#5087) has been open for almost seven
years for this discrepancy.

=head2 UTF-8 revamp

The handling of Unicode is unclean in many places. For example, the regexp
engine matches in Unicode semantics whenever the string or the pattern is
flagged as UTF-8, but that should not be dependent on an internal storage
detail of the string. Likewise, case folding behaviour is dependent on the
UTF8 internal flag being on or off.

=head2 Properly Unicode safe tokeniser and pads.

The tokeniser isn't actually very UTF-8 clean. C<use utf8;> is a hack -
variable names are stored in stashes as raw bytes, without the utf-8 flag
set. The pad API only takes a C<char *> pointer, so that's all bytes too. The
tokeniser ignores the UTF-8-ness of C<PL_rsfp>, or any SVs returned from
source filters.  All this could be fixed.

=head2 state variable initialization in list context

Currently this is illegal:

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

In Perl 6, C<state ($a) = foo();> and C<(state $a) = foo();> have different
semantics, which is tricky to implement in Perl 5 as currently they produce
the same opcode trees. The Perl 6 design is firm, so it would be good to
implement the necessary code in Perl 5. There are comments in
C<Perl_newASSIGNOP()> that show the code paths taken by various assignment
constructions involving state variables.

=head2 Implement $value ~~ 0 .. $range

It would be nice to extend the syntax of the C<~~> operator to also
understand numeric (and maybe alphanumeric) ranges.

=head2 A does() built-in

Like ref(), only useful. It would call the C<DOES> method on objects; it
would also tell whether something can be dereferenced as an
array/hash/etc., or used as a regexp, etc.
L<http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/2007-03/msg00481.html>

=head2 Tied filehandles and write() don't mix

There is no method on tied filehandles to allow them to be called back by
formats.

=head2 Attach/detach debugger from running program

The old perltodo notes "With C<gdb>, you can attach the debugger to a running
program if you pass the process ID. It would be good to do this with the Perl
debugger on a running Perl program, although I'm not sure how it would be
done." ssh and screen do this with named pipes in /tmp. Maybe we can too.

=head2 Optimize away empty destructors

Defining an empty DESTROY method might be useful (notably in
AUTOLOAD-enabled classes), but it's still a bit expensive to call. That
could probably be optimized.

=head2 LVALUE functions for lists

The old perltodo notes that lvalue functions don't work for list or hash
slices. This would be good to fix.

=head2 LVALUE functions in the debugger

The old perltodo notes that lvalue functions don't work in the debugger. This
would be good to fix.

=head2 regexp optimiser optional

The regexp optimiser is not optional. It should configurable to be, to allow
its performance to be measured, and its bugs to be easily demonstrated.

=head2 delete &function

Allow to delete functions. One can already undef them, but they're still
in the stash.

=head2 C</w> regex modifier

That flag would enable to match whole words, and also to interpolate
arrays as alternations. With it, C</P/w> would be roughly equivalent to:

    do { local $"='|'; /\b(?:P)\b/ }

See L<http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/2007-01/msg00400.html>
for the discussion.

=head2 optional optimizer

Make the peephole optimizer optional. Currently it performs two tasks as
it walks the optree - genuine peephole optimisations, and necessary fixups of
ops. It would be good to find an efficient way to switch out the
optimisations whilst keeping the fixups.

=head2 You WANT *how* many

Currently contexts are void, scalar and list. split has a special mechanism in
place to pass in the number of return values wanted. It would be useful to
have a general mechanism for this, backwards compatible and little speed hit.
This would allow proposals such as short circuiting sort to be implemented
as a module on CPAN.

=head2 lexical aliases

Allow lexical aliases (maybe via the syntax C<my \$alias = \$foo>.

=head2 entersub XS vs Perl

At the moment pp_entersub is huge, and has code to deal with entering both
perl and XS subroutines. Subroutine implementations rarely change between 
perl and XS at run time, so investigate using 2 ops to enter subs (one for
XS, one for perl) and swap between if a sub is redefined.

=head2 Self-ties

Self-ties are currently illegal because they caused too many segfaults. Maybe
the causes of these could be tracked down and self-ties on all types
reinstated.

=head2 Optimize away @_

The old perltodo notes "Look at the "reification" code in C<av.c>".

=head2 Virtualize operating system access

Implement a set of "vtables" that virtualizes operating system access
(open(), mkdir(), unlink(), readdir(), getenv(), etc.)  At the very
least these interfaces should take SVs as "name" arguments instead of
bare char pointers; probably the most flexible and extensible way
would be for the Perl-facing interfaces to accept HVs.  The system
needs to be per-operating-system and per-file-system
hookable/filterable, preferably both from XS and Perl level
(L<perlport/"Files and Filesystems"> is good reading at this point,
in fact, all of L<perlport> is.)

This has actually already been implemented (but only for Win32),
take a look at F<iperlsys.h> and F<win32/perlhost.h>.  While all Win32
variants go through a set of "vtables" for operating system access,
non-Win32 systems currently go straight for the POSIX/UNIX-style
system/library call.  Similar system as for Win32 should be
implemented for all platforms.  The existing Win32 implementation
probably does not need to survive alongside this proposed new
implementation, the approaches could be merged.

What would this give us?  One often-asked-for feature this would
enable is using Unicode for filenames, and other "names" like %ENV,
usernames, hostnames, and so forth.
(See L<perlunicode/"When Unicode Does Not Happen">.)

But this kind of virtualization would also allow for things like
virtual filesystems, virtual networks, and "sandboxes" (though as long
as dynamic loading of random object code is allowed, not very safe
sandboxes since external code of course know not of Perl's vtables).
An example of a smaller "sandbox" is that this feature can be used to
implement per-thread working directories: Win32 already does this.

See also L</"Extend PerlIO and PerlIO::Scalar">.

=head2 Investigate PADTMP hash pessimisation

The peephole optimier converts constants used for hash key lookups to shared
hash key scalars. Under ithreads, something is undoing this work.
See http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/2007-09/msg00793.html

=head2 Store the current pad in the OP slab allocator

=for clarification
I hope that I got that "current pad" part correct

Currently we leak ops in various cases of parse failure. I suggested that we
could solve this by always using the op slab allocator, and walking it to
free ops. Dave comments that as some ops are already freed during optree
creation one would have to mark which ops are freed, and not double free them
when walking the slab. He notes that one problem with this is that for some ops
you have to know which pad was current at the time of allocation, which does
change. I suggested storing a pointer to the current pad in the memory allocated
for the slab, and swapping to a new slab each time the pad changes. Dave thinks
that this would work.

=head2 repack the optree

Repacking the optree after execution order is determined could allow
removal of NULL ops, and optimal ordering of OPs with respect to cache-line
filling.  The slab allocator could be reused for this purpose.  I think that
the best way to do this is to make it an optional step just before the
completed optree is attached to anything else, and to use the slab allocator
unchanged, so that freeing ops is identical whether or not this step runs.
Note that the slab allocator allocates ops downwards in memory, so one would
have to actually "allocate" the ops in reverse-execution order to get them
contiguous in memory in execution order.

See http://www.nntp.perl.org/group/perl.perl5.porters/2007/12/msg131975.html

Note that running this copy, and then freeing all the old location ops would
cause their slabs to be freed, which would eliminate possible memory wastage if
the previous suggestion is implemented, and we swap slabs more frequently.

=head2 eliminate incorrect line numbers in warnings

This code

    use warnings;
    my $undef;
    
    if ($undef == 3) {
    } elsif ($undef == 0) {
    }

used to produce this output:

    Use of uninitialized value in numeric eq (==) at wrong.pl line 4.
    Use of uninitialized value in numeric eq (==) at wrong.pl line 4.

where the line of the second warning was misreported - it should be line 5.
Rafael fixed this - the problem arose because there was no nextstate OP
between the execution of the C<if> and the C<elsif>, hence C<PL_curcop> still
reports that the currently executing line is line 4. The solution was to inject
a nextstate OPs for each C<elsif>, although it turned out that the nextstate
OP needed to be a nulled OP, rather than a live nextstate OP, else other line
numbers became misreported. (Jenga!)

The problem is more general than C<elsif> (although the C<elsif> case is the
most common and the most confusing). Ideally this code

    use warnings;
    my $undef;
    
    my $a = $undef + 1;
    my $b
      = $undef
      + 1;

would produce this output

    Use of uninitialized value $undef in addition (+) at wrong.pl line 4.
    Use of uninitialized value $undef in addition (+) at wrong.pl line 7.

(rather than lines 4 and 5), but this would seem to require every OP to carry
(at least) line number information.

What might work is to have an optional line number in memory just before the
BASEOP structure, with a flag bit in the op to say whether it's present.
Initially during compile every OP would carry its line number. Then add a late
pass to the optimiser (potentially combined with L</repack the optree>) which
looks at the two ops on every edge of the graph of the execution path. If
the line number changes, flags the destination OP with this information.
Once all paths are traced, replace every op with the flag with a
nextstate-light op (that just updates C<PL_curcop>), which in turn then passes
control on to the true op. All ops would then be replaced by variants that
do not store the line number. (Which, logically, why it would work best in
conjunction with L</repack the optree>, as that is already copying/reallocating
all the OPs)

(Although I should note that we're not certain that doing this for the general
case is worth it)

=head2 optimize tail-calls

Tail-calls present an opportunity for broadly applicable optimization;
anywhere that C<< return foo(...) >> is called, the outer return can
be replaced by a goto, and foo will return directly to the outer
caller, saving (conservatively) 25% of perl's call&return cost, which
is relatively higher than in C.  The scheme language is known to do
this heavily.  B::Concise provides good insight into where this
optimization is possible, ie anywhere entersub,leavesub op-sequence
occurs.

 perl -MO=Concise,-exec,a,b,-main -e 'sub a{ 1 }; sub b {a()}; b(2)'

Bottom line on this is probably a new pp_tailcall function which
combines the code in pp_entersub, pp_leavesub.  This should probably
be done 1st in XS, and using B::Generate to patch the new OP into the
optrees.

=head1 Big projects

Tasks that will get your name mentioned in the description of the "Highlights
of 5.12"

=head2 make ithreads more robust

Generally make ithreads more robust. See also L</iCOW>

This task is incremental - even a little bit of work on it will help, and
will be greatly appreciated.

One bit would be to write the missing code in sv.c:Perl_dirp_dup.

Fix Perl_sv_dup, et al so that threads can return objects.

=head2 iCOW

Sarathy and Arthur have a proposal for an improved Copy On Write which
specifically will be able to COW new ithreads. If this can be implemented
it would be a good thing.

=head2 (?{...}) closures in regexps

Fix (or rewrite) the implementation of the C</(?{...})/> closures.

=head2 A re-entrant regexp engine

This will allow the use of a regex from inside (?{ }), (??{ }) and
(?(?{ })|) constructs.

=head2 Add class set operations to regexp engine

Apparently these are quite useful. Anyway, Jeffery Friedl wants them.

demerphq has this on his todo list, but right at the bottom.