X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperltodo.pod;h=30f17b04cbc8ad30d90482654ac035b0deed2e84;hb=00ca1d0d42208c5980e79d63906648d2926498ec;hp=34cd5a3dff84a53ea02cafdac744797a28ccf878;hpb=539f2c5426314080e02054800a435b5de46b293e;p=p5sagit%2Fp5-mst-13.2.git

diff --git a/pod/perltodo.pod b/pod/perltodo.pod
index 34cd5a3..30f17b0 100644
--- a/pod/perltodo.pod
+++ b/pod/perltodo.pod
@@ -4,10 +4,11 @@ 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. By all means contact a pumpking privately first if you prefer.
+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
@@ -22,11 +23,61 @@ programming languages offer you 1 line of immortality?
 
 =head1 Tasks that only need Perl knowledge
 
-=head2 common test code for timed bail out
+=head2 Smartmatch design issues
 
-Write portable self destruct code for tests to stop them burning CPU in
-infinite loops. This needs to avoid using alarm, as some of the tests are
-testing alarm/sleep or timers.
+In 5.10.0 the smartmatch operator C<~~> isn't working quite "right". But
+before we can fix the implementation, we need to define what "right" is.
+The first problem is that Robin Houston implemented the Perl 6 smart match
+spec as of February 2006, when smart match was axiomatically symmetrical:
+L<http://groups.google.com/group/perl.perl6.language/msg/bf2b486f089ad021>
+
+Since then the Perl 6 target moved, but the Perl 5 implementation did not.
+
+So it would be useful for someone to compare the Perl 6 smartmatch table
+as of February 2006 L<http://svn.perl.org/viewvc/perl6/doc/trunk/design/syn/S03.pod?view=markup&pathrev=7615>
+and the current table L<http://svn.perl.org/viewvc/perl6/doc/trunk/design/syn/S03.pod?revision=14556&view=markup>
+and tabulate the differences in Perl 6. The annotated view of changes is
+L<http://svn.perl.org/viewvc/perl6/doc/trunk/design/syn/S03.pod?view=annotate> and the diff is
+C<svn diff -r7615:14556 http://svn.perl.org/perl6/doc/trunk/design/syn/S03.pod>
+-- search for C<=head1 Smart matching>. (In theory F<viewvc> can generate that,
+but in practice when I tried it hung forever, I assume "thinking")
+
+With that done and published, someone (else) can then map any changed Perl 6
+semantics back to Perl 5, based on how the existing semantics map to Perl 5:
+L<http://search.cpan.org/~rgarcia/perl-5.10.0/pod/perlsyn.pod#Smart_matching_in_detail>
+
+
+There are also some questions that need answering:
+
+=over 4
+
+=item *
+
+How do you negate one?  (documentation issue)
+http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/2008-01/msg00071.html
+
+=item *
+
+Array behaviors
+http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/2007-12/msg00799.html
+
+* Should smart matches be symmetrical? (Perl 6 says no)
+
+* Other differences between Perl 5 and Perl 6 smart match?
+
+=item *
+
+Objects and smart match
+http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/2007-12/msg00865.html
+
+=back
+
+=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
 
@@ -40,36 +91,24 @@ 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.)
 
-The core regression test suite is getting ever more comprehensive, which has
-the side effect that it takes longer to run. This isn't so good. Investigate
-whether it would be feasible to give the harness script the B<option> of
-running sets of tests in parallel. This would be useful for tests in
-F<t/op/*.t> and F<t/uni/*.t> and maybe some sets of tests in F<lib/>.
-
-Questions to answer
-
-=over 4
-
-=item 1
-
-How does screen layout work when you're running more than one test?
-
-=item 2
-
-How does the caller of test specify how many tests to run in parallel?
-
-=item 3
-
-How do setup/teardown tests identify themselves?
-
-=back
-
-Pugs already does parallel testing - can their approach be re-used?
+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
 
@@ -133,10 +172,11 @@ 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 Improving C<threads::shared>
+=head2 Bundle dual life modules in ext/
 
-Investigate whether C<threads::shared> could share aggregates properly with
-only Perl level changes to shared.pm
+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
 
@@ -175,6 +215,13 @@ 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
@@ -327,11 +374,100 @@ 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>.
+
+=head2 Split "linker" from "compiler"
+
+Right now, Configure probes for two commands, and sets two variables:
+
+=over 4
+
+=item * C<cc (cc.U)>
+
+This variable holds the name of a command to execute a C compiler which
+can resolve multiple global references that happen to have the same
+name.  Usual values are F<cc> and F<gcc>.
+Fervent ANSI compilers may be called F<c89>.  AIX has F<xlc>.
+
+=item * ld (dlsrc.U)
+
+This variable indicates the program to be used to link
+libraries for dynamic loading.  On some systems, it is F<ld>.
+On ELF systems, it should be C<$cc>.  Mostly, we'll try to respect
+the hint file setting.
+
+=back
+
+There is an implicit historical assumption from around Perl5.000alpha
+something, that C<$cc> is also the correct command for linking object files
+together to make an executable. This may be true on Unix, but it's not true
+on other platforms, and there are a maze of work arounds in other places (such
+as F<Makefile.SH>) to cope with this.
+
+Ideally, we should create a new variable to hold the name of the executable
+linker program, probe for it in F<Configure>, and centralise all the special
+case logic there or in hints files.
+
+A small bikeshed issue remains - what to call it, given that C<$ld> is already
+taken (arguably for the wrong thing now, but on SunOS 4.1 it is the command
+for creating dynamically-loadable modules) and C<$link> could be confused with
+the Unix command line executable of the same name, which does something
+completely different. Andy Dougherty makes the counter argument "In parrot, I
+tried to call the command used to link object files and  libraries into an
+executable F<link>, since that's what my vaguely-remembered DOS and VMS
+experience suggested. I don't think any real confusion has ensued, so it's
+probably a reasonable name for perl5 to use."
+
+"Alas, I've always worried that introducing it would make things worse, 
+since now the module building utilities would have to look for 
+C<$Config{link}> and institute a fall-back plan if it weren't found."
+Although I can see that as confusing, given that C<$Config{d_link}> is true
+when (hard) links are available.
+
 =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
@@ -372,6 +508,8 @@ 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.
@@ -414,12 +552,152 @@ 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;
@@ -493,17 +771,6 @@ 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 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 Audit the code for destruction ordering assumptions
 
 Change 25773 notes
@@ -547,24 +814,80 @@ 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(); 
 
-The current Perl 6 design is that C<state ($a) = foo();> and
-C<(state $a) = foo();> have different semantics, which is tricky to implement
-in Perl 5 as currently the produce the same opcode trees. It would be useful
-to clarify that the Perl 6 design is firm, and then implement the necessary
-code in Perl 5. There are comments in C<Perl_newASSIGNOP()> that show the
-code paths taken by various assignment constructions involving state variables.
+In Perl 6, C<state ($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
 
@@ -652,34 +975,16 @@ 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
+=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 re-
-instated.
+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 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 The yada yada yada operators
-
-Perl 6's Synopsis 3 says:
-
-I<The ... operator is the "yada, yada, yada" list operator, which is used as
-the body in function prototypes. It complains bitterly (by calling fail)
-if it is ever executed. Variant ??? calls warn, and !!! calls die.>
-
-Those would be nice to add to Perl 5. That could be done without new ops.
-
 =head2 Virtualize operating system access
 
 Implement a set of "vtables" that virtualizes operating system access
@@ -715,6 +1020,122 @@ 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