=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
=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<$Is_Foo> checks. We can safely put this
+into a file, change it to build an C<%Is> hash and require it. Maybe just put
+it into F<test.pl>. Throw in the handy tainting subroutines.
=head2 POD -E<gt> HTML conversion in the core still sucks
The addition of C<Pod::Simple> and its related modules may make this task
easier to complete.
+=head2 merge checkpods and podchecker
+
+F<pod/checkpods.PL> (and C<make check> in the F<pod/> subdirectory)
+implements a very basic check for pod files, but the errors it discovers
+aren't found by podchecker. Add this check to podchecker, get rid of
+checkpods and have C<make check> use podchecker.
+
=head2 Parallel testing
(This probably impacts much more than the core: also the Test::Harness
and TAP::* modules on CPAN.)
-The core regression test suite is getting ever more comprehensive, which has
-the side effect that it takes longer to run. This isn't so good. Investigate
-whether it would be feasible to give the harness script the B<option> of
-running sets of tests in parallel. This would be useful for tests in
-F<t/op/*.t> and F<t/uni/*.t> and maybe some sets of tests in F<lib/>.
-
-Questions to answer
-
-=over 4
-
-=item 1
-
-How does screen layout work when you're running more than one test?
-
-=item 2
-
-How does the caller of test specify how many tests to run in parallel?
-
-=item 3
-
-How do setup/teardown tests identify themselves?
-
-=back
-
-Pugs already does parallel testing - can their approach be re-used?
+All of the tests in F<t/> can now be run in parallel, if C<$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
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
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
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
not see the same filesystem(s), the $Config{run} may need to do some
file/directory copying back and forth.
+=head2 roffitall
+
+Make F<pod/roffitall> be updated by F<pod/buildtoc>.
+
+=head2 Split "linker" from "compiler"
+
+Right now, Configure probes for two commands, and sets two variables:
+
+=over 4
+
+=item * C<cc> (in F<cc.U>)
+
+This variable holds the name of a command to execute a C compiler which
+can resolve multiple global references that happen to have the same
+name. Usual values are F<cc> and F<gcc>.
+Fervent ANSI compilers may be called F<c89>. AIX has F<xlc>.
+
+=item * C<ld> (in F<dlsrc.U>)
+
+This variable indicates the program to be used to link
+libraries for dynamic loading. On some systems, it is F<ld>.
+On ELF systems, it should be C<$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)
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.
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 __FUNCTION__ for MSVC-pre-7.0
+=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.
-Jarkko notes that one can things morally equivalent to C<__FUNCTION__>
-(or C<__func__>) even in MSVC-pre-7.0, contrary to popular belief.
-See L<http://www.codeproject.com/debug/extendedtrace.asp> if you feel like
-making C<PERL_MEM_LOG> more useful on Win32.
=head1 Tasks that need a knowledge of XS
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;
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
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 error reporting of [$a ; $b]
+
+Using C<;> inside brackets is a syntax error, and we don't propose to change
+that by giving it any meaning. However, it's not reported very helpfully:
+
+ $ perl -e '$a = [$b; $c];'
+ syntax error at -e line 1, near "$b;"
+ syntax error at -e line 1, near "$c]"
+ Execution of -e aborted due to compilation errors.
+
+It should be possible to hook into the tokeniser or the lexer, so that when a
+C<;> is parsed where it is not legal as a statement terminator (ie inside
+C<{}> used as a hashref, C<[]> or C<()>) it issues an error something like
+I<';' isn't legal inside an expression - if you need multiple statements use a
+do {...} block>. See the thread starting at
+http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/2008-09/msg00573.html
+
+=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
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
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
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
https://perl5.git.perl.org / perl5.git / blobdiff