# Copyright (C) 2001-2004, The Perl Foundation.
# $Id: faq.pod 14632 2006-09-16 17:39:14Z coke $

=head1 NAME

docs/faq.pod - Parrot FAQ

=head1 GENERAL QUESTIONS

=head2 What is Parrot?

Parrot is the new interpreter being designed from scratch to support the
upcoming Perl 6 language. It is being designed as a standalone virtual machine
that can be used to execute bytecode compiled dynamic languages such as Perl 6,
but also Perl 5. Ideally, Parrot can be used to support other dynamic,
bytecode-compiled languages such as Python, Ruby and Tcl.

=head2 Why "Parrot"?

The name "Parrot" relates to Simon Cozens's L<April Fool's Joke|"LINKS"> where
Larry Wall and Guido van Rossum announced the merger of the Perl and Python
languages.

As penance, Simon spent time as Parrot's lead developer, but he's gotten
better.

=head2 Is Parrot the same as Perl 6?

No. Parrot is an implementation that is expected to be used to execute Perl 6
programs. The Perl 6 language definition is currently being crafted by Larry
Wall. While the true nature of Perl 6 is still unknown, it will be substantially
similar to Perl as we know it today, and will need a runtime system. For more
information on the nascent Perl 6 language definition, check out Larry's
L<apocalypses|"LINKS">.

=head2 Can I use Parrot today?

Yes.

Parrot is in the early phases of its implementation.  The primary way to use
Parrot is to write Parrot assembly code, described in L<PDD6|pdds/pdd6.pod>.

You can also create dynamic content within Apache using Ask Bjorn Hansen's
mod_parrot module.  You are strongly advised that mod_parrot is a toy, and
should not be used with any production code.

=head2 Why should I program in Parrot Assembly language?

Lots of reasons, actually.  :^)

=over 4

=item *

All the L<cool kids|"LINKS"> are doing it.

=item *

It's a neat hack.

=item *

You get all the pleasure of programming in assembly language without any of the
requisite system crashes.

=back

Seriously, though, programming in Parrot assembly language is an interesting
challenge.  It's also one of the best ways to write test cases for Parrot.

=head2 When can I expect to use Parrot with a I<real> programming language?

It depends on what you mean by I<real>.  :^)

=over 4

=item *

Leon Brocard has released a proof-of-concept L<Java bytecode to Parrot
bytecode|"LINKS"> compiler.

=item *

Gregor Purdy is working on a little language called Jako that targets Parrot
bytecode directly.  (Available with the Parrot distribution.)

=item *

Dan Sugalski and Jeff Goff have started work on compiling Scheme down to Parrot
bytecode.  (Available with the Parrot distribution.)

=item *

Clint Pierce wrote an Integer Basic implementation in parrot assembly, which is
shipped with the parrot distribution, as are a few example programs. (Including
Hunt the Wumpus and Eliza)

=item *

There's a Befunge interpreter in the languages directory

=item *

There's an (ahem)BF interpreter in the languages directory. Be aware that BF is
not, strictly speaking, the language's name, merely its initials.

=item *

There is a prototype Perl 6 implementation in the languages directory as well,
though it's only as complete as the Perl 6 spec. (Which, at this writing, isn't
sufficiently complete)

=back

=head2 What language is Parrot written in?

C.

=head2 For the love of God, man, why?!?!?!?

Because it's the best we've got.

=head2 That's sad.

So true.  Regardless, C's available pretty much everywhere.  Perl 5's in C, so
we can potentially build any place Perl 5 builds.

=head2 Why not write it in I<insert favorite language here>?

Because of one of:

=over 4

=item *

Not available everywhere.

=item *

Limited talent pool for core programmers.

=item *

Not fast enough.

=back

=head2 Why aren't you using external tool or library I<X>?

The most common issues are:

=over 4

=item *
License compatibility.

Parrot has an odd license -- it currently uses the same license as Perl 5,
which is the disjunction of the GNU GPL and the Artistic License, which can be
written (Artistic|GPL) for short.  Thus, Parrot's license is compatible with
the GNU GPL, which means you can combine Parrot with GPL'ed code.

Code accepted into the core interpreter must fall under the same terms as
parrot. Library code (for example the ICU library we're using for Unicode) we
link into the interpreter can be covered by other licenses so long as their
terms don't prohibit this.

=item *
Platform compatibility.

Parrot has to work on most of Perl 5's platforms, as well as a few of its own.
Perl 5 runs on eighty platforms; Parrot must run on Unix, Windows, Mac OS (X
and Classic), VMS, Crays, Windows CE, and Palm OS, just to name a few.  Among
its processor architectures will be x86, SPARC, Alpha, IA-64, ARM, and 68x00
(Palms and old Macs).  If something doesn't work on all of these, we can't use
it in Parrot.

=item *
Speed, size, and flexibility.

Not only does Parrot have to run on all those platforms, but it must also run
efficiently.  Parrot's core size is currently between 250K and 700K, depending
on compiler.  That's pushing it on the handheld platforms.  Any library used by
Parrot must be fast enough to have a fairly small performance impact, small
enough to have little impact on core size, and flexible enough to handle the
varying demands of Perl, Python, Tcl, Ruby, Scheme, and whatever else some
clever or twisted hacker throws at Parrot.

=back

These tests are very hard to pass; currently we're expecting we'll probably
have to write everything but the Unicode stuff.

=head2 Why your own virtual machine?  Why not compile to JVM/.NET?

Those VMs are designed for statically typed languages. That's fine, since Java,
C#, and lots of other languages are statically typed. Perl isn't.  For a
variety of reasons, it means that Perl would run more slowly there than on an
interpreter geared towards dynamic languages.

The .NET VM didn't even exist when we started development, or at least we
didn't know about it when we were working on the design. We do now, though it's
still not suitable.

=head2 So you won't run on JVM/.NET?

Sure we will. They're just not our first target. We build our own
interpreter/VM, then when that's working we start in on the JVM and/or .NET
back ends.

=head2 What about I<insert other VM here>

While I'm sure that's a perfectly nice, fast VM, it's probably got the same
issues as do the languages in the "Why not something besides C" question does.
I realize that the Scheme-48 interpreter's darned fast, for example, but we're
looking at the same sort of portability and talent pool problems that we are
with, say, Erlang or Haskell as an implementation language.

=head2 Why is the development list called perl6-internals?

It's not anymore.  As of July 2006, the list is called parrot-porters
to reflect the growing list of languages and platforms embraced by
Parrot.  The old perl6-internals list forwards to the new one.

=head2 Pugs is going great shakes - why not just toss Parrot and run
Perl 6 on Pugs?

Perl 5 is highly portable, and builds on around 50 different systems, many far
removed from Unix or MS Windows. We'd like Perl 6 to be able run everywhere
that Perl 5 runs, so we need to keep Parrot as portable as possible.  The
Glasgow Haskell Compiler is a pain to build on minor systems, and downright
impossible on small systems. So by going with Pugs and GHC we'd be sacrificing
portability.

As well, other languages apart from Perl 6 are being targeted to Parrot.
Significant parts of Python, Tcl, and Basic have already been implemented,
and others are on the way. Running multiple languages on the same
Parrot engine allows them to be cross-language compatible-- in other words, one
targeted language could directly invoke the methods of another at the bytecode
level.

Finally there is a reason the Parrot design keeps talking about running
bytecode direct from disk rather than relying on doing compiling (from Perl or
with a JIT) in memory. It's all very well doing such operations when running
one program, but think what happens on a multi-user system when 300 people fire
up "parrot order.pbc" - 300 parrot processes all fighting for resources.  To
quote Dan,

  non-jit vss/rss is 29784 17312, JIT vss/rss is 122032 108916. A not
  insignificant difference :)

With read only bytecode shared between processes, much of that "non-jit"
resident memory is going to be shared. So much less swapping. And don't think
that this won't matter to you because you don't have 300 users all running the
same program - consider what happens if each Perl 6 module is compiled to
bytecode. With read only bytecode 300 different Perl scripts all share the same
memory for Carp.pbc, warnings.pbc, etc. Without, and they're all swapping like
crazy...

=head2 OK, so Parrot is fast... Pugs can back-end to Parrot, right?

There was an early attempt for Pugs to target Parrot, but at the moment
it is not actively maintained; instead, look into the F<languages/perl6/>
directory in the Parrot source tree for an implementation that does not
depend on either GHC or Perl 5's runtime.

Still, the ultimate goal is for the Perl 6 compiler to be self-hosting
(that is, written in itself) in order to improve introspection, debugger
capabilities and compile-time semantic modulation.

See L<http://dev.perl.org/perl6/doc/design/syn/S01.html#Project_Plan>
for further discussions on Perl 6's project plan.

=head1 PARROT AND PERL

=head2 Why re-implement Perl?

Good question.

At The Perl Conference 4.0, in the summer of 2000, Larry Wall
L<announced|"LINKS"> that it was time to recreate Perl from the ground up. This
included the Perl language, the implementation of that language, the community
of open source developers who volunteer to implement and maintain the language,
and the larger community of programmers who use Perl.

A variety of reasons were given for embarking on this project:

=over 4

=item *

Perl 5 is a stable, reliable, robust platform for developing software; it's not
going away for a long time, even after Perl 6 is released. (Proof: Perl 4 is
still out there, no matter how much we all want it to go away.)

=item *

We have the ability to translate Perl 5 into Perl 6 if necessary. This preserves
backward compatibility with a large body of existing Perl code, which is
I<very> important.

=item *

The language can stand some revision: formats don't really belong in the core
language, and typeglobs have outlived their usefulness. By revising the
language now, we can make Perl better.

=item *

Some warts really should be removed: C<system> should return I<true> instead of
I<false> on success, and C<localtime> should return the year, not the year -
1900.

=item *

It would be nice to write the Perl to Bytecode compiler in Perl, instead of C.
That would make it much easier for Perl hackers to hack on Perl.

=back

=head2 You want to write the Perl compiler in Perl?

Sure.  Why not?  C, Java, Lisp, Scheme, and practically every other language is
self-hoisting.  Why not?

=head2 Isn't there a bootstrapping problem?

No, not really.  Don't forget that we can use Perl 5 to run Perl 5 programs,
such as a Perl 5 to Parrot compiler.

=head2 How will Parrot handle both Perl 5 and Perl 6?

We don't know yet, since it depends on the Perl 6 language definition.  But we
I<could> use the more appropriate of two Perl compilers, depending of whether
we're compiling Perl 5 or Perl 6.  Larry has mumbled something about a
C<package> statement declaring that the file is Perl 5, but we're still not
quite sure on how that fits in.

=head2 Is this how Parrot will run Python, Ruby, and Tcl code?

Probably.

=head2 Latin and Klingon too?

No, Parrot won't be twisted enough for Damian.  Perhaps when Parrot is ported
to a pair of supercooled calcium ions, though...

=head2 Huh?

You had to L<be there|"LINKS">.

=head1 PARROT IMPLEMENTATION ISSUES

=head2 What's with the whole register thing machine?

Not much, why do you ask?

=head2 Don't you know that stack machines are the way to go in software?

No, in fact, I don't.

=head2 But look at all the successful stack-based VMs!

Like what?  There's just the JVM.

=head2 What about all the others?

B<What> others?  That's it, unless you count Perl, Python, or Ruby.

=head2 Yeah them!

Yeah, right.  You never thought of them as VMs, admit it.  :^)

Seriously, we're already running with a faster opcode dispatch than any of them
are, and having registers just decreases the amount of stack thrash we get.

=head2 Right, smarty.  Then name a successful register-based VM!

The 68K emulator Apple ships with all its PPC-enabled versions of Mac OS.

=head2 Really?

L<Really.|"LINKS">

=head2 You're not using reference counting. Why not?

Reference counting has three big issues.

=over 4

=item Code complexity

Every single place where an object is referenced, and every single place where
a reference is dropped, I<must> properly alter the refcount of the objects
being manipulated. One mistake and an object (and everything it references,
directly or indirectly) lives forever or dies prematurely. Since a lot of code
references objects, that's a lot of places to scatter reference counting code.
While some of it can be automated, that's a lot of discipline that has to be
maintained.

It's enough of a problem to track down garbage collection systems as it is, and
when your garbage collection system is scattered across your entire source
base, and possibly across all your extensions, it's a massive annoyance. More
sophisticated garbage collection systems, on the other hand, involve much less
code. It is, granted, trickier code, but it's a small chunk of code, contained
in one spot. Once you get that one chunk correct, you don't have to bother with
the garbage collector any more.

=item Cost

For reference counting to work right, you need to twiddle reference counts
every time an object is referenced, or unreferenced. This generally includes
even short-lived objects that will exist only briefly before dying. The cost of
a reference counting scheme is directly linked to the number of times code
references, or unreferences, objects. A tracing system of one sort or another
(and there are many) has an average-case cost that's based on the number of
live objects.

There are a number of hidden costs in a reference-counting scheme. Since the
code to manipulate the reference counts I<must> be scattered throughout the
interpreter, the interpreter code is less dense than it would be without
reference counts. That means that more of the processor's cache is dedicated to
reference count code, code that is ultimately just interpreter bookkeeping, and
not dedicated to running your program. The data is also less dense, as there
has to be a reference count embedded in it. Once again, that means more cache
used for each object during normal running, and lower cache density.

A tracing collector, on the other hand, has much denser code, since all it's
doing is running through active objects in a tight loop. If done right, the
entire tracing system will fit nicely in a processor's L1 cache, which is about
as tight as you can get. The data being accessed is also done in a linear
fashion, at least in part, which lends itself well to processor's prefetch
mechanisms where they exist. The garbage collection data can also be put in a
separate area and designed in a way that's much tighter and more cache-dense.

Having said that, the worst-case performance for a tracing garbage collecting
system is worse than that of a reference counting system. Luckily the
pathological cases are quite rare, and there are a number of fairly good
techniques to deal with those. Refcounting schemes are also more deterministic
than tracing systems, which can be an advantage in some cases. Making a tracing
collector deterministic can be somewhat expensive.

=item Self-referential structures live forever

Or nearly forever. Since the only time an object is destroyed is when its
refcount drops to zero, data in a self-referential structure will live on
forever. It's possible to detect this and clean it up, of course... by
implementing a full tracing garbage collector. That means that you have two
full garbage collection systems rather than one, which adds to the code
complexity.

=back

=head2 Could we do a partial refcounting scheme?

Well... no. It's all or nothing. If we were going to do a partial scheme we
might as well do a full scheme. (A partial refcounting scheme is actually more
expensive, since partial schemes check to see whether refcounts need twiddling,
and checks are more expensive than you might think)

=head2 Why are there so many opcodes?

Whether we have a lot or not actually depends on how you count.  In absolute,
unique op numbers we have more than pretty much any other processor, but that
is in part because we have *no* runtime op variance.

It's also important to note that there's no less code involved (or, for the
hardware, complexity) doing it our way or the decode-at-runtime way -- all the
code is still there in every case, since we all have to do the same things (add
a mix of ints, floats, and objects, with a variety of ways of finding them) so
there's no real penalty to doing it our way. It actually simplifies the JIT
some (no need to puzzle out the parameter types), so in that we get a win over
other platforms since JIT expenses are paid by the user every run, while our
form of decoding's only paid when you compile.

Finally, there's the big "does it matter, and to whom?" question. As someone
actually writing parrot assembly, it looks like parrot only has one "add" op --
when emitting pasm or pir you use the "add" mnemonic. That it gets qualified
and assembles down to one variant or another based on the (fixed at assemble
time) parameters is just an implementation detail. For those of us writing op
bodies, it just looks like we've got an engine with full signature-based
dispatching (which, really, we do -- it's just a static variant), so rather
than having to have a big switch statement or chain of ifs at the beginning of
the add op we just write the specific variants identified by function prototype
and leave it to the engine to choose the right variant.

Heck, we could, if we chose, switch over to a system with a single add op with
tagged parameter types and do runtime decoding without changing the source for
the ops at all -- the op preprocessor could glob them all together and
autogenerate the big switch/if ladder at the head of the function. (We're not
going to, of course, but we could.)

=head2 What are the criteria for adding and deleting opcodes?

As for what the rationale is... well, it's a combination of whim and necessity
for adding them, and brutal reality for deleting them.

Our ops fall into two basic categories. The first, like add, are just basic
operations that any engine has to perform. The second, like time, are low-level
library functions.

For something like hardware, splitting standard library from the CPU makes
sense -- often the library requires resources that the hardware doesn't have
handy.  Hardware is also often bit-limited -- opcodes need to fit in 8 or 9
bits.

Parrot, on the other hand, *isn't* bit-limited, since our ops are 32 bits. (A
more efficient design on RISC systems where byte-access is expensive.) That
opens things up a bunch.

If you think about it, the core opcode functions and the core low-level
libraries are *always* available. Always. The library functions also have a
very fixed parameter list. Fixed parameter list, guaranteed availability...
looks like an opcode function to me.  So they are. We could make them library
functions instead, but all that'd mean would be that they'd be more expensive
to call (our sub/method call is a bit heavyweight) and that you'd have to do
more work to find and call the functions. Seemed silly.

Or, I suppose, you could think of it as if we had *no* opcodes at all other
than end and loadoplib. Heck, we've a loadable opcode system -- it'd not be too
much of a stretch to consider all the opcode functions other than those two as
just functions with a fast-path calling system. The fact that a while bunch of
'em are available when you start up's just a convenience for you.

See L<http://www.nntp.perl.org/group/perl.perl6.internals/22003> for more
details.

=head1 LINKS

April Fool's Joke: L<http://www.perl.com/pub/a/2001/04/01/parrot.htm>

apocalypses: L<http://dev.perl.org/perl6/apocalypse/>

exegeses: L<http://dev.perl.org/perl6/exegesis/>

synopses: L<http://dev.perl.org/perl6/synopsis/>

Java bytecode to Parrot bytecode:
L<http://archive.develooper.com/perl6-internals@perl.org/msg03864.html>

L<http://www.perl.com/pub/a/2000/10/23/soto2000.html>

be there: L<http://www.csse.monash.edu.au/~damian/papers/#Superpositions>

Really.:
L<http://developer.apple.com/techpubs/mac/PPCSoftware/PPCSoftware-13.html>

=cut

=head1 VERSION

The FAQ is now in version control and "Revision" isn't really being tracked.
The most recent SVN ID is C<$Id: faq.pod 14632 2006-09-16 17:39:14Z coke $>

=over 4

=item Revision 0.5 - 04 September 2002

=item Revision 0.4 - 26 August 2002

Fixed up the licensing bits

=item Revision 0.3 - 13 March 2002

Translated to POD and added "Why aren't we using external tool or library
I<X>?"

=item Revision 0.2 - 03 December 2001

Added the "Parrot and Perl" section and "Why Re-implement Perl".  Incorporated
Dan's Q&A items.

=item Revision 0.1 - 03 December 2001

Adopted from Simon Cozens's article, "Parrot: A Cross-Language Virtual Machine
Architecture".

=back

