# Copyright (C) 2001-2007, The Perl Foundation.
# $Id: faq.pod 20475 2007-08-04 20:04:06Z chromatic $

=head1 NAME

docs/faq.pod - Parrot FAQ

=head1 GENERAL QUESTIONS

=head2 What is Parrot?

Parrot is a virtual machine for dynamic languages such as PHP, Perl, Python,
Ruby, Scheme, Tcl, etc. It compiles and executes bytecode, but is also designed
to act as an interpreter.

=head2 Why did you call it "Parrot"?

The name "Parrot" started with Simon Cozens's April Fool's Joke
(L<http://www.perl.com/pub/a/2001/04/01/parrot.htm>) where Larry Wall and Guido
van Rossum announced the merger of the Perl and Python languages.

A year later, when we were looking for a name for our virtual machine that
could run both Perl and Python, it seemed like a perfect fit.

=head2 Is Parrot the same as Perl 6?

No. Perl 6 is just one of the languages that will run on Parrot. For
information about Perl 6 on Parrot, see L<languages/perl6/>.

=head2 Can I use Parrot today?

Yes.

Although Parrot is currently still under development, Parrot has been usable for
a long time. The primary way to use Parrot is to write Parrot Intermediate
Representation (PIR), described in L<PDD19|docs/pdds/draft/pdd19_pir.pod>.
PIR is a high-level assembly language. PIR maps to the more low-level
Parrot Assembly language (PASM), which is harder to write and read.
Although you could write PASM instead of PIR, PIR is the recommended way
to program for Parrot. See the examples to see what both PIR and PASM
looks like.

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 PIR?

Lots of reasons, actually.  :^)

=over 4

=item *

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

=item *

It's easy to write and read.

=item *

You get all the pleasure of programming in assembly language (albeit a high-level
assembly language) without any of the requisite system crashes.

=back

Seriously, though, PIR is generally easy to learn if you have a background
in dynamic languages. Programming in PIR is an effective way to write
libraries for Parrot, and 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?

You can already today! There are quite a few high level languages being
targeted to Parrot. Since the introduction of the Parrot Compiler Tools (the
Parrot Grammar Engine (PGE) and the Tree Grammar Engine (TGE)), targeting a
language to Parrot has become a snap! Please note that, although some languages
have come a long way, due to the fact that Parrot is still under active
development, most library development is still done in PIR.

Below is a list of some languages that are actively worked on.

=over 4

=item *

Will Coleda and Matt Diephouse are working hard on their Tcl port to Parrot
(called ParTcl). Will also created an APL implementation with Patrick R.
Michaud.

=item *

Patrick R. Michaud is working on a Perl 6 implementation using the Parrot
Compiler Tools. (although the Perl 6 specification is not finished yet).

=item *

François Perrad is working on a Lua implementation for Parrot.

=item *

Allison Randal is working on a Perl 1 port to Parrot, called Punie.

=item *

Jonathan Worthington has been working on a .NET to Parrot translator.

=item *

Many other languages are worked on, some more actively than others. See
C<http://www.parrotcode.org/languages/> for a complete list.

=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 uses the Artistic License 2.0, which is compatible with
the GNU GPL. This 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.

=head1 PARROT IMPLEMENTATION ISSUES

=head2 Why a register-based VM instead of a stack-based one?

The JVM and the CLR (Mono and .NET) are two successful stack-based
virtual machines. Many interpreters such as Perl, Python, and Ruby are
also internally stack-based.

On the other hand, most hardware is register-based, as are several virtual
machines designed to emulate hardware (such as the 68K
emulator--L<http://en.wikipedia.org/wiki/Mac_68k_emulator>--Apple shipped with
its PPC-enabled versions of Mac OS).

A few reasons we chose a register-based architecture:

=over 4

=item *

Executing opcodes on a register-based VM takes fewer instructions (since
you eliminate all the steps to push items on to the stack and pull them
off again), which means less CPU time.

=item *

One class of security problems in modern software are a result of
problems with the stack (stack overflows, stack smashing). We can't
entirely eliminate these (since Parrot is written in C, which is
stack-based), but we can significantly minimize them.

=item *

A register-based VM is far more pleasant to work with than a stack-based VM.

=item *

In the early days of Parrot one motivating factor was taking advantage of
decades of register-based hardware research. We've now moved away from a
fixed number of registers to a variable (and potentially unlimited) number
of registers per sub. This change opens up many new possibilities, but it
does push us past the limits of most research into register-based hardware.

=item *

See also L<http://www.usenix.org/publications/library/proceedings/vee05/full_papers/p153-yunhe.pdf>.

We're pushing forward the state-of-the art in virtual machines.
Innovation often involves breaking with tradition. We're pleased
with the results so far, and we're not finished yet.

=back


=head2 Why aren't you using reference counting?

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 (or, for the
hardware, complexity) involved in 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. To 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 C<end> and C<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.

=cut
