# Copyright (C) 2001-2008, The Perl Foundation.
# $Id: /mirror/trunk/docs/pdds/pdd07_codingstd.pod 34725 2008-12-31T09:53:48.289739Z bernhard  $

=head1 NAME

docs/pdds/pdd07_codingstd.pod - Conventions and Guidelines for Parrot Source
Code

=head1 VERSION

$Revision: 34725 $

=head1 ABSTRACT

This document describes the various rules, guidelines and advice for those
wishing to contribute to the source code of Parrot, in such areas as code
structure, naming conventions, comments etc.

=head1 SYNOPSIS

Not applicable.

=head1 DESCRIPTION

One of the criticisms of Perl 5 is that its source code is impenetrable to
newcomers, due to such things as inconsistent or obscure variable naming
conventions, lack of comments in the source code, and so on.  We don't intend
to make the same mistake when writing Parrot. Hence this document.

We define three classes of conventions:

=over 4

=item I<"must">

Items labelled I<must> are mandatory; and code will not be accepted (apart
from in exceptional circumstances) unless it obeys them.

=item I<"should">

Items labelled I<should> are strong guidelines that should normally be
followed unless there is a sensible reason to do otherwise.

=item I<"may">

Items labelled I<may> are tentative suggestions to be used at your discretion.

=back

Note that since Parrot is substantially implemented in C, these rules apply to
C language source code unless otherwise specified.

=head1 IMPLEMENTATION


=head2 Language Standards and Portability

=over 4

=item *

C code must generally depend on only those language and library features
specified by the ISO C89 standard.

In addition, C code may assume that any pointer value can be coerced to an
integral type (no smaller than typedef C<INTVAL> in Parrot), then back to its
original type, without loss.

Also C code may assume that there is a single NULL pointer representation
and that it consists of a number, usually 4 or 8, of '\0' chars in memory.

C code that makes assumptions beyond these must depend on the configuration
system, either to not compile an entire non-portable source where it will not
work, or to provide an appropriate #ifdef macro.

=item *

Perl code must be written for Perl 5.8.0 and all later versions.

Perl code may use features not available in Perl 5.8.0 only if it is not vital
to Parrot, and if it uses C<$^O> and C<$]> to degrade or fail gracefully when
it is run where the features it depends on are not available.

=back


=head2 Code Formatting

The following I<must> apply:

=over 4

=item *

Source line width is limited to 100 characters.  Exceptions can be made for
technical requirements, but not for style reasons.  And please bear in mind
that very long lines I<can> be hard to read.

=item *

Indentation must consist only of spaces.  (Tab characters just complicate
things.)

=item *

C and Perl code must be indented four columns per nesting level.

=item *

Preprocessor #directives must be indented two columns per nesting level, with
two exceptions: neither PARROT_IN_CORE nor the outermost _GUARD #ifdefs cause
the level of indenting to increase.

=item *

Labels (including case labels) must be outdented two columns relative to the
code they label.

=item *

Closing braces for control structures must line up vertically with the
start of the control structures; e.g. C<}> that closes an C<if> must
line up with the C<if>.

=item *

Long lines, when split, must use at least one extra level of indentation on
the continued line.

=item *

Cuddled C<else>s are forbidden: i.e. avoid  C<} else {> .

=item *

C macro parameters must be parenthesized in macro bodies, to allow expressions
passed as arguments; e.g.:

  #define PMC_pmc_val(pmc)      (pmc)->obj.u._ptrs._pmc_val

=back


The following I<should> apply:

=over 4

=item *

In function definitions, the function name must be on the left margin, with
the return type on the previous line.

=item *

In function declarations (e.g. in header files), the function name must be on
the same line as the return type.

=item *

Pointer types should be written with separation between the star and the base
type, e.g. C<Interp *foo>, but not C<Interp* foo>.

=item *

To distinguish keywords from function calls visually, there should be at least
one space between a C keyword and any subsequent open parenthesis, e.g.
C<return (x+y)*2>.  There should be no space between a function name and the
following open parenthesis, e.g. C<z = foo(x+y)*2>

=item *

Use patterns of formatting to indicate patterns of semantics.  Similar items
should look similar, as the language permits.  Note that some dimensions of
similarity are incidental, not worth emphasizing; e.g. "these are all ints".

=item *

Binary operators (except C<.> and C<< -> >>) should have at least one space on
either side; there should be no space between unary operators and their
operands; parentheses should not have space immediately after the opening
parenthesis nor immediately before the closing parenthesis; commas should have
at least one space after, but not before; e.g.:

        x = (a-- + b) * f(c, d / e.f)

=item *

Use vertical alignment for clarity of parallelism.  Compare this (bad):

     foo = 1 + 100;
     x = 100 + 1;
     whatever = 100 + 100;

... to this (good):

     foo      =   1 + 100;
     x        = 100 +   1;
     whatever = 100 + 100;

=item *

Do not routinely put single statements in statement blocks.

(Note that formatting consistency trumps this rule.  For example, a long
C<if>/C<else if> chain is easier to read if all (or none) of the conditional
code is in blocks.)

=item *

Return values should not be parenthesized without need.  It may be necessary
to parenthesize a long return expression so that a smart editor will properly
indent it.

{{ RT#45365: Modify parrot.el so this rule is no longer required. }}

=item *

When assigning inside a conditional, use extra parentheses,
e.g. C<if (a && (b = c)) ...> or C<if ((a = b)) ...>.

=item *

When splitting a long line at a binary operator (other than comma), the split
should be I<before> the operator, so that the continued line looks like one,
e.g.:

    something_long_here = something_very_long + something_else_also_long
            - something_which_also_could_be_long;

=item *

When splitting a long line inside parentheses (or brackets), the continuation
should be indented to the right of the innermost unclosed punctuation, e.g.:

    z = foo(bar + baz(something_very_long_here
                       * something_else_very_long),
            corge);

=back


=head2 Code Structure

The following I<must> apply:

=over 4

=item *

C code must use C-style comments only, i.e. C</* comment */>.  (Not all C
compilers handle C++-style comments.)

=item *

Structure types must have tags.

=item *

Functions must have prototypes in scope at the point of use.  Prototypes for
extern functions must appear only in header files. If static functions are
defined before use, their definitions serve as prototypes.

=item *

Parameters in function prototypes must be named. These names should match the
parameters in the function definition.

=item *

Variable names must be included for all function parameters in the function
declarations.

=item *

Header files must be wrapped with guard macros to prevent header redefinition.
The guard macro must begin with C<PARROT_>, followed by unique and descriptive
text identifying the header file (usually the directory path and filename),
and end with a C<_GUARD> suffix.  The matching C<#endif> must have the guard
macro name in a comment, to prevent confusion.  For example, a file named
F<parrot/foo.h> might look like:

    #ifndef PARROT_FOO_H_GUARD
    #define PARROT_FOO_H_GUARD

    #include "parrot/config.h"
    #ifdef PARROT_HAS_FEATURE_FOO
    #  define FOO_TYPE bar
    typedef struct foo {
        ...
    } foo_t;
    #endif /* PARROT_HAS_FEATURE_FOO */

    #endif /* PARROT_FOO_H_GUARD */

=back


The following I<should> apply

=over 4

=item *

Structure types should have typedefs with the same name as their tags, e.g.:

    typedef struct Foo {
        ...
    } Foo;

=item *

Avoid double negatives, e.g. C<#ifndef NO_FEATURE_FOO>.

=item *

Do not compare directly against NULL, 0, or FALSE.  Instead, write a boolean
test, e.g. C<if (!foo) ...>.

However, the sense of the expression being checked must be a boolean.
Specifically, C<strcmp()> and its brethren are three-state returns.

    if ( !strcmp(x,y) )     # BAD, checks for if x and y match, but looks
                            # like it is checking that they do NOT match.
    if ( strcmp(x,y) == 0 ) # GOOD, checks proper return value
    if ( STREQ(x,y) )       # GOOD, uses boolean wrapper macro

(Note: C<PMC *> values should be checked for nullity with the C<PMC_IS_NULL>
macro, unfortunately leading to violations of the double-negative rule.)

=item *

Avoid dependency on "FIXME" and "TODO" labels: use the external bug tracking
system.  If a bug must be fixed soon, use "XXX" B<and> put a ticket in the
bug tracking system.  This means that each "XXX" should have an RT ticket
number next to it.

=back


=head2 Smart Editor Style Support

All developers using Emacs must ensure that their Emacs instances load the
elisp source file F<editor/parrot.el> before opening Parrot source files.
See L<editor/README.pod> for instructions.

All source files must end with an editor instruction coda:

=over 4

=item *

C source files, and files largely consisting of C (e.g. yacc, lex, PMC, and
opcode source files), must end with this coda:

   /*
    * Local variables:
    *   c-file-style: "parrot"
    * End:
    * vim: expandtab shiftwidth=4:
    */

=item *

Perl source files must end with this coda:

    # Local Variables:
    #   mode: cperl
    #   cperl-indent-level: 4
    #   fill-column: 100
    # End:
    # vim: expandtab shiftwidth=4:

B<Exception>: Files with C<__END__> or C<__DATA__> blocks do not require the
coda.  This is at least until there is some consensus as to how solve the
issue of using editor hints in files with such blocks.

=item *

PIR source files should end with this coda:

    # Local Variables:
    #   mode: pir
    #   fill-column: 100
    # End:
    # vim: expandtab shiftwidth=4 ft=pir:

=back

{{ XXX - Proper formatting and syntax coloring of C code under Emacs requires
that Emacs know about typedefs.  We should provide a simple script to update a
list of typedefs, and parrot.el should read it or contain it. }}


=head2 Portability

Parrot runs on many, many platforms, and will no doubt be ported to ever more
bizarre and obscure ones over time.  You should never assume an operating
system, processor architecture, endian-ness, size of standard type, or
anything else that varies from system to system.

Since most of Parrot's development uses GNU C, you might accidentally depend
on a GNU feature without noticing.  To avoid this, know what features of gcc
are GNU extensions, and use them only when they're protected by #ifdefs.


=head2 Defensive Programming

=head3 Use Parrot data structures instead of C strings and arrays

C arrays, including strings, are very sharp tools without safety guards, and
Parrot is a large program maintained by many people.  Therefore:

Don't use a C<char *> when a Parrot STRING would suffice.  Don't use a C array
when a Parrot array PMC would suffice.  If you do use a C<char *> or C array,
check and recheck your code for even the slightest possibility of buffer
overflow or memory leak.

Note that efficiency of some low-level operations may be a reason to break
this rule.  Be prepared to justify your choices to a jury of your peers.

=head3 Pass only C<unsigned char> to C<isxxx()> and C<toxxx()>

Pass only values in the range of C<unsigned char> (and the special value -1,
a.k.a. C<EOF>) to the isxxx() and toxxx() library functions.  Passing signed
characters to these functions is a very common error and leads to incorrect
behavior at best and crashes at worst.  And under most of the compilers Parrot
targets, C<char> I<is> signed.

=head3 The C<const> keyword on arguments

Use the C<const> keyword as often as possible on pointers.  It lets
the compiler know when you intend to modify the contents of something.
For example, take this definition:

    int strlen(const char *p);

The C<const> qualifier tells the compiler that the argument will not be
modified.  The compiler can then tell you that this is an uninitialized
variable:

    char *p;
    int n = strlen(p);

Without the C<const>, the compiler has to assume that C<strlen()> is
actually initializing the contents of C<p>.

=head3 The C<const> keyword on variables

If you're declaring a temporary pointer, declare it C<const>, with the
const to the right of the C<*>, to indicate that the pointer should not
be modified.

    Wango * const w = get_current_wango();
    w->min  = 0;
    w->max  = 14;
    w->name = "Ted";

This prevents you from modifying C<w> inadvertantly.

    new_wango = w++; /* Error */

If you're not going to modify the target of the pointer, put a C<const>
to the left of the type, as in:

    const Wango * const w = get_current_wango();
    if (n < wango->min || n > wango->max) {
        /* do something */
    }

=head3 Localizing variables

Declare variables in the innermost scope possible.

    if (foo) {
        int i;
        for (i = 0; i < n; i++)
            do_something(i);
    }

Don't reuse unrelated variables.  Localize as much as possible, even if
the variables happen to have the same names.

    if (foo) {
        int i;
        for (i = 0; i < n; i++)
            do_something(i);
    }
    else {
        int i;
        for (i = 14; i > 0; i--)
            do_something_else(i * i);
    }

You could hoist the C<int i;> outside the test, but then you'd have an
C<i> that's visible after it's used, which is confusing at best.

=head2 Subversion properties

=head3 svn:ignore

Sometimes new files will be created in the configuration and build process of
Parrot. These files should not show up when checking the distribution with

    svn status

or

    perl tools/dev/manicheck.pl

The list of these ignore files can be set up with:

    svn propedit svn:ignore <PATH>

In order to keep the two different checks synchronized,
the MANIFEST and MANIFEST.SKIP files should be regenerated with:

    perl tools/dev/mk_manifest_and_skip.pl

and the files then committed to the Parrot svn repository.

=head3 svn:mime-type

The C<svn:mime-type> property must be set to C<text/plain> for all test
files, and may be set to C<text/plain> for other source code files in
the repository. Using I<auto-props>, Subversion can automatically set
this property for you on test files.  To enable this option, add the
following to your F<~/.subversion/config>:

    [miscellany]
    enable-auto-props = yes
    [auto-props]
    *.t = svn:mime-type=text/plain

The F<t/distro/file_metadata.t> test checks that the files needing
this property have it set.

=head3 svn:keywords

The C<svn:keywords> property should be set to:

    Author Date Id Revision

on each file with a mime-type of C<text/plain>.  Do this with the command:

    svn propset svn:keywords "Author Date Id Revision" <filename>

The F<t/distro/file_metadata.t> test checks that the files needing
this property have it set.

=head3 svn:eol-style

The C<svn:eol-style> property makes sure that whenever a file is checked out
of subversion it has the correct end-of-line characters appropriate for
the given platform.  Therefore, most files should have their
C<svn:eol-style> property set to C<native>.  However, this is not true
for all files.  Some input files to tests (such as the C<*.input> and
C<*.output> files for PIR tests) need to have C<LF> as their
C<svn:eol-style> property.  The current list of such files is described in
F<t/distro/file_metadata.t>.

Set the C<svn:eol-style> property to C<native> with the command:

    svn propset svn:eol-style "native" <filename>

Set the C<svn:eol-style> property to C<LF> with the command:

    svn propset svn:eol-style "LF" <filename>

The F<t/distro/file_metadata.t> test checks that the files needing
this property have it set.

=head2 Naming conventions

=over 4

=item Filenames

Filenames must be assumed to be case-insensitive, in the sense that that you
may not have two different files called F<Foo> and F<foo>. Normal source-code
filenames should be all lower-case; filenames with upper-case letters in them
are reserved for notice-me-first files such as F<README>, and for files which
need some sort of pre-processing applied to them or which do the preprocessing
- e.g. a script F<foo.SH> might read F<foo.TEMPLATE> and output F<foo.c>.

The characters making up filenames must be chosen from the ASCII set
A-Z,a-z,0-9 plus .-_

An underscore should be used to separate words rather than a hyphen (-).
A file should not normally have more than a single '.' in it, and this
should be used to denote a suffix of some description. The filename must
still be unique if the main part is truncated to 8 characters and any
suffix truncated to 3 characters. Ideally, filenames should restricted
to 8.3 in the first place, but this is not essential.

Each subsystem I<foo> should supply the following files. This
arrangement is based on the assumption that each subsystem will -- as
far as is practical -- present an opaque interface to all other
subsystems within the core, as well as to extensions and embeddings.

=over 4

=item C<foo.h>

This contains all the declarations needed for external users of that API
(and nothing more), i.e. it defines the API. It is permissible for the
API to include different or extra functionality when used by other parts
of the core, compared with its use in extensions and embeddings. In this
case, the extra stuff within the file is enabled by testing for the
macro C<PARROT_IN_CORE>.

=item C<foo_private.h>

This contains declarations used internally by that subsystem, and which
must only be included within source files associated the subsystem. This
file defines the macro C<PARROT_IN_FOO> so that code knows when it is
being used within that subsystem. The file will also contain all the
'convenience' macros used to define shorter working names for functions
without the perl prefix (see below).

=item C<foo_globals.h>

This file contains the declaration of a single structure containing the
private global variables used by the subsystem (see the section on
globals below for more details).

=item C<foo_bar.[ch]> etc.

All other source files associated with the subsystem will have the
prefix C<foo_>.

=back

=item Names of code entities

Code entities such as variables, functions, macros etc. (apart from strictly
local ones) should all follow these general guidelines.

=over 4

=item *

Multiple words or components should be separated with underscores rather
than using tricks such as capitalization, e.g. C<new_foo_bar> rather
than C<NewFooBar> or (gasp) C<newfoobar>.

=item *

The names of entities should err on the side of verbosity, e.g.
C<create_foo_from_bar()> in preference to C<ct_foo_bar()>. Avoid cryptic
abbreviations wherever possible.

=item *

All entities should be prefixed with the name of the subsystem in which they
appear, e.g. C<pmc_foo()>, C<struct io_bar>.

=item *

Functions with external visibility should be of the form C<Parrot_foo>,
and should only use typedefs with external visibility (or types defined
in C89).  Generally these functions should not be used inside the core,
but this is not a hard and fast rule.

=item *

Variables and structure names should be all lower-case, e.g. C<pmc_foo>.

=item *

Structure elements should be all lower-case, and the first component of the
name should incorporate the structure's name or an abbreviation of it.

=item *

Typedef names should be lower-case except for the first letter, e.g.
C<Foo_bar>.  The exception to this is when the first component is a
short abbreviation, in which case the whole first component may be made
uppercase for readability purposes, e.g. C<IO_foo> rather than
C<Io_foo>.  Structures should generally be typedefed.

=item *

Macros should have their first component uppercase, and the majority of
the remaining components should be likewise. Where there is a family of
macros, the variable part can be indicated in lowercase, e.g.
C<PMC_foo_FLAG>, C<PMC_bar_FLAG>, ....

=item *

A macro which defines a flag bit should be suffixed with C<_FLAG>, e.g.
C<PMC_readonly_FLAG> (although you probably want to use an C<enum> instead.)

=item *

A macro which tests a flag bit should be suffixed with C<_TEST>, e.g. C<if
(PMC_readonly_TEST(foo)) ...>

=item *

A macro which sets a flag bit should be suffixed with C<_SET>, e.g.
C<PMC_readonly_SET(foo);>

=item *

A macro which clears a flag bit should be suffixed with C<_CLEAR>, e.g.
C<PMC_readonly_CLEAR(foo);>

=item *

A macro defining a mask of flag bits should be suffixed with C<_MASK>, e.g.
C<foo &= ~PMC_STATUS_MASK> (but see notes on extensibility below).

=item *

Macros can be defined to cover common flag combinations, in which case they
should have C<_SETALL>, C<_CLEARALL>, C<_TESTALL> or C<_TESTANY> suffixes as
appropriate, to indicate aggregate bits, e.g. C<PMC_valid_CLEARALL(foo)>.

=item *

A macro defining an auto-configuration value should be prefixed with C<HAS_>,
e.g. C<HAS_BROKEN_FLOCK>, C<HAS_EBCDIC>.

=item *

A macro indicating the compilation 'location' should be prefixed with
C<IN_>, e.g. C<PARROT_IN_CORE>, C<PARROT_IN_PMC>, C<PARROT_IN_X2P>.
Individual include file visitations should be marked with C<PARROT_IN_FOO_H>
for file C<foo.h>

=item *

A macro indicating major compilation switches should be prefixed with C<USE_>,
e.g. C<PARROT_USE_STDIO>, C<USE_MULTIPLICITY>.

=item *

A macro that may declare stuff and thus needs to be at the start of a
block should be prefixed with C<DECL_>, e.g. C<DECL_SAVE_STACK>. Note
that macros which implicitly declare and then use variables are strongly
discouraged, unless it is essential for portability or extensibility.
The following are in decreasing preference style-wise, but increasing
preference extensibility-wise.

    { Stack sp = GETSTACK;  x = POPSTACK(sp) ... /* sp is an auto variable */
    { DECL_STACK(sp);  x = POPSTACK(sp); ... /* sp may or may not be auto */
    { DECL_STACK; x = POPSTACK; ... /* anybody's guess */


=back

=item Global Variables

Global variables must never be accessed directly outside the subsystem
in which they are used. Some other method, such as accessor functions,
must be provided by that subsystem's API. (For efficiency the 'accessor
functions' may occasionally actually be macros, but then the rule still
applies in spirit at least).

All global variables needed for the internal use of a particular subsystem
should all be declared within a single struct called C<foo_globals> for
subsystem C<foo>. This structure's declaration is placed in the file
C<foo_globals.h>. Then somewhere a single compound structure will be
declared which has as members the individual structures from each subsystem.
Instances of this structure are then defined as a one-off global variable,
or as per-thread instances, or whatever is required.

[Actually, three separate structures may be required, for global,
per-interpreter and per-thread variables.]

Within an individual subsystem, macros are defined for each global variable of
the form C<GLOBAL_foo> (the name being deliberately clunky). So we might for
example have the following macros:

    /* perl_core.h or similar */

    #ifdef HAS_THREADS
    #  define GLOBALS_BASE (aTHX_->globals)
    #else
    #  define GLOBALS_BASE (Parrot_globals)
    #endif

    /* pmc_private.h */

    #define GLOBAL_foo   GLOBALS_BASE.pmc.foo
    #define GLOBAL_bar   GLOBALS_BASE.pmc.bar
    ... etc ...

=back


=head2 Code comments

The importance of good code documentation cannot be stressed enough. To make
your code understandable by others (and indeed by yourself when you come to
make changes a year later), the following conventions apply to all source
files.

=over 4

=item Developer files

Each source file (e.g. a F<foo.c>, F<foo.h> pair), should contain inline
POD documentation containing information on the implementation decisions
associated with the source file. (Note that this is in contrast to PDDs,
which describe design decisions). In addition, more discussive
documentation can be placed in F<*.pod> files in the F<docs/dev>
directory. This is the place for mini-essays on how to avoid overflows
in unsigned arithmetic, or on the pros and cons of differing hash
algorithms, and why the current one was chosen, and how it works.

In principle, someone coming to a particular source file for the first time
should be able to read the inline documentation file and gain an immediate
overview of what the source file is for, the algorithms it implements, etc.

The POD documentation should follow the standard POD layout:

=over 4

=item Copyright

The Parrot copyright statement.

=item SVN

A SVN id string.

=item NAME

src/foo.c - Foo

=item SYNOPSIS

When appropriate, some simple examples of usage.

=item DESCRIPTION

A description of the contents of the file, how the implementation works, data
structures and algorithms, and anything that may be of interest to your
successors, e.g. benchmarks of differing hash algorithms, essays on how to do
integer arithmetic.

=item SEE ALSO

Links to pages and books that may contain useful information relevant to the
stuff going on in the code -- e.g. the book you stole the hash function from.

=back

Don't include author information in individual files. Author information
can be added to the CREDITS file. (Languages are an exception to this rule,
and may follow whatever convention they choose.)

=item Per-section comments

If there is a collection of functions, structures or whatever which are
grouped together and have a common theme or purpose, there should be a
general comment at the start of the section briefly explaining their
overall purpose. (Detailed essays should be left to the developer file).
If there is really only one section, then the top-of-file comment
already satisfies this requirement.

=item Per-entity comments

Every non-local named entity, be it a function, variable, structure, macro or
whatever, must have an accompanying comment explaining its purpose.  This
comment must be in the special format described below, in order to allow
automatic extraction by tools - for example, to generate per API man pages,
B<perldoc -f> style utilities and so on.

Often the comment need only be a single line explaining its purpose, but
sometimes more explanation may be needed. For example, "return an Integer Foo
to its allocation pool" may be enough to demystify the function C<del_I_foo()>

Each comment should be of the form

    /*

    =item C<function(arguments)>

    Description.

    =cut

    */

This inline POD documentation is parsed to HTML by running:

    % perl tools/docs/write_docs.pl -s

=item Optimizations

Whenever code has deliberately been written in an odd way for performance
reasons, you should point this out - if nothing else, to avoid some poor
schmuck trying subsequently to replace it with something 'cleaner'.

    /* The loop is partially unrolled here as it makes it a lot faster.
     * See the file in docs/dev for the full details
     */

=item General comments

While there is no need to go mad commenting every line of code, it is
immensely helpful to provide a "running commentary" every 10 lines or so
if nothing else, this makes it easy to quickly locate a specific chunk
of code. Such comments are particularly useful at the top of each major
branch, e.g.

    if (FOO_bar_BAZ(**p+*q) <= (r-s[FOZ & FAZ_MASK]) || FLOP_2(z99)) {
        /* we're in foo mode: clean up lexicals */
        ... (20 lines of gibberish) ...
    }
    else if (...) {
        /* we're in bar mode: clean up globals */
        ... (20 more lines of gibberish) ...
    }
    else {
        /* we're in baz mode: self-destruct */
        ....
    }

=back

=head2 Extensibility

Over the lifetime of Parrot, the source code will undergo many major changes
never envisaged by its original authors. To this end, your code should balance
out the assumptions that make things possible, fast or small, with the
assumptions that make it difficult to change things in future. This is
especially important for parts of the code which are exposed through APIs --
the requirements of source or binary compatibility for such things as
extensions can make it very hard to change things later on.

For example, if you define suitable macros to set/test flags in a struct, then
you can later add a second word of flags to the struct without breaking source
compatibility. (Although you might still break binary compatibility if you're
not careful.) Of the following two methods of setting a common combination of
flags, the second doesn't assume that all the flags are contained within a
single field:

    foo->flags |= (FOO_int_FLAG | FOO_num_FLAG | FOO_str_FLAG);
    FOO_valid_value_SETALL(foo);

Similarly, avoid using a C<char*> (or C<{char*,length}>) if it is feasible
to later use a C<PMC *> at the same point: c.f. UTF-8 hash keys in Perl 5.

Of course, private code hidden behind an API can play more fast and loose than
code which gets exposed.

=head2 Performance

We want Parrot to be fast. Very fast. But we also want it to be portable and
extensible. Based on the 90/10 principle, (or 80/20, or 95/5, depending on who
you speak to), most performance is gained or lost in a few small but critical
areas of code. Concentrate your optimization efforts there.

Note that the most overwhelmingly important factor in performance is in
choosing the correct algorithms and data structures in the first place. Any
subsequent tweaking of code is secondary to this. Also, any tweaking that is
done should as far as possible be platform independent, or at least likely to
cause speed-ups in a wide variety of environments, and do no harm elsewhere.

If you do put an optimization in, time it on as many architectures as
you can, and be suspicious of it if it slows down on any of them!
Perhaps it will be slow on other architectures too (current and future).
Perhaps it wasn't so clever after all? If the optimization is platform
specific, you should probably put it in a platform-specific function in
a platform-specific file, rather than cluttering the main source with
zillions of #ifdefs.

And remember to document it.

=head1 EXEMPTIONS

Not all files can strictly fall under these guidelines as they are
automatically generated by other tools, or are external files included in
the Parrot repository for convenience.  Such files include the C header and
source files automatically generated by (f)lex and yacc/bison, and some of
the Perl modules under the F<lib/> directory.

To exempt a file (or directory of files) from checking by the coding
standards tests, one must edit the appropriate exemption list within
C<lib/Parrot/Distribution.pm> (in either of the methods C<is_c_exemption()>
or C<is_perl_exemption()>).  One can use wildcards in the list to exempt,
for example, all files under a given directory.

=head1 REFERENCES

none

=cut

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