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NAME
perlcompile - Introduction to the Perl Compiler-Translator
DESCRIPTION
Perl has always had a compiler: your source is compiled into an inter-
nal form (a parse tree) which is then optimized before being run.
Since version 5.005, Perl has shipped with a module capable of inspect-
ing the optimized parse tree ("B"), and this has been used to write
many useful utilities, including a module that lets you turn your Perl
into C source code that can be compiled into a native executable.
The "B" module provides access to the parse tree, and other modules
("back ends") do things with the tree. Some write it out as bytecode,
C source code, or a semi-human-readable text. Another traverses the
parse tree to build a cross-reference of which subroutines, formats,
and variables are used where. Another checks your code for dubious
constructs. Yet another back end dumps the parse tree back out as Perl
source, acting as a source code beautifier or deobfuscator.
Because its original purpose was to be a way to produce C code corre-
sponding to a Perl program, and in turn a native executable, the "B"
module and its associated back ends are known as "the compiler", even
though they don't really compile anything. Different parts of the com-
piler are more accurately a "translator", or an "inspector", but people
want Perl to have a "compiler option" not an "inspector gadget". What
can you do?
This document covers the use of the Perl compiler: which modules it
comprises, how to use the most important of the back end modules, what
problems there are, and how to work around them.
Layout
The compiler back ends are in the "B::" hierarchy, and the front-end
(the module that you, the user of the compiler, will sometimes interact
with) is the O module. Some back ends (e.g., "B::C") have programs
(e.g., perlcc) to hide the modules' complexity.
Here are the important back ends to know about, with their status
expressed as a number from 0 (outline for later implementation) to 10
(if there's a bug in it, we're very surprised):
B::Bytecode
Stores the parse tree in a machine-independent format, suitable for
later reloading through the ByteLoader module. Status: 5 (some
things work, some things don't, some things are untested).
B::C
Creates a C source file containing code to rebuild the parse tree
and resume the interpreter. Status: 6 (many things work ade-
quately, including programs using Tk).
B::CC
Creates a C source file corresponding to the run time code path in
the parse tree. This is the closest to a Perl-to-C translator
there is, but the code it generates is almost incomprehensible
because it translates the parse tree into a giant switch structure
that manipulates Perl structures. Eventual goal is to reduce
(given sufficient type information in the Perl program) some of the
Perl data structure manipulations into manipulations of C-level
ints, floats, etc. Status: 5 (some things work, including uncom-
plicated Tk examples).
B::Lint
Complains if it finds dubious constructs in your source code. Sta-
tus: 6 (it works adequately, but only has a very limited number of
areas that it checks).
B::Deparse
Recreates the Perl source, making an attempt to format it coher-
ently. Status: 8 (it works nicely, but a few obscure things are
missing).
B::Xref
Reports on the declaration and use of subroutines and variables.
Status: 8 (it works nicely, but still has a few lingering bugs).
Using The Back Ends
The following sections describe how to use the various compiler back
ends. They're presented roughly in order of maturity, so that the most
stable and proven back ends are described first, and the most experi-
mental and incomplete back ends are described last.
The O module automatically enabled the -c flag to Perl, which prevents
Perl from executing your code once it has been compiled. This is why
all the back ends print:
myperlprogram syntax OK
before producing any other output.
The Cross Referencing Back End
The cross referencing back end (B::Xref) produces a report on your pro-
gram, breaking down declarations and uses of subroutines and variables
(and formats) by file and subroutine. For instance, here's part of the
report from the pod2man program that comes with Perl:
Subroutine clear_noremap
Package (lexical)
$ready_to_print i1069, 1079
Package main
$& 1086
$. 1086
$0 1086
$1 1087
$2 1085, 1085
$3 1085, 1085
$ARGV 1086
%HTML_Escapes 1085, 1085
This shows the variables used in the subroutine "clear_noremap". The
variable $ready_to_print is a my() (lexical) variable, introduced
(first declared with my()) on line 1069, and used on line 1079. The
variable $& from the main package is used on 1086, and so on.
A line number may be prefixed by a single letter:
i Lexical variable introduced (declared with my()) for the first
time.
& Subroutine or method call.
s Subroutine defined.
r Format defined.
The most useful option the cross referencer has is to save the report
to a separate file. For instance, to save the report on myperlprogram
to the file report:
$ perl -MO=Xref,-oreport myperlprogram
The Decompiling Back End
The Deparse back end turns your Perl source back into Perl source. It
can reformat along the way, making it useful as a de-obfuscator. The
most basic way to use it is:
$ perl -MO=Deparse myperlprogram
You'll notice immediately that Perl has no idea of how to paragraph
your code. You'll have to separate chunks of code from each other with
newlines by hand. However, watch what it will do with one-liners:
$ perl -MO=Deparse -e '$op=shift||die "usage: $0
code [...]";chomp(@ARGV=<>)unless@ARGV; for(@ARGV){$was=$_;eval$op;
die$@ if$@; rename$was,$_ unless$was eq $_}'
-e syntax OK
$op = shift @ARGV || die("usage: $0 code [...]");
chomp(@ARGV = <ARGV>) unless @ARGV;
foreach $_ (@ARGV) {
$was = $_;
eval $op;
die $@ if $@;
rename $was, $_ unless $was eq $_;
}
The decompiler has several options for the code it generates. For
instance, you can set the size of each indent from 4 (as above) to 2
with:
$ perl -MO=Deparse,-si2 myperlprogram
The -p option adds parentheses where normally they are omitted:
$ perl -MO=Deparse -e 'print "Hello, world\n"'
-e syntax OK
print "Hello, world\n";
$ perl -MO=Deparse,-p -e 'print "Hello, world\n"'
-e syntax OK
print("Hello, world\n");
See B::Deparse for more information on the formatting options.
The Lint Back End
The lint back end (B::Lint) inspects programs for poor style. One pro-
grammer's bad style is another programmer's useful tool, so options let
you select what is complained about.
To run the style checker across your source code:
$ perl -MO=Lint myperlprogram
To disable context checks and undefined subroutines:
$ perl -MO=Lint,-context,-undefined-subs myperlprogram
See B::Lint for information on the options.
The Simple C Back End
This module saves the internal compiled state of your Perl program to a
C source file, which can be turned into a native executable for that
particular platform using a C compiler. The resulting program links
against the Perl interpreter library, so it will not save you disk
space (unless you build Perl with a shared library) or program size.
It may, however, save you startup time.
The "perlcc" tool generates such executables by default.
perlcc myperlprogram.pl
The Bytecode Back End
This back end is only useful if you also have a way to load and execute
the bytecode that it produces. The ByteLoader module provides this
functionality.
To turn a Perl program into executable byte code, you can use "perlcc"
with the "-b" switch:
perlcc -b myperlprogram.pl
The byte code is machine independent, so once you have a compiled mod-
ule or program, it is as portable as Perl source (assuming that the
user of the module or program has a modern-enough Perl interpreter to
decode the byte code).
See B::Bytecode for information on options to control the optimization
and nature of the code generated by the Bytecode module.
The Optimized C Back End
The optimized C back end will turn your Perl program's run time code-
path into an equivalent (but optimized) C program that manipulates the
Perl data structures directly. The program will still link against the
Perl interpreter library, to allow for eval(), "s///e", "require", etc.
The "perlcc" tool generates such executables when using the -opt
switch. To compile a Perl program (ending in ".pl" or ".p"):
perlcc -opt myperlprogram.pl
To produce a shared library from a Perl module (ending in ".pm"):
perlcc -opt Myperlmodule.pm
For more information, see perlcc and B::CC.
Module List for the Compiler Suite
B This module is the introspective ("reflective" in Java terms) mod-
ule, which allows a Perl program to inspect its innards. The back
end modules all use this module to gain access to the compiled
parse tree. You, the user of a back end module, will not need to
interact with B.
O This module is the front-end to the compiler's back ends. Normally
called something like this:
$ perl -MO=Deparse myperlprogram
This is like saying "use O 'Deparse'" in your Perl program.
B::Asmdata
This module is used by the B::Assembler module, which is in turn
used by the B::Bytecode module, which stores a parse-tree as byte-
code for later loading. It's not a back end itself, but rather a
component of a back end.
B::Assembler
This module turns a parse-tree into data suitable for storing and
later decoding back into a parse-tree. It's not a back end itself,
but rather a component of a back end. It's used by the assemble
program that produces bytecode.
B::Bblock
This module is used by the B::CC back end. It walks "basic
blocks". A basic block is a series of operations which is known to
execute from start to finish, with no possibility of branching or
halting.
B::Bytecode
This module is a back end that generates bytecode from a program's
parse tree. This bytecode is written to a file, from where it can
later be reconstructed back into a parse tree. The goal is to do
the expensive program compilation once, save the interpreter's
state into a file, and then restore the state from the file when
the program is to be executed. See "The Bytecode Back End" for
details about usage.
B::C
This module writes out C code corresponding to the parse tree and
other interpreter internal structures. You compile the correspond-
ing C file, and get an executable file that will restore the inter-
nal structures and the Perl interpreter will begin running the pro-
gram. See "The Simple C Back End" for details about usage.
B::CC
This module writes out C code corresponding to your program's oper-
ations. Unlike the B::C module, which merely stores the inter-
preter and its state in a C program, the B::CC module makes a C
program that does not involve the interpreter. As a consequence,
programs translated into C by B::CC can execute faster than normal
interpreted programs. See "The Optimized C Back End" for details
about usage.
B::Concise
This module prints a concise (but complete) version of the Perl
parse tree. Its output is more customizable than the one of
B::Terse or B::Debug (and it can emulate them). This module useful
for people who are writing their own back end, or who are learning
about the Perl internals. It's not useful to the average program-
mer.
B::Debug
This module dumps the Perl parse tree in verbose detail to STDOUT.
It's useful for people who are writing their own back end, or who
are learning about the Perl internals. It's not useful to the
average programmer.
B::Deparse
This module produces Perl source code from the compiled parse tree.
It is useful in debugging and deconstructing other people's code,
also as a pretty-printer for your own source. See "The Decompiling
Back End" for details about usage.
B::Disassembler
This module turns bytecode back into a parse tree. It's not a back
end itself, but rather a component of a back end. It's used by the
disassemble program that comes with the bytecode.
B::Lint
This module inspects the compiled form of your source code for
things which, while some people frown on them, aren't necessarily
bad enough to justify a warning. For instance, use of an array in
scalar context without explicitly saying "scalar(@array)" is some-
thing that Lint can identify. See "The Lint Back End" for details
about usage.
B::Showlex
This module prints out the my() variables used in a function or a
file. To get a list of the my() variables used in the subroutine
mysub() defined in the file myperlprogram:
$ perl -MO=Showlex,mysub myperlprogram
To get a list of the my() variables used in the file myperlprogram:
$ perl -MO=Showlex myperlprogram
[BROKEN]
B::Stackobj
This module is used by the B::CC module. It's not a back end
itself, but rather a component of a back end.
B::Stash
This module is used by the perlcc program, which compiles a module
into an executable. B::Stash prints the symbol tables in use by a
program, and is used to prevent B::CC from producing C code for the
B::* and O modules. It's not a back end itself, but rather a com-
ponent of a back end.
B::Terse
This module prints the contents of the parse tree, but without as
much information as B::Debug. For comparison, "print "Hello,
world."" produced 96 lines of output from B::Debug, but only 6
from B::Terse.
This module is useful for people who are writing their own back
end, or who are learning about the Perl internals. It's not useful
to the average programmer.
B::Xref
This module prints a report on where the variables, subroutines,
and formats are defined and used within a program and the modules
it loads. See "The Cross Referencing Back End" for details about
usage.
KNOWN PROBLEMS
The simple C backend currently only saves typeglobs with alphanumeric
names.
The optimized C backend outputs code for more modules than it should
(e.g., DirHandle). It also has little hope of properly handling "goto
LABEL" outside the running subroutine ("goto &sub" is okay). "goto
LABEL" currently does not work at all in this backend. It also creates
a huge initialization function that gives C compilers headaches.
Splitting the initialization function gives better results. Other
problems include: unsigned math does not work correctly; some opcodes
are handled incorrectly by default opcode handling mechanism.
BEGIN{} blocks are executed while compiling your code. Any external
state that is initialized in BEGIN{}, such as opening files, initiating
database connections etc., do not behave properly. To work around
this, Perl has an INIT{} block that corresponds to code being executed
before your program begins running but after your program has finished
being compiled. Execution order: BEGIN{}, (possible save of state
through compiler back-end), INIT{}, program runs, END{}.
AUTHOR
This document was originally written by Nathan Torkington, and is now
maintained by the perl5-porters mailing list perl5-porters@perl.org.
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