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PERLCOMPILE(1)
NAME
perlcompile - Introduction to the Perl Compiler-Translator
DESCRIPTION
Perl has always had a compiler: your source is compiled into an internal
form (a parse tree) which is then optimized before being run. Since
version 5.005, Perl has shipped with a module capable of inspecting 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
corresponding 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 compiler
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 adequately,
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 uncomplicated Tk
examples).
B::Lint
Complains if it finds dubious constructs in your source code. Status:
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 coherently.
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 experimental 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 program,
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
programmer'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 module 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 -O switch. To
compile a Perl program (ending in ".pl" or ".p"):
perlcc -O myperlprogram.pl
To produce a shared library from a Perl module (ending in ".pm"):
perlcc -O 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) module,
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 bytecode 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 corresponding C file,
and get an executable file that will restore the internal structures
and the Perl interpreter will begin running the program. See "The
Simple C Back End" for details about usage.
B::CC
This module writes out C code corresponding to your program's
operations. Unlike the B::C module, which merely stores the
interpreter 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 programmer.
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 something 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 component 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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