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PERLCOMPILE(1) Perl Programmers Reference Guide 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 any-
thing. 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, break-
ing 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. How-
ever, 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 com-
plained 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 byte-
code 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 pro-
gram, 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 decod-
ing 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 inter-
preter internal structures. You compile the corresponding C file, and get an
executable file that will restore the internal structures and the Perl inter-
preter 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 inter-
preter. 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 use-
ful 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 exe-
cutable. 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 informa-
tion 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., DirHan-
dle). It also has little hope of properly handling "goto LABEL" outside the run-
ning 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 com-
pilers 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 AT perl.org.
perl v5.8.8 2006-01-07 PERLCOMPILE(1)
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