perlcompile - Introduction to the Perl Compiler-Translator
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-humanreadable
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 [Toc] [Back]
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).
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 [Toc] [Back]
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, world0'
-e syntax OK
print "Hello, world0;
$ perl -MO=Deparse,-p -e 'print "Hello, world0'
-e syntax OK
print("Hello, world0);
See B::Deparse for more information on the formatting
options.
The Lint Back End [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back] 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 prettyprinter
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.
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{}.
This document was originally written by Nathan Torkington,
and is now maintained by the perl5-porters mailing list
[email protected].
perl v5.8.5 2002-11-06 9 [ Back ] |
