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 ("backends") 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 backend 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 backends 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 backend modules, what problems there are, and how to work around them.

Other perl to exe compilers

Maybe you want to look for the free PAR module or some commercial products, like perl2exe at http://www.indigostar.com/perl2exe.htm and perlapp as PerlDevKit from ActiveState at http://www.activestate.com/Products/perl_dev_kit/

These are technically no compilers, just source packagers with a simple native code unpacker. Run-time behaviour is actually slower than with a normal perl source or real compiler, because of the additional unpacking and check steps. It's just convenient to have single file applications.

The simpliest windows "compiler" would be then pl2exe.pl in C::DynaLib.

Several years ago the undump functionality used to work on several platforms. See perlrun for -u.

Layout

The compiler backends 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 backends (e.g., B::C) have programs (e.g., perlcc) to hide the modules' complexity.

Since Perl 5.10 the three code-producing backends (B::C, B::CC and B::Bytecode), aka the compiler, have been removed from CORE Perl and are available as seperate CPAN module http://search.cpan.org/dist/B-C/.

Here are the important backends to know about, with their status expressed as a number from 0 (outline for later implementation) to 10:

The B::Bytecode backend

Stores the parse tree in a machine-independent format, suitable for later reloading through the "ByteLoader" module. Status: 5 (some things work until 5.8.x, some things don't, some things are untested).

The B::C backend

Creates a C source file containing code to rebuild the parse tree and resume the interpreter.

Status: 6 (many things work adequately until 5.8.x, including programs using Tk).

5.6: works perfectly 5.8: minor bugs (qr//) 5.10: two major bugs: autoloaded subs, magic GVs (sort $a, tie FETCH) 5.11: order of evaluation in regex replacements, split

The B::CC backend

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 until 5.8.x, including uncomplicated Tk examples).

Same bugs as in B::C, plus 5.6: none 5.8: none 5.10 and higher: eval (test 12)

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 Backends

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 backends are described first, and the most experimental and incomplete backends 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 backends print:

myperlprogram syntax OK

before producing any other output.

The Cross Referencing Backend

The cross referencing backend (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 Backend

The Deparse backend 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 Backend

The lint backend 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 Backend

The B::C 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

C Backend Invocation

If there are any non-option arguments, they are taken to be names of objects to be saved (probably doesn't work properly yet). Without extra arguments, it saves the main program.

        -q 		Be quiet. STDOUT goes to $O::BEGIN_output
        -qq		Be very quiet. Also suppress "Syntax OK"
	-ofilename	Output to filename instead of STDOUT
	-v		Be verbose. Currently gives a few compilation statistics.
	--		Force end of options
	-uPackname	Force apparently unused subs from package Packname to
			be compiled. This allows programs to use eval "foo()"
			even when sub foo is never seen to be used at compile
			time. The down side is that any subs which really are
			never used also have code generated. This option is
			necessary, for example, if you have a signal handler
			foo which you initialise with $SIG{BAR} = "foo".
			A better fix, though, is just to change it to
			$SIG{BAR} = \&foo. You can have multiple -u options.
	-e ARG		Eval ARG at startup
NYI	-w		Warn on undefined SYMs
	-l LIMIT	Force max linelength to LIMIT (e.g. MSVC to 2048)
	-D		Debug options (concat or separate flags like perl -D)
		o	Print walkoptree OPs
		O	Prints more OP information
		c	COPs, prints COPs as processed (incl. file & line num)
		S	prints SV/RE information on saving
		A	prints AV information on saving
		C	prints CV information on saving
		M	prints MAGIC information on saving
		G	prints GV information on saving
		u       Do not print -D information when parsing unused subs.
	-f		Force optimisations on or off one at a time.
		cog		Copy-on-grow: PVs declared and initialised statically
		no-cog		No copy-on-grow
		save-data	Save package::DATA filehandles 
				( only available with PerlIO )
		ppaddr		Optimize the initialization of op_ppaddr.
		warn-sv		Optimize the initialization of cop_warnings.
		av-init		Faster initialization of AVs
		use-script-name	Use the script name instead of the program name as $0.
		save-sig-hash	Save compile-time modifications to the %SIG hash.
		cop		Omit COP, no file+line info for warnings
	-On		Optimisation level (n = 0, 1, 2, ...). -O means -O1.
	  	-O1	-fcog
		-O2	-O1 -fcog -fppaddr -fwarn-sv -fav-init
		-O3     -O2 -fsave-sig-hash -fsave-data 
		-O4     -O3 -fcop

C Examples perl -MO=C foo.pl > foo.c perl cc_harness -o foo foo.c

perl -MO=C,-v,-DcA bar.pl > /dev/null

For more information, see perlcc and B::C.

The Bytecode Backend

This backend 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).

Bytecode Backend Invocation

If there are any non-option arguments, they are taken to be names of objects to be saved (probably doesn't work properly yet). Without extra arguments, it saves the main program.

        -q 		Be quiet. STDOUT goes to $O::BEGIN_output
        -qq		Be very quiet. Also suppress "Syntax OK"
	-ofilename	Output to filename instead of STDOUT.
NYI	-v              Be verbose.
	--		Force end of options.
NYI	-f		Force optimisations on or off one at a time.
			Each can be preceded by no- to turn the option off.
		compress-nullops
			Only fills in the necessary fields of ops which have
			been optimised away by perl's internal compiler.
		omit-sequence-numbers
			Leaves out code to fill in the op_seq field of all ops
			which is only used by perl's internal compiler.
		bypass-nullops
			If op->op_next ever points to a NULLOP, replaces the
			op_next field with the first non-NULLOP in the path
			of execution.
	-s              strip-syntax-tree
			Leaves out code to fill in the pointers which link the
			internal syntax tree together. They're not needed at
			run-time but leaving them out will make it impossible
			to recompile or disassemble the resulting program.
			It will also stop "goto label" statements from working.
NYI	-On		Optimisation level (n = 0, 1, 2, ...). -O means -O1.
			-O1 sets -fcompress-nullops -fomit-sequence numbers.
			-O6 adds -fstrip-syntax-tree.
NYI	-D		Debug options (concat or separate flags like perl -D)
		O	OPs, prints each OP as it's processed.
		b	print debugging information about bytecompiler progress
		a	tells the assembler to include source assembler lines
			in its output as bytecode comments.
		C	prints each CV taken from the final symbol tree walk.
	-S		Output assembler source rather than piping it
			through the assembler and outputting bytecode.
	-H		add #! perl shebang header
	-s		scan and keep keep syntax tree if goto op found.
			scan the script for C<# line ..> directives and for <goto LABEL>
			expressions. When gotos are found keep the syntax tree.
	-b		Save all the BEGIN blocks. Normally only BEGIN blocks that require
			other files (ex. use Foo;) are saved.
	-k		keep syntax tree to disassemble the plc.
			it is stripped by default.
	-TI		testing, dump the @INC av
	-TF     file	testing, sets COP::file	
   	-m		Compile as a module rather than a standalone program.
			Currently this just means that the bytecodes for
			initialising main_start, main_root and curpad are
			omitted.

Bytecode Invocation Examples

perl -MO=Bytecode,-O6,-H,-ofoo.plc foo.pl
./foo.plc

perl -MO=Bytecode,-S foo.pl > foo.S
assemble foo.S > foo.plc
perl -MByteLoader foo.plc

perl -MO=Bytecode,-m,-oFoo.pmc Foo.pm

The Optimized C Backend

The B::CC optimized C backend 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

CC Backend Invocation

If there are any non-option arguments, they are taken to be names of subs to be saved. Without extra arguments, it saves the main program.

        -q 		Be quiet. STDOUT goes to $O::BEGIN_output
        -qq		Be very quiet. Also suppress "Syntax OK"
	-ofilename	Output to filename instead of STDOUT
	-v              Be verbose.
	--		Force end of options
NYI	-pn		Generate code for perl version n (e.g. 5.008007).
			The default is to generate C code which will link
			with the currently executing version of perl,
			running the perl compiler.
	-mModulename	Instead of generating source for a runnable executable,
			generate source for an XSUB module. The
			boot_Modulename function (which DynaLoader can look
			for) does the appropriate initialisation and runs the
			main part of the Perl source that is being compiled.
	-uPackname	Force apparently unused subs from package Packname to
			be compiled. This allows programs to use eval "foo()"
			even when sub foo is never seen to be used at compile
			time. The down side is that any subs which really are
			never used also have code generated. This option is
			necessary, for example, if you have a signal handler
			foo which you initialise with $SIG{BAR} = "foo".
			A better fix, though, is just to change it to
			$SIG{BAR} = \&foo. You can have multiple -u options.
	-e ARG		Eval ARG at startup
NYI	-w		Warn on undefined SYMs
	-l LIMIT	Force max linelength to LIMIT (e.g. MSVC to 2048)
	-D		Debug options (concat or separate flags like perl -D)
		o	Enable B debugging
		r	Writes debugging output to STDERR just as it's about
			to write to the program's runtime. Otherwise writes
			debugging info as comments in its C output.
		O	Outputs each OP as it's compiled
		s	Outputs the contents of the shadow stack at each OP
		p	Outputs the contents of the shadow pad of lexicals as
			it's loaded for each sub or the main program.
		q	Outputs the name of each fake PP function in the queue
			as it's about to processes.
		l	Output the filename and line number of each original
			line of Perl code as it's processed (pp_nextstate).
		t	Outputs timing information of compilation stages
	-f		Force optimisations on or off one at a time.
		cog	Copy-on-grow: PVs declared and initialised statically
                freetmps-each-bblock   Delays FREETMPS from the end of each
                                       statement to the end of the each basic
                                       block.
                freetmps-each-loop     Delays FREETMPS from the end of each
                                       statement to the end of the group of
                                       basic blocks forming a loop. At most
                                       one of the freetmps-each-* options can
                                       be used.
                no-inline-ops          Turn off aggressive inlining of ops
                omit-taint             Omits generating code for handling
                                       perl's tainting mechanism.
	-On		Optimisation level (n = 0, 1, 2, ...). -O means -O1.
	  	-O1	-ffreetmps-each-bblock
		-O2	-O1 -ffreetmps-each-loop

The following B::C optimisations are automatically used: -fwarn-sv -fsave-data -fav-init -fsave-sig-hash and for Perl < 5.10 -fcog.

CC Invocation Example

perl -MO=CC,-O2,-ofoo.c foo.pl
perl cc_harness -o foo foo.c

perl -MO=CC,-mFoo,-oFoo.c Foo.pm
perl cc_harness -shared -c -o Foo.so Foo.c

perlcc -O myperlprogram.pl
perlcc -O MyperlModule.pm

Backends For Debugging

perl -MO=Terse,exec foo.pl
perl -MO=Debug bar.pl

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 backend modules all use this module to gain access to the compiled parse tree. You, the user of a backend module, will not need to interact with B.

O

This module is the front-end to the compiler's backends. Normally called something like this:

$ perl -MO=Deparse,-q myperlprogram

This is like saying use O 'Deparse' qw(-q) in your Perl program.

Used with "perl -MO=Backend,-foo,-obar prog.pl" to invoke the backend B::Backend with options -foo and -obar. O invokes the sub B::Backend::compile() with arguments -foo and -obar at BEGIN time. That compile() sub must do any inital argument processing replied. If unsuccessful, it should return a string which O arranges to be printed as an error message followed by a clean error exit. In the normal case where any option processing in compile() is successful, it should return a sub ref (usually a closure) to perform the actual compilation. When O regains control, it ensures that the "-c" option is forced (so that the program being compiled doesn't end up running) and registers a CHECK block to call back the sub ref returned from the backend's compile(). Perl then continues by parsing prog.pl (just as it would with "perl -c prog.pl") and after doing so, assuming there are no parse-time errors, the CHECK block of O gets called and the actual backend compilation happens. Phew.

ByteLoader

This run-time module parses and executes the binary bytecode produced by "B::Bytecode". These are normally .plc for scripts and .pmc files for modules.

Note that Perl CORE favors .pmc over .pm files, so it would be wise to add the ByteLoader module in advance. Either statically linked into your perl (see Config{static_ext}) or with -MByteLoader on the command line.

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 backend itself, but rather a component of a backend.

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 backend itself, but rather a component of a backend. It's used by the assemble program that produces .plc bytecode.

B::Bblock

This module is used by the B::CC backend. 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 or jumps into inner ops.

B::Bytecode

This module is a backend that generates bytecode from a program's parse tree. This bytecode is written to a .plc 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 Backend" for details about usage.

With the -M switch you can also produce bytecode compiled modules as .pmc files, which if pesent in the @INC patch are favored over normal .pm files. You need to load the "ByteLoader" module then also, which is a problem, because it is not in CORE anymore.

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 Backend" 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 Backend" 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 backend, 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 backend, 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 Backend" for details about usage.

B::Disassembler

This module decodes .plc bytecode back into a readable parse-tree, the reverse of the "B::Assembler". It's not a backend itself, but rather a component of a backend. It's used by the disassemble program that produces 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 Backend" 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::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 backend, 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 Backend" for details about usage.

KNOWN PROBLEMS

BEGIN{} blocks are executed before 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{}.

CC backend: goto, sort with non-default comparison. last for non-loop blocks.

improve XSUB handling (both static and dynamic)

sv_magic can do SvREFCNT_inc(obj) which messes up precalculated refcounts.

allocation of XPV[INAHC]V structures needs fixing: Perl tries to free them, whereas the compiler expects them to be linked to a xpv[inahc]v_root

list the same as X[IPR]V structures.

ref counts

perl_parse replacement

fix cstring for long strings

compile-time initialisation of AvARRAYs

signed/unsigned problems with NV (and IV?) initialisation and elsewhere?

CvOUTSIDE for ordinary subs

DATA filehandle for standalone Bytecode program (easy)

DATA filehandle for multiple bytecode-compiled modules (harder)

DATA filehandle for C-compiled program (yet harder)

pad panics since 5.10

AUTHOR

This document was originally written by Nathan Torkington, and was maintained by the perl5-porters mailing list perl5-porters@perl.org up to Perl version 5.8.

This version with all the compiler options is now part of the B::C compiler module, maintained by Reini Urban rurban@cpan.org.

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