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, "optree") which is then optimized before being run. Since version 5.005, Perl has shipped with a module capable of inspecting the optimized optree (B), and this has been used to write many useful utilities, including the B::C and B::CC modules 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 optree, and other modules ("backends") do things with the tree. Some write it out as bytecode B::Bytecode, C source code B::C, or a semi-human-readable text. Another B::Xref traverses the parse tree to build a cross-reference of which subroutines, formats, and variables are used where. Another B::Lint checks your code for dubious constructs. B::Deparse 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, mainly B::C, B::CC and B::Bytecode are known as "the compiler", even though they don't really compile anything.

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.

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: 7 (5.18 and higher is broken).

The B::C backend

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

Status: 9 (most applications work out of the box, some need cmdline flags)

5.14 works best. See STATUS

B::C is stable and used in production for -O0 and -O3. For some programs not all methods in certain packages or eval strings might be detected, you'd need to add them manually via -u<packagename>.

Still missing in general are:

  - attribute handlers (i.e. run-time attributes)
  - compile-time perlio layers
  - re-eval groups (?{})
  - smartmatch subrefs
  - compile-time stash-magic delete renames to ANON

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 applications work).

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"
        -o<filename>    Output to filename instead of STDOUT
        -v              Be verbose. Currently gives a few compilation statistics.
        --              Force end of options
        -u<package>     use package or filename. Force apparently unused subs from
                        package to be compiled. This allows programs to use run-time
                        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 or -U options.
        -U<package>     Unuse package or filename. Ignore all subs from package to be compiled.
                        Certain packages might not be needed at run-time, even if the
                        pessimistic walker detects it. If required those packages will be
                        run-time loaded then.
        -e ARG          Eval ARG at startup
        -c              Check and abort (used to print warnings)
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::scalar )
                ppaddr          Optimize the initialization of op_ppaddr.
                warn-sv         Optimize the initialization of cop_warnings.
                av-init         Faster initialization of AVs.
                av-init2        Initialization of AVs via ptmalloc3 independent_comalloc().
                use-script-name Use the script name instead of the program name as $0.
                ro-inc          Readonly @INC and %INC pathnames.
                const-strings   Declares static readonly strings as const.
                save-sig-hash   Save compile-time modifications to the %SIG hash.
                no-destruct     Faster destruction.
                no-fold         Do not compile unicode foldings tables, needed for m//i
                no-warnings     Do not compile warnings hashes.
                stash           Add all stash hashes even if not used.
                no-delete-pkg   Do not delete compiler-internal and dependent packages.
                no-dyn-padlist  Disable dynamic padlists (5.18). This is faster but might cause
                                die and exit to crash.
                cop             Omit COP, no file+line info for warnings
        -On             Optimisation level (n = 0, 1, 2, ...). -O means -O1.
                -O1     -fcog -fav-init2/-fav-init -fppaddr -fwarn-sv
                -O2     -O1 -fro-inc -fsave-data
                -O3     -O2 -fsave-sig-hash -fno-destruct fconst-strings
                -O4     -O3 -fcop -fno-dyn-padlist

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. B::C takes all B::C options plus a few new ones:

        -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

All B::C -O3 optimisations are automatically used.

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 main code in use'd modules, 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{}.

Yet unsupported:

  - attribute handlers (i.e. run-time attributes)
  - compile-time perlio layers
  - re-eval groups (?{})
  - smartmatch subrefs
  - compile-time stash-magic delete renames to ANON

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

See STATUS

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". Work is planned to revive "undump".

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.