MirOS Manual: perlcompile(1)


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 utili-
     ties, 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 con-
     structs.  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 com-
     pile 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

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         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 manipu-
         lates Perl structures.  Eventual goal is to reduce
         (given sufficient type information in the Perl program)
         some of the Perl data structure manipulations into mani-
         pulations 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 com-
     piler 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.

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     The Cross Referencing Back End

     The cross referencing back end (B::Xref) produces a report
     on your program, breaking down declarations and uses of sub-
     routines 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

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     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 gen-
     erates.  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 omit-
     ted:

       $ 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

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     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"):

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       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 pro-
         gram.

     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

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         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 pro-
         gram, 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 nor-
         mal 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 inter-
         nals.  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

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         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 com-
         piles 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 com-
         ponent of a back end.

     B::Terse
         This module prints the contents of the parse tree, but
         without as much information as B::Debug.  For com-
         parison, "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 inter-
         nals.  It's not useful to the average programmer.

     B::Xref
         This module prints a report on where the variables, sub-
         routines, and formats are defined and used within a pro-
         gram and the modules it loads.  See "The Cross Referenc-
         ing 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 subrou-
     tine ("goto &sub" is okay). "goto LABEL" currently does not
     work at all in this backend. It also creates a huge initial-
     ization function that gives C compilers headaches.

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     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 open-
     ing 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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