python-peps/pep-0339.txt

PEP: 339
Title: How to Change CPython's Bytecode
Version: $Revision$
Last-Modified: $Date$
Author: Brett Cannon <brett@python.org>
Status: Active
Type: Informational
Content-Type: text/x-rst
Created: 02-Feb-2005
Post-History: 02-Feb-2005


Abstract
========

Python source code is compiled down to something called bytecode.  This
bytecode must implement enough semantics to perform the actions required by the
Language Reference [#lang_ref]_.  As such, knowing how to add, remove, or change
the bytecode is important to do properly when changing the abilities of the
Python language.
This PEP covers how to accomplish this in the CPython implementation of the
language (referred to as simply "Python" for the rest of this PEP).

.. warning::
     The guidelines outlined in this PEP apply to Python 2.4 and earlier.
     Current plans for Python 2.5 will lead to a significant change in how
     Python's bytecode is handled.
     This PEP will be updated once these planned changes are committed into
     CVS.


Rationale
=========

While changing Python's bytecode is not a frequent occurence, it still happens.
Having the required steps documented in a single location should make
experimentation with the bytecode easier since it is not necessarily obvious
what the steps are to change the bytecode.

This PEP, paired with PEP 306 [#PEP-306]_, should provide enough basic
guidelines for handling any changes performed to the Python language itself in
terms of syntactic changes that introduce new semantics.


Checklist
=========

This is a rough checklist of what files need to change and how they are
involved with the bytecode.  All paths are given from the viewpoint of
``/cvsroot/python/dist/src`` from CVS).  This list should not be considered
exhaustive nor to cover all possible situations.

- ``Include/opcode.h``
	This include file lists all known opcodes and associates each opcode
	name with
	a unique number.  When adding a new opcode it is important to take note
	of the ``HAVE_ARGUMENT`` value.  This ``#define``'s value specifies the
	value at which all opcodes greater than ``HAVE_ARGUMENT`` are expected
	to take an argument to the opcode.

- ``Lib/opcode.py``
	Lists all of the opcodes and their associated value.  Used by the dis
	module [#dis]_ to map bytecode values to their names.

- ``Python/ceval.c``
	Contains the main interpreter loop.  Code to handle the evalution of an
	opcode goes here.

- ``Python/compile.c``
	To make sure an opcode is actually used, this file must be altered.
	The emitting of all bytecode occurs here.

- ``Lib/compiler/pyassem.py``, ``Lib/compiler/pycodegen.py``
	The 'compiler' package [#compiler]_ needs to be altered to also reflect
	any changes to the bytecode.

- ``Doc/lib/libdis.tex``
	The documentation [#opcode_list]_ for the dis module contains a complete
	list of all the opcodes.

- ``Python/import.c``
	Defines the magic word (named ``MAGIC``) used in .pyc files to detect if
	the bytecode used matches the one used by the version of Python running.
	This number needs to be changed to make sure that the running
	interpreter does not try to execute bytecode that it does not know
	about.


Suggestions for bytecode development
====================================

A few things can be done to make sure that development goes smoothly when
experimenting with Python's bytecode.  One is to delete all .py(c|o) files
after each semantic change to Python/compile.c .  That way all files will use
any bytecode changes.

Make sure to run the entire testing suite [#test-suite]_.  Since the
``regrtest.py`` driver recompiles all source code before a test is run it acts
a good test to make sure that no existing semantics are broken.

Running parrotbench [#parrotbench]_ is also a good way to make sure existing
semantics are not broken; this benchmark is practically a compliance test.


Previous experiments
====================
This section lists known bytecode experiments that have not gone into Python.

Skip Montanaro presented a paper at a Python workshop on a peephole optimizer
[#skip-peephole]_.

Michael Hudson has a non-active SourceForge project named Bytecodehacks
[#Bytecodehacks]_ that provides functionality for playing with bytecode
directly.

An opcode to combine the functionality of LOAD_ATTR/CALL_FUNCTION was created
named CALL_ATTR [#CALL_ATTR]_.  Currently only works for classic classes and
for new-style classes rough benchmarking showed an actual slowdown thanks to
having to support both classic and new-style classes.


References
==========

.. [#lang_ref] Python Language Reference, van Rossum & Drake
   (http://docs.python.org/ref/ref.html)

.. [#PEP-306] PEP 306, How to Change Python's Grammar, Hudson
   (http://www.python.org/peps/pep-0306.html)

.. [#dis] dis Module
   (http://docs.python.org/lib/module-dis.html)

.. [#compiler] 'compiler' Package
   (http://docs.python.org/lib/module-compiler.html)

.. [#test-suite] 'test' Package
   (http://docs.python.org/lib/module-test.html)

.. [#opcode_list] Python Byte Code Instructions
   (http://docs.python.org/lib/bytecodes.html)

.. [#parrotbench] Parrotbench
   (ftp://ftp.python.org/pub/python/parrotbench/parrotbench.tgz,
   http://mail.python.org/pipermail/python-dev/2003-December/041527.html)

.. [#skip-peephole] Skip Montanaro's Peephole Optimizer Paper
   (http://www.foretec.com/python/workshops/1998-11/proceedings/papers/montanaro/montanaro.html)

.. [#Bytecodehacks] Bytecodehacks Project
   (http://bytecodehacks.sourceforge.net/bch-docs/bch/index.html)

.. [#CALL_ATTR] CALL_ATTR opcode
   (http://www.python.org/sf/709744)


Copyright
=========

This document has been placed in the public domain.



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