[Python-Dev] proto-pep: How to change CPython's bytecode

Brett C. bac at OCF.Berkeley.EDU
Thu Jan 6 20:50:22 CET 2005

OK, latest update with all suggest revisions (mention this is for CPython, 
section for known previous bytecode work).

If no one has any revisions I will submit to David for official PEP acceptance 
this weekend.


Title: How to change CPython's bytecode
Version: $Revision: 1.4 $
Last-Modified: $Date: 2003/09/22 04:51:50 $
Author: Brett Cannoon <brett at python.org>
Status: Draft
Type: Informational
Content-Type: text/x-rst
Created: XX-XXX-XXXX
Post-History: XX-XXX-XXXX


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).


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.


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 that have a value 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 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

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
Skip Montanaro presented a paper at a Python workshop on a peephole optimizer

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


.. [#lang-ref] Python Language Reference, van Rossum & Drake

.. [#PEP-306] PEP 306, How to Change Python's Grammar, Hudson

.. [#dis] dis Module

.. [#test-suite] 'test' package

.. [#parrotbench] Parrotbench

.. [#opcode-list] Python Byte Code Instructions

.. [#skip-peephole]

.. [#Bytecodehacks]


This document has been placed in the public domain.

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