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.
PEP: XXX Title: How to change CPython's bytecode Version: $Revision: 1.4 $ Last-Modified: $Date: 2003/09/22 04:51:50 $ Author: Brett Cannoon firstname.lastname@example.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 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 ==================== 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.
.. [#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)
.. [#test-suite] 'test' package (http://docs.python.org/lib/module-test.html)
.. [#parrotbench] Parrotbench (ftp://ftp.python.org/pub/python/parrotbench/parrotbench.tgz, http://mail.python.org/pipermail/python-dev/2003-December/041527.html)
.. [#opcode-list] Python Byte Code Instructions (http://docs.python.org/lib/bytecodes.html)
.. [#Bytecodehacks] http://bytecodehacks.sourceforge.net/bch-docs/bch/index.html
This document has been placed in the public domain.
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