.. _compiler:
Design of CPython's Compiler
============================
Abstract
--------
In CPython, the compilation from source code to bytecode involves several steps:
1. Parse source code into a parse tree (:file:`Parser/pgen.c`)
2. Transform parse tree into an Abstract Syntax Tree (:file:`Python/ast.c`)
3. Transform AST into a Control Flow Graph (:file:`Python/compile.c`)
4. Emit bytecode based on the Control Flow Graph (:file:`Python/compile.c`)
The purpose of this document is to outline how these steps of the process work.
This document does not touch on how parsing works beyond what is needed
to explain what is needed for compilation. It is also not exhaustive
in terms of the how the entire system works. You will most likely need
to read some source to have an exact understanding of all details.
Parse Trees
-----------
Python's parser is an LL(1) parser mostly based off of the
implementation laid out in the Dragon Book [Aho86]_.
The grammar file for Python can be found in :file:`Grammar/Grammar` with the
numeric value of grammar rules stored in :file:`Include/graminit.h`. The
numeric values for types of tokens (literal tokens, such as ``:``,
numbers, etc.) are kept in :file:`Include/token.h`. The parse tree is made up
of ``node *`` structs (as defined in :file:`Include/node.h`).
Querying data from the node structs can be done with the following
macros (which are all defined in :file:`Include/node.h`):
``CHILD(node *, int)``
Returns the nth child of the node using zero-offset indexing
``RCHILD(node *, int)``
Returns the nth child of the node from the right side; use
negative numbers!
``NCH(node *)``
Number of children the node has
``STR(node *)``
String representation of the node; e.g., will return ``:`` for a
``COLON`` token
``TYPE(node *)``
The type of node as specified in :file:`Include/graminit.h`
``REQ(node *, TYPE)``
Assert that the node is the type that is expected
``LINENO(node *)``
retrieve the line number of the source code that led to the
creation of the parse rule; defined in :file:`Python/ast.c`
For example, consider the rule for 'while'::
while_stmt: 'while' test ':' suite ['else' ':' suite]
The node representing this will have ``TYPE(node) == while_stmt`` and
the number of children can be 4 or 7 depending on whether there is an
'else' statement. ``REQ(CHILD(node, 2), COLON)`` can be used to access
what should be the first ``:`` and require it be an actual ``:`` token.
Abstract Syntax Trees (AST)
---------------------------
.. sidebar:: Green Tree Snakes
See also `Green Tree Snakes - the missing Python AST docs
`_ by Thomas Kluyver.
The abstract syntax tree (AST) is a high-level representation of the
program structure without the necessity of containing the source code;
it can be thought of as an abstract representation of the source code. The
specification of the AST nodes is specified using the Zephyr Abstract
Syntax Definition Language (ASDL) [Wang97]_.
The definition of the AST nodes for Python is found in the file
:file:`Parser/Python.asdl`.
Each AST node (representing statements, expressions, and several
specialized types, like list comprehensions and exception handlers) is
defined by the ASDL. Most definitions in the AST correspond to a
particular source construct, such as an 'if' statement or an attribute
lookup. The definition is independent of its realization in any
particular programming language.
The following fragment of the Python ASDL construct demonstrates the
approach and syntax::
module Python
{
stmt = FunctionDef(identifier name, arguments args, stmt* body,
expr* decorators)
| Return(expr? value) | Yield(expr? value)
attributes (int lineno)
}
The preceding example describes two different kinds of statements and an
expression: function definitions, return statements, and yield expressions.
All three kinds are considered of type ``stmt`` as shown by ``|`` separating
the various kinds. They all take arguments of various kinds and amounts.
Modifiers on the argument type specify the number of values needed; ``?``
means it is optional, ``*`` means 0 or more, while no modifier means only one
value for the argument and it is required. ``FunctionDef``, for instance,
takes an ``identifier`` for the *name*, ``arguments`` for *args*, zero or more
``stmt`` arguments for *body*, and zero or more ``expr`` arguments for
*decorators*.
Do notice that something like 'arguments', which is a node type, is
represented as a single AST node and not as a sequence of nodes as with
stmt as one might expect.
All three kinds also have an 'attributes' argument; this is shown by the
fact that 'attributes' lacks a '|' before it.
The statement definitions above generate the following C structure type:
.. code-block:: c
typedef struct _stmt *stmt_ty;
struct _stmt {
enum { FunctionDef_kind=1, Return_kind=2, Yield_kind=3 } kind;
union {
struct {
identifier name;
arguments_ty args;
asdl_seq *body;
} FunctionDef;
struct {
expr_ty value;
} Return;
struct {
expr_ty value;
} Yield;
} v;
int lineno;
}
Also generated are a series of constructor functions that allocate (in
this case) a ``stmt_ty`` struct with the appropriate initialization. The
``kind`` field specifies which component of the union is initialized. The
``FunctionDef()`` constructor function sets 'kind' to ``FunctionDef_kind`` and
initializes the *name*, *args*, *body*, and *attributes* fields.
Memory Management
-----------------
Before discussing the actual implementation of the compiler, a discussion of
how memory is handled is in order. To make memory management simple, an arena
is used. This means that a memory is pooled in a single location for easy
allocation and removal. What this gives us is the removal of explicit memory
deallocation. Because memory allocation for all needed memory in the compiler
registers that memory with the arena, a single call to free the arena is all
that is needed to completely free all memory used by the compiler.
In general, unless you are working on the critical core of the compiler, memory
management can be completely ignored. But if you are working at either the
very beginning of the compiler or the end, you need to care about how the arena
works. All code relating to the arena is in either :file:`Include/pyarena.h` or
:file:`Python/pyarena.c`.
``PyArena_New()`` will create a new arena. The returned ``PyArena`` structure
will store pointers to all memory given to it. This does the bookkeeping of
what memory needs to be freed when the compiler is finished with the memory it
used. That freeing is done with ``PyArena_Free()``. This only needs to be
called in strategic areas where the compiler exits.
As stated above, in general you should not have to worry about memory
management when working on the compiler. The technical details have been
designed to be hidden from you for most cases.
The only exception comes about when managing a PyObject. Since the rest
of Python uses reference counting, there is extra support added
to the arena to cleanup each PyObject that was allocated. These cases
are very rare. However, if you've allocated a PyObject, you must tell
the arena about it by calling ``PyArena_AddPyObject()``.
Parse Tree to AST
-----------------
The AST is generated from the parse tree (see :file:`Python/ast.c`) using the
function ``PyAST_FromNode()``.
The function begins a tree walk of the parse tree, creating various AST
nodes as it goes along. It does this by allocating all new nodes it
needs, calling the proper AST node creation functions for any required
supporting functions, and connecting them as needed.
Do realize that there is no automated nor symbolic connection between
the grammar specification and the nodes in the parse tree. No help is
directly provided by the parse tree as in yacc.
For instance, one must keep track of which node in the parse tree
one is working with (e.g., if you are working with an 'if' statement
you need to watch out for the ':' token to find the end of the conditional).
The functions called to generate AST nodes from the parse tree all have
the name ``ast_for_xx`` where *xx* is the grammar rule that the function
handles (``alias_for_import_name`` is the exception to this). These in turn
call the constructor functions as defined by the ASDL grammar and
contained in :file:`Python/Python-ast.c` (which was generated by
:file:`Parser/asdl_c.py`) to create the nodes of the AST. This all leads to a
sequence of AST nodes stored in ``asdl_seq`` structs.
Function and macros for creating and using ``asdl_seq *`` types as found
in :file:`Python/asdl.c` and :file:`Include/asdl.h` are as follows:
``_Py_asdl_seq_new(Py_ssize_t, PyArena *)``
Allocate memory for an ``asdl_seq`` for the specified length
``asdl_seq_GET(asdl_seq *, int)``
Get item held at a specific position in an ``asdl_seq``
``asdl_seq_SET(asdl_seq *, int, stmt_ty)``
Set a specific index in an ``asdl_seq`` to the specified value
``asdl_seq_LEN(asdl_seq *)``
Return the length of an ``asdl_seq``
If you are working with statements, you must also worry about keeping
track of what line number generated the statement. Currently the line
number is passed as the last parameter to each ``stmt_ty`` function.
Control Flow Graphs
-------------------
A control flow graph (often referenced by its acronym, CFG) is a
directed graph that models the flow of a program using basic blocks that
contain the intermediate representation (abbreviated "IR", and in this
case is Python bytecode) within the blocks. Basic blocks themselves are
a block of IR that has a single entry point but possibly multiple exit
points. The single entry point is the key to basic blocks; it all has
to do with jumps. An entry point is the target of something that
changes control flow (such as a function call or a jump) while exit
points are instructions that would change the flow of the program (such
as jumps and 'return' statements). What this means is that a basic
block is a chunk of code that starts at the entry point and runs to an
exit point or the end of the block.
As an example, consider an 'if' statement with an 'else' block. The
guard on the 'if' is a basic block which is pointed to by the basic
block containing the code leading to the 'if' statement. The 'if'
statement block contains jumps (which are exit points) to the true body
of the 'if' and the 'else' body (which may be ``NULL``), each of which are
their own basic blocks. Both of those blocks in turn point to the
basic block representing the code following the entire 'if' statement.
CFGs are usually one step away from final code output. Code is directly
generated from the basic blocks (with jump targets adjusted based on the
output order) by doing a post-order depth-first search on the CFG
following the edges.
AST to CFG to Bytecode
----------------------
With the AST created, the next step is to create the CFG. The first step
is to convert the AST to Python bytecode without having jump targets
resolved to specific offsets (this is calculated when the CFG goes to
final bytecode). Essentially, this transforms the AST into Python
bytecode with control flow represented by the edges of the CFG.
Conversion is done in two passes. The first creates the namespace
(variables can be classified as local, free/cell for closures, or
global). With that done, the second pass essentially flattens the CFG
into a list and calculates jump offsets for final output of bytecode.
The conversion process is initiated by a call to the function
``PyAST_Compile()`` in :file:`Python/compile.c`. This function does both the
conversion of the AST to a CFG and outputting final bytecode from the CFG.
The AST to CFG step is handled mostly by two functions called by
``PyAST_Compile()``; ``PySymtable_Build()`` and ``compiler_mod()``. The former
is in :file:`Python/symtable.c` while the latter is in :file:`Python/compile.c`.
``PySymtable_Build()`` begins by entering the starting code block for the
AST (passed-in) and then calling the proper ``symtable_visit_xx`` function
(with *xx* being the AST node type). Next, the AST tree is walked with
the various code blocks that delineate the reach of a local variable
as blocks are entered and exited using ``symtable_enter_block()`` and
``symtable_exit_block()``, respectively.
Once the symbol table is created, it is time for CFG creation, whose
code is in :file:`Python/compile.c`. This is handled by several functions
that break the task down by various AST node types. The functions are
all named ``compiler_visit_xx`` where *xx* is the name of the node type (such
as stmt, expr, etc.). Each function receives a ``struct compiler *``
and ``xx_ty`` where *xx* is the AST node type. Typically these functions
consist of a large 'switch' statement, branching based on the kind of
node type passed to it. Simple things are handled inline in the
'switch' statement with more complex transformations farmed out to other
functions named ``compiler_xx`` with *xx* being a descriptive name of what is
being handled.
When transforming an arbitrary AST node, use the ``VISIT()`` macro.
The appropriate ``compiler_visit_xx`` function is called, based on the value
passed in for (so ``VISIT(c, expr, node)`` calls
``compiler_visit_expr(c, node)``). The ``VISIT_SEQ`` macro is very similar,
but is called on AST node sequences (those values that were created as
arguments to a node that used the '*' modifier). There is also
``VISIT_SLICE()`` just for handling slices.
Emission of bytecode is handled by the following macros:
``ADDOP(struct compiler *, int)``
add a specified opcode
``ADDOP_I(struct compiler *, int, Py_ssize_t)``
add an opcode that takes an argument
``ADDOP_O(struct compiler *, int, PyObject *, PyObject *)``
add an opcode with the proper argument based on the position of the
specified PyObject in PyObject sequence object, but with no handling of
mangled names; used for when you
need to do named lookups of objects such as globals, consts, or
parameters where name mangling is not possible and the scope of the
name is known
``ADDOP_NAME(struct compiler *, int, PyObject *, PyObject *)``
just like ``ADDOP_O``, but name mangling is also handled; used for
attribute loading or importing based on name
``ADDOP_JABS(struct compiler *, int, basicblock *)``
create an absolute jump to a basic block
``ADDOP_JREL(struct compiler *, int, basicblock *)``
create a relative jump to a basic block
Several helper functions that will emit bytecode and are named
``compiler_xx()`` where *xx* is what the function helps with (list, boolop,
etc.). A rather useful one is ``compiler_nameop()``.
This function looks up the scope of a variable and, based on the
expression context, emits the proper opcode to load, store, or delete
the variable.
As for handling the line number on which a statement is defined, this is
handled by ``compiler_visit_stmt()`` and thus is not a worry.
In addition to emitting bytecode based on the AST node, handling the
creation of basic blocks must be done. Below are the macros and
functions used for managing basic blocks:
``NEXT_BLOCK(struct compiler *)``
create an an implicit jump from the current block
to the new block
``compiler_new_block(struct compiler *)``
create a block but don't use it (used for generating jumps)
Once the CFG is created, it must be flattened and then final emission of
bytecode occurs. Flattening is handled using a post-order depth-first
search. Once flattened, jump offsets are backpatched based on the
flattening and then a ``PyCodeObject`` is created. All of this is
handled by calling ``assemble()``.
Introducing New Bytecode
------------------------
Sometimes a new feature requires a new opcode. But adding new bytecode is
not as simple as just suddenly introducing new bytecode in the AST ->
bytecode step of the compiler. Several pieces of code throughout Python depend
on having correct information about what bytecode exists.
First, you must choose a name and a unique identifier number. The official
list of bytecode can be found in :file:`Include/opcode.h`. If the opcode is to
take an argument, it must be given a unique number greater than that assigned to
``HAVE_ARGUMENT`` (as found in :file:`Include/opcode.h`).
Once the name/number pair has been chosen and entered in Include/opcode.h,
you must also enter it into :file:`Lib/opcode.py` and
:file:`Doc/library/dis.rst`.
With a new bytecode you must also change what is called the magic number for
.pyc files. The variable ``MAGIC`` in :file:`Python/import.c` contains the
number.
Changing this number will lead to all .pyc files with the old ``MAGIC``
to be recompiled by the interpreter on import.
Finally, you need to introduce the use of the new bytecode. Altering
:file:`Python/compile.c` and :file:`Python/ceval.c` will be the primary places
to change. But you will also need to change the 'compiler' package.
The key files to do that are :file:`Lib/compiler/pyassem.py` and
:file:`Lib/compiler/pycodegen.py`.
If you make a change here that can affect the output of bytecode that
is already in existence and you do not change the magic number constantly, make
sure to delete your old .py(c|o) files! Even though you will end up changing
the magic number if you change the bytecode, while you are debugging your work
you will be changing the bytecode output without constantly bumping up the
magic number. This means you end up with stale .pyc files that will not be
recreated.
Running ``find . -name '*.py[co]' -exec rm -f {} ';'`` should delete all .pyc
files you have, forcing new ones to be created and thus allow you test out your
new bytecode properly.
Code Objects
------------
The result of ``PyAST_Compile()`` is a ``PyCodeObject`` which is defined in
:file:`Include/code.h`. And with that you now have executable Python bytecode!
The code objects (byte code) are executed in :file:`Python/ceval.c`. This file
will also need a new case statement for the new opcode in the big switch
statement in ``PyEval_EvalFrameDefault()``.
Important Files
---------------
+ Parser/
Python.asdl
ASDL syntax file
asdl.py
Parser for ASDL definition files. Reads in an ASDL description
and parses it into an AST that describes it.
asdl_c.py
"Generate C code from an ASDL description." Generates
:file:`Python/Python-ast.c` and :file:`Include/Python-ast.h`.
+ Python/
Python-ast.c
Creates C structs corresponding to the ASDL types. Also
contains code for marshalling AST nodes (core ASDL types have
marshalling code in :file:`asdl.c`). "File automatically generated by
:file:`Parser/asdl_c.py`". This file must be committed separately
after every grammar change is committed since the ``__version__``
value is set to the latest grammar change revision number.
asdl.c
Contains code to handle the ASDL sequence type. Also has code
to handle marshalling the core ASDL types, such as number and
identifier. Used by :file:`Python-ast.c` for marshalling AST nodes.
ast.c
Converts Python's parse tree into the abstract syntax tree.
ceval.c
Executes byte code (aka, eval loop).
compile.c
Emits bytecode based on the AST.
symtable.c
Generates a symbol table from AST.
pyarena.c
Implementation of the arena memory manager.
import.c
Home of the magic number (named ``MAGIC``) for bytecode versioning
+ Include/
Python-ast.h
Contains the actual definitions of the C structs as generated by
:file:`Python/Python-ast.c`.
"Automatically generated by :file:`Parser/asdl_c.py`".
asdl.h
Header for the corresponding :file:`Python/ast.c`.
ast.h
Declares ``PyAST_FromNode()`` external (from :file:`Python/ast.c`).
code.h
Header file for :file:`Objects/codeobject.c`; contains definition of
``PyCodeObject``.
symtable.h
Header for :file:`Python/symtable.c`. struct symtable and
``PySTEntryObject`` are defined here.
pyarena.h
Header file for the corresponding :file:`Python/pyarena.c`.
opcode.h
Master list of bytecode; if this file is modified you must modify
several other files accordingly (see "`Introducing New Bytecode`_")
+ Objects/
codeobject.c
Contains PyCodeObject-related code (originally in
:file:`Python/compile.c`).
+ Lib/
opcode.py
One of the files that must be modified if :file:`Include/opcode.h` is.
Known Compiler-related Experiments
----------------------------------
This section lists known experiments involving the compiler (including
bytecode).
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
----------
.. [Aho86] Alfred V. Aho, Ravi Sethi, Jeffrey D. Ullman.
`Compilers: Principles, Techniques, and Tools`,
https://www.amazon.com/exec/obidos/tg/detail/-/0201100886/104-0162389-6419108
.. [Wang97] Daniel C. Wang, Andrew W. Appel, Jeff L. Korn, and Chris
S. Serra. `The Zephyr Abstract Syntax Description Language.`_
In Proceedings of the Conference on Domain-Specific Languages, pp.
213--227, 1997.
.. _The Zephyr Abstract Syntax Description Language.:
http://www.cs.princeton.edu/research/techreps/TR-554-97
.. [#skip-peephole] Skip Montanaro's Peephole Optimizer Paper
(https://drive.google.com/open?id=0B2InO7qBBGRXQXlDM3FVdWZxQWc)
.. [#Bytecodehacks] Bytecodehacks Project
(http://bytecodehacks.sourceforge.net/bch-docs/bch/index.html)
.. [#CALL_ATTR] CALL_ATTR opcode
(https://bugs.python.org/issue709744)