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Compiler design
===============
Abstract
--------
In CPython, the compilation from source code to bytecode involves several steps:
1. Tokenize the source code [Parser/lexer/](../Parser/lexer)
and [Parser/tokenizer/](../Parser/tokenizer).
2. Parse the stream of tokens into an Abstract Syntax Tree
[Parser/parser.c](../Parser/parser.c).
3. Transform AST into an instruction sequence
[Python/compile.c](../Python/compile.c).
4. Construct a Control Flow Graph and apply optimizations to it
[Python/flowgraph.c](../Python/flowgraph.c).
5. Emit bytecode based on the Control Flow Graph
[Python/assemble.c](../Python/assemble.c).
This document outlines how these steps of the process work.
This document only describes parsing in enough depth to explain what is needed
for understanding compilation. This document provides a detailed, though not
exhaustive, view of the how the entire system works. You will most likely need
to read some source code to have an exact understanding of all details.
Parsing
=======
As of Python 3.9, Python's parser is a PEG parser of a somewhat
unusual design. It is unusual in the sense that the parser's input is a stream
of tokens rather than a stream of characters which is more common with PEG
parsers.
The grammar file for Python can be found in
[Grammar/python.gram](../Grammar/python.gram).
The definitions for literal tokens (such as `:`, numbers, etc.) can be found in
[Grammar/Tokens](../Grammar/Tokens). Various C files, including
[Parser/parser.c](../Parser/parser.c) are generated from these.
See Also:
* [Guide to the parser](parser.md)
for a detailed description of the parser.
* [Changing CPython’s grammar](changing_grammar.md)
for a detailed description of the grammar.
Abstract syntax trees (AST)
===========================
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) [^1], [^2].
The definition of the AST nodes for Python is found in the file
[Parser/Python.asdl](../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:
```
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.
See also [Green Tree Snakes - The missing Python AST docs](
https://greentreesnakes.readthedocs.io/en/latest) by Thomas Kluyver.
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 that pools memory in a single location for easy
allocation and removal. This enables 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
[Include/internal/pycore_pyarena.h](../Include/internal/pycore_pyarena.h)
or [Python/pyarena.c](../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 of memory
management 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()`.
Source code to AST
==================
The AST is generated from source code using the function
`_PyParser_ASTFromString()` or `_PyParser_ASTFromFile()`
[Parser/peg_api.c](../Parser/peg_api.c).
After some checks, a helper function in
[Parser/parser.c](../Parser/parser.c)
begins applying production rules on the source code it receives; converting source
code to tokens and matching these tokens recursively to their corresponding rule. The
production rule's corresponding rule function is called on every match. These rule
functions follow the format `xx_rule`. Where *xx* is the grammar rule
that the function handles and is automatically derived from
[Grammar/python.gram](../Grammar/python.gram) by
[Tools/peg_generator/pegen/c_generator.py](../Tools/peg_generator/pegen/c_generator.py).
Each rule function in turn creates an AST node as it goes along. It does this
by allocating all the new nodes it needs, calling the proper AST node creation
functions for any required supporting functions and connecting them as needed.
This continues until all nonterminal symbols are replaced with terminals. If an
error occurs, the rule functions backtrack and try another rule function. If
there are no more rules, an error is set and the parsing ends.
The AST node creation helper functions have the name `_PyAST_{xx}`
where *xx* is the AST node that the function creates. These are defined by the
ASDL grammar and contained in [Python/Python-ast.c](../Python/Python-ast.c)
(which is generated by [Parser/asdl_c.py](../Parser/asdl_c.py)
from [Parser/Python.asdl](../Parser/Python.asdl)).
This all leads to a sequence of AST nodes stored in `asdl_seq` structs.
To demonstrate everything explained so far, here's the
rule function responsible for a simple named import statement such as
`import sys`. Note that error-checking and debugging code has been
omitted. Removed parts are represented by `...`.
Furthermore, some comments have been added for explanation. These comments
may not be present in the actual code.
```
// This is the production rule (from python.gram) the rule function
// corresponds to:
// import_name: 'import' dotted_as_names
static stmt_ty
import_name_rule(Parser *p)
{
...
stmt_ty _res = NULL;
{ // 'import' dotted_as_names
...
Token * _keyword;
asdl_alias_seq* a;
// The tokenizing steps.
if (
(_keyword = _PyPegen_expect_token(p, 513)) // token='import'
&&
(a = dotted_as_names_rule(p)) // dotted_as_names
)
{
...
// Generate an AST for the import statement.
_res = _PyAST_Import ( a , ...);
...
goto done;
}
...
}
_res = NULL;
done:
...
return _res;
}
```
To improve backtracking performance, some rules (chosen by applying a
`(memo)` flag in the grammar file) are memoized. Each rule function checks if
a memoized version exists and returns that if so, else it continues in the
manner stated in the previous paragraphs.
There are macros for creating and using `asdl_xx_seq *` types, where *xx* is
a type of the ASDL sequence. Three main types are defined
manually -- `generic`, `identifier` and `int`. These types are found in
[Python/asdl.c](../Python/asdl.c) and its corresponding header file
[Include/internal/pycore_asdl.h](../Include/internal/pycore_asdl.h).
Functions and macros for creating `asdl_xx_seq *` types are as follows:
* `_Py_asdl_generic_seq_new(Py_ssize_t, PyArena *)`:
Allocate memory for an `asdl_generic_seq` of the specified length
* `_Py_asdl_identifier_seq_new(Py_ssize_t, PyArena *)`:
Allocate memory for an `asdl_identifier_seq` of the specified length
* `_Py_asdl_int_seq_new(Py_ssize_t, PyArena *)`:
Allocate memory for an `asdl_int_seq` of the specified length
In addition to the three types mentioned above, some ASDL sequence types are
automatically generated by [Parser/asdl_c.py](../Parser/asdl_c.py) and found in
[Include/internal/pycore_ast.h](../Include/internal/pycore_ast.h).
Macros for using both manually defined and automatically generated ASDL
sequence types are as follows:
* `asdl_seq_GET(asdl_xx_seq *, int)`:
Get item held at a specific position in an `asdl_xx_seq`
* `asdl_seq_SET(asdl_xx_seq *, int, stmt_ty)`:
Set a specific index in an `asdl_xx_seq` to the specified value
Untyped counterparts exist for some of the typed macros. These are useful
when a function needs to manipulate a generic ASDL sequence:
* `asdl_seq_GET_UNTYPED(asdl_seq *, int)`:
Get item held at a specific position in an `asdl_seq`
* `asdl_seq_SET_UNTYPED(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` or `asdl_xx_seq`
Note that typed macros and functions are recommended over their untyped
counterparts. Typed macros carry out checks in debug mode and aid
debugging errors caused by incorrectly casting from `void *`.
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.
See also [PEP 617: New PEG parser for CPython](https://peps.python.org/pep-0617/).
Control flow graphs
===================
A **control flow graph** (often referenced by its acronym, **CFG**) is a
directed graph that models the flow of a program. A node of a CFG is
not an individual bytecode instruction, but instead represents a
sequence of bytecode instructions that always execute sequentially.
Each node is called a *basic block* and must always execute from
start to finish, with a single entry point at the beginning and a
single exit point at the end. If some bytecode instruction *a* needs
to jump to some other bytecode instruction *b*, then *a* must occur at
the end of its basic block, and *b* must occur at the start of its
basic block.
As an example, consider the following code snippet:
```python
if x < 10:
f1()
f2()
else:
g()
end()
```
The `x < 10` guard is represented by its own basic block that
compares `x` with `10` and then ends in a conditional jump based on
the result of the comparison. This conditional jump allows the block
to point to both the body of the `if` and the body of the `else`. The
`if` basic block contains the `f1()` and `f2()` calls and points to
the `end()` basic block. The `else` basic block contains the `g()`
call and similarly points to the `end()` block.
Note that more complex code in the guard, the `if` body, or the `else`
body may be represented by multiple basic blocks. For instance,
short-circuiting boolean logic in a guard like `if x or y:`
will produce one basic block that tests the truth value of `x`
and then points both (1) to the start of the `if` body and (2) to
a different basic block that tests the truth value of y.
CFGs are useful as an intermediate representation of the code because
they are a convenient data structure for optimizations.
AST to CFG to bytecode
======================
The conversion of an `AST` to bytecode is initiated by a call to the function
`_PyAST_Compile()` in [Python/compile.c](../Python/compile.c).
The first step is to construct the symbol table. This is implemented by
`_PySymtable_Build()` in [Python/symtable.c](../Python/symtable.c).
This function 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, the `AST` is transformed by `compiler_codegen()`
in [Python/compile.c](../Python/compile.c) into a sequence of pseudo instructions.
These are similar to bytecode, but in some cases they are more abstract, and are
resolved later into actual bytecode. The construction of this instruction sequence
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 <node type> (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).
Emission of bytecode is handled by the following macros:
* `ADDOP(struct compiler *, location, int)`:
add a specified opcode
* `ADDOP_IN_SCOPE(struct compiler *, location, int)`:
like `ADDOP`, but also exits current scope; used for adding return value
opcodes in lambdas and closures
* `ADDOP_I(struct compiler *, location, int, Py_ssize_t)`:
add an opcode that takes an integer argument
* `ADDOP_O(struct compiler *, location, int, PyObject *, TYPE)`:
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; *TYPE* is the name of PyObject sequence
(`names` or `varnames`)
* `ADDOP_N(struct compiler *, location, int, PyObject *, TYPE)`:
just like `ADDOP_O`, but steals a reference to PyObject
* `ADDOP_NAME(struct compiler *, location, int, PyObject *, TYPE)`:
just like `ADDOP_O`, but name mangling is also handled; used for
attribute loading or importing based on name
* `ADDOP_LOAD_CONST(struct compiler *, location, PyObject *)`:
add the `LOAD_CONST` opcode with the proper argument based on the
position of the specified PyObject in the consts table.
* `ADDOP_LOAD_CONST_NEW(struct compiler *, location, PyObject *)`:
just like `ADDOP_LOAD_CONST_NEW`, but steals a reference to PyObject
* `ADDOP_JUMP(struct compiler *, location, int, basicblock *)`:
create a jump to a basic block
The `location` argument is a struct with the source location to be
associated with this instruction. It is typically extracted from an
`AST` node with the `LOC` macro. The `NO_LOCATION` can be used
for *synthetic* instructions, which we do not associate with a line
number at this stage. For example, the implicit `return None`
which is added at the end of a function is not associated with any
line in the source code.
There are several helper functions that will emit pseudo-instructions
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.
Once the instruction sequence is created, it is transformed into a CFG
by `_PyCfg_FromInstructionSequence()`. Then `_PyCfg_OptimizeCodeUnit()`
applies various peephole optimizations, and
`_PyCfg_OptimizedCfgToInstructionSequence()` converts the optimized `CFG`
back into an instruction sequence. These conversions and optimizations are
implemented in [Python/flowgraph.c](../Python/flowgraph.c).
Finally, the sequence of pseudo-instructions is converted into actual
bytecode. This includes transforming pseudo instructions into actual instructions,
converting jump targets from logical labels to relative offsets, and
construction of the [exception table](exception_handling.md) and
[locations table](code_objects.md#source-code-locations).
The bytecode and tables are then wrapped into a `PyCodeObject` along with additional
metadata, including the `consts` and `names` arrays, information about function
reference to the source code (filename, etc). All of this is implemented by
`_PyAssemble_MakeCodeObject()` in [Python/assemble.c](../Python/assemble.c).
Code objects
============
The result of `_PyAST_Compile()` is a `PyCodeObject` which is defined in
[Include/cpython/code.h](../Include/cpython/code.h).
And with that you now have executable Python bytecode!
The code objects (byte code) are executed in `_PyEval_EvalFrameDefault()`
in [Python/ceval.c](../Python/ceval.c).
Important files
===============
* [Parser/](../Parser)
* [Parser/Python.asdl](../Parser/Python.asdl):
ASDL syntax file.
* [Parser/asdl.py](../Parser/asdl.py):
Parser for ASDL definition files.
Reads in an ASDL description and parses it into an AST that describes it.
* [Parser/asdl_c.py](../Parser/asdl_c.py):
Generate C code from an ASDL description. Generates
[Python/Python-ast.c](../Python/Python-ast.c) and
[Include/internal/pycore_ast.h](../Include/internal/pycore_ast.h).
* [Parser/parser.c](../Parser/parser.c):
The new PEG parser introduced in Python 3.9. Generated by
[Tools/peg_generator/pegen/c_generator.py](../Tools/peg_generator/pegen/c_generator.py)
from the grammar [Grammar/python.gram](../Grammar/python.gram).
Creates the AST from source code. Rule functions for their corresponding production
rules are found here.
* [Parser/peg_api.c](../Parser/peg_api.c):
Contains high-level functions which are used by the interpreter to create
an AST from source code.
* [Parser/pegen.c](../Parser/pegen.c):
Contains helper functions which are used by functions in
[Parser/parser.c](../Parser/parser.c) to construct the AST. Also contains
helper functions which help raise better error messages when parsing source code.
* [Parser/pegen.h](../Parser/pegen.h):
Header file for the corresponding [Parser/pegen.c](../Parser/pegen.c).
Also contains definitions of the `Parser` and `Token` structs.
* [Python/](../Python)
* [Python/Python-ast.c](../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
[Python/asdl.c](../Python/asdl.c)).
File automatically generated by [Parser/asdl_c.py](../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.
* [Python/asdl.c](../Python/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 [Python/Python-ast.c](../Python/Python-ast.c)
for marshalling AST nodes.
* [Python/ast.c](../Python/ast.c):
Used for validating the AST.
* [Python/ast_opt.c](../Python/ast_opt.c):
Optimizes the AST.
* [Python/ast_unparse.c](../Python/ast_unparse.c):
Converts the AST expression node back into a string (for string annotations).
* [Python/ceval.c](../Python/ceval.c):
Executes byte code (aka, eval loop).
* [Python/symtable.c](../Python/symtable.c):
Generates a symbol table from AST.
* [Python/pyarena.c](../Python/pyarena.c):
Implementation of the arena memory manager.
* [Python/compile.c](../Python/compile.c):
Emits pseudo bytecode based on the AST.
* [Python/flowgraph.c](../Python/flowgraph.c):
Implements peephole optimizations.
* [Python/assemble.c](../Python/assemble.c):
Constructs a code object from a sequence of pseudo instructions.
* [Python/instruction_sequence.c](../Python/instruction_sequence.c):
A data structure representing a sequence of bytecode-like pseudo-instructions.
* [Include/](../Include)
* [Include/cpython/code.h](../Include/cpython/code.h)
: Header file for [Objects/codeobject.c](../Objects/codeobject.c);
contains definition of `PyCodeObject`.
* [Include/opcode.h](../Include/opcode.h)
: One of the files that must be modified whenever
[Lib/opcode.py](../Lib/opcode.py) is.
* [Include/internal/pycore_ast.h](../Include/internal/pycore_ast.h)
: Contains the actual definitions of the C structs as generated by
[Python/Python-ast.c](../Python/Python-ast.c).
Automatically generated by [Parser/asdl_c.py](../Parser/asdl_c.py).
* [Include/internal/pycore_asdl.h](../Include/internal/pycore_asdl.h)
: Header for the corresponding [Python/ast.c](../Python/ast.c).
* [Include/internal/pycore_ast.h](../Include/internal/pycore_ast.h)
: Declares `_PyAST_Validate()` external (from [Python/ast.c](../Python/ast.c)).
* [Include/internal/pycore_symtable.h](../Include/internal/pycore_symtable.h)
: Header for [Python/symtable.c](../Python/symtable.c).
`struct symtable` and `PySTEntryObject` are defined here.
* [Include/internal/pycore_parser.h](../Include/internal/pycore_parser.h)
: Header for the corresponding [Parser/peg_api.c](../Parser/peg_api.c).
* [Include/internal/pycore_pyarena.h](../Include/internal/pycore_pyarena.h)
: Header file for the corresponding [Python/pyarena.c](../Python/pyarena.c).
* [Include/opcode_ids.h](../Include/opcode_ids.h)
: List of opcodes. Generated from [Python/bytecodes.c](../Python/bytecodes.c)
by
[Tools/cases_generator/opcode_id_generator.py](../Tools/cases_generator/opcode_id_generator.py).
* [Objects/](../Objects)
* [Objects/codeobject.c](../Objects/codeobject.c)
: Contains PyCodeObject-related code.
* [Objects/frameobject.c](../Objects/frameobject.c)
: Contains the `frame_setlineno()` function which should determine whether it is allowed
to make a jump between two points in a bytecode.
* [Lib/](../Lib)
* [Lib/opcode.py](../Lib/opcode.py)
: opcode utilities exposed to Python.
* [Include/core/pycore_magic_number.h](../Include/internal/pycore_magic_number.h)
: Home of the magic number (named `MAGIC_NUMBER`) for bytecode versioning.
Objects
=======
* [Locations](code_objects.md#source-code-locations): Describes the location table
* [Frames](frames.md): Describes frames and the frame stack
* [Objects/object_layout.md](../Objects/object_layout.md): Describes object layout for 3.11 and later
* [Exception Handling](exception_handling.md): Describes the exception table
References
==========
[^1]: 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.
[^2]: The Zephyr Abstract Syntax Description Language.:
https://www.cs.princeton.edu/research/techreps/TR-554-97
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