PEP: 744
Title: JIT Compilation
Author: Brandt Bucher <brandt@python.org>,
        Savannah Ostrowski <savannah@python.org>,
Discussions-To: https://discuss.python.org/t/pep-744-jit-compilation/50756
Status: Draft
Type: Informational
Created: 11-Apr-2024
Python-Version: 3.13
Post-History: `11-Apr-2024 <https://discuss.python.org/t/pep-744-jit-compilation/50756>`__

Abstract
========

Earlier this year, an `experimental "just-in-time" compiler
<https://github.com/python/cpython/pull/113465>`__ was merged into CPython's
``main`` development branch. While recent CPython releases have included other
substantial internal changes, this addition represents a particularly
significant departure from the way CPython has traditionally executed Python
code. As such, it deserves wider discussion.

This PEP aims to summarize the design decisions behind this addition, the
current state of the implementation, and future plans for making the JIT a
permanent, non-experimental part of CPython. It does *not* seek to provide a
comprehensive overview of *how* the JIT works, instead focusing on the
particular advantages and disadvantages of the chosen approach, as well as
answering many questions that have been asked about the JIT since its
introduction.

Readers interested in learning more about the new JIT are encouraged to consult
the following resources:

- The `presentation <https://youtu.be/HxSHIpEQRjs>`__ which first introduced the
  JIT at the 2023 CPython Core Developer Sprint. It includes relevant
  background, a light technical introduction to the "copy-and-patch" technique
  used, and an open discussion of its design amongst the core developers
  present. Slides for this talk can be found on `GitHub <https://github.com/brandtbucher/brandtbucher/blob/master/2023/10/10/a_jit_compiler_for_cpython.pdf>`__.

- The `open access paper <https://dl.acm.org/doi/10.1145/3485513>`__ originally
  describing copy-and-patch.

- The `blog post <https://sillycross.github.io/2023/05/12/2023-05-12>`__ by the
  paper's author detailing the implementation of a copy-and-patch JIT compiler
  for Lua. While this is a great low-level explanation of the approach, note
  that it also incorporates other techniques and makes implementation decisions
  that are not particularly relevant to CPython's JIT.

- The `implementation <#reference-implementation>`__ itself.

Motivation
==========

Until this point, CPython has always executed Python code by compiling it to
bytecode, which is interpreted at runtime. This bytecode is a more-or-less
direct translation of the source code: it is untyped, and largely unoptimized.

Since the Python 3.11 release, CPython has used a "specializing adaptive
interpreter" (:pep:`659`), which `rewrites these bytecode instructions in-place
<https://youtu.be/shQtrn1v7sQ>`__ with type-specialized versions as they run.
This new interpreter delivers significant performance improvements, despite the
fact that its optimization potential is limited by the boundaries of individual
bytecode instructions. It also collects a wealth of new profiling information:
the types flowing though a program, the memory layout of particular objects, and
what paths through the program are being executed the most. In other words,
*what* to optimize, and *how* to optimize it.

Since the Python 3.12 release, CPython has generated this interpreter from a
`C-like domain-specific language
<https://github.com/python/cpython/blob/main/Python/bytecodes.c>`__ (DSL). In
addition to taming some of the complexity of the new adaptive interpreter, the
DSL also allows CPython's maintainers to avoid hand-writing tedious boilerplate
code in many parts of the interpreter, compiler, and standard library that must
be kept in sync with the instruction definitions. This ability to generate large
amounts of runtime infrastructure from a single source of truth is not only
convenient for maintenance; it also unlocks many possibilities for expanding
CPython's execution in new ways. For instance, it makes it feasible to
automatically generate tables for translating a sequence of instructions into an
equivalent sequence of smaller "micro-ops", generate an optimizer for sequences
of these micro-ops, and even generate an entire second interpreter for executing
them.

In fact, since early in the Python 3.13 release cycle, all CPython builds have
included this exact micro-op translation, optimization, and execution machinery.
However, it is disabled by default; the overhead of interpreting even optimized
traces of micro-ops is just too large for most code. Heavier optimization
probably won't improve the situation much either, since any efficiency gains
made by new optimizations will likely be offset by the interpretive overhead of
even smaller, more complex micro-ops.

The most obvious strategy to overcome this new bottleneck is to statically
compile these optimized traces. This presents opportunities to avoid several
sources of indirection and overhead introduced by interpretation. In particular,
it allows the removal of dispatch overhead between micro-ops (by replacing a
generic interpreter with a straight-line sequence of hot code), instruction
decoding overhead for individual micro-ops (by "burning" the values or addresses
of arguments, constants, and cached values directly into machine instructions),
and memory traffic (by moving data off of heap-allocated Python frames and into
physical hardware registers).

Since much of this data varies even between identical runs of a program and the
existing optimization pipeline makes heavy use of runtime profiling information,
it doesn't make much sense to compile these traces ahead of time and would be a
substantial redesign of the existing specification and micro-op tracing infrastructure
that has already been implemented. As has been demonstrated for many other dynamic
languages (`and even Python itself <https://www.pypy.org>`__), the most promising
approach is to compile the optimized micro-ops "just in time" for execution.

Rationale
=========

Despite their reputation, JIT compilers are not magic "go faster" machines.
Developing and maintaining any sort of optimizing compiler for even a single
platform, let alone all of CPython's most popular supported platforms, is an
incredibly complicated, expensive task. Using an existing compiler framework
like LLVM can make this task simpler, but only at the cost of introducing heavy
runtime dependencies and significantly higher JIT compilation overhead.

It's clear that successfully compiling Python code at runtime requires not only
high-quality Python-specific optimizations for the code being run, *but also*
quick generation of efficient machine code for the optimized program. The Python
core development team has the necessary skills and experience for the former (a
middle-end tightly coupled to the interpreter), and copy-and-patch compilation
provides an attractive solution for the latter.

In a nutshell, copy-and-patch allows a high-quality template JIT compiler to be
generated from the same DSL used to generate the rest of the interpreter. For a
widely-used, volunteer-driven project like CPython, this benefit cannot be
overstated: CPython's maintainers, by merely editing the bytecode definitions,
will also get the JIT backend updated "for free", for *all* JIT-supported
platforms, at once. This is equally true whether instructions are being added,
modified, or removed.

Like the rest of the interpreter, the JIT compiler is generated at build time,
and has no runtime dependencies. It supports a wide range of platforms (see the
`Support`_ section below), and has comparatively low maintenance burden. In all,
the current implementation is made up of about 900 lines of build-time Python
code and 500 lines of runtime C code.

Specification
=============

The JIT is currently not part of the default build configuration, and it is
likely to remain that way for the foreseeable future (though official binaries
may include it). That said, the JIT will become non-experimental once all of
the following conditions are met:

#. It provides a meaningful performance improvement for at least one popular
   platform (realistically, on the order of 5%).

#. It can be built, distributed, and deployed with minimal disruption.

#. The Steering Council, upon request, has determined that it would provide more
   value to the community if enabled than if disabled (considering tradeoffs
   such as maintenance burden, memory usage, or the feasibility of alternate
   designs).

These criteria should be considered a starting point, and may be expanded over
time. For example, discussion of this PEP may reveal that additional
requirements (such as multiple committed maintainers, a security audit,
documentation in the devguide, support for out-of-process debugging, or a
runtime option to disable the JIT) should be added to this list.

Until the JIT is non-experimental, it should *not* be used in production, and
may be broken or removed at any time without warning.

Once the JIT is no longer experimental, it should be treated in much the same
way as other build options such as ``--enable-optimizations`` or ``--with-lto``.
It may be a recommended (or even default) option for some platforms, and release
managers *may* choose to enable it in official releases.

Support
-------

The JIT has been developed for all of :pep:`11`'s current tier one platforms,
most of its tier two platforms, and one of its tier three platforms.
Specifically, CPython's ``main`` branch has `CI
<https://github.com/python/cpython/blob/main/.github/workflows/jit.yml>`__
building and testing the JIT for both release and debug builds on:

- ``aarch64-apple-darwin/clang``

- ``aarch64-pc-windows/msvc`` [#untested]_

- ``aarch64-unknown-linux-gnu/clang`` [#emulated]_

- ``aarch64-unknown-linux-gnu/gcc`` [#emulated]_

- ``i686-pc-windows-msvc/msvc``

- ``x86_64-apple-darwin/clang``

- ``x86_64-pc-windows-msvc/msvc``

- ``x86_64-unknown-linux-gnu/clang``

- ``x86_64-unknown-linux-gnu/gcc``

It's worth noting that some platforms, even future tier one platforms, may never
gain JIT support. This can be for a variety of reasons, including insufficient
LLVM support (``powerpc64le-unknown-linux-gnu/gcc``), inherent limitations of
the platform (``wasm32-unknown-wasi/clang``), or lack of developer interest
(``x86_64-unknown-freebsd/clang``).

Once JIT support for a platform is added (meaning, the JIT builds successfully
without displaying warnings to the user), it should be treated in much the same
way as :pep:`11` prescribes: it should have reliable CI/buildbots, and JIT
failures on tier one and tier two platforms should block releases. Though it's
not necessary to update :pep:`11` to specify JIT support, it may be helpful to
do so anyway. Otherwise, a list of supported platforms should be maintained in
`the JIT's README
<https://github.com/python/cpython/blob/main/Tools/jit/README.md>`__.

Since it should always be possible to build CPython without the JIT, removing
JIT support for a platform should *not* be considered a backwards-incompatible
change. However, if it is reasonable to do so, the normal deprecation process
should be followed as outlined in :pep:`387`.

The JIT's build-time dependencies may be changed between releases, within
reason.

Backwards Compatibility
=======================

Due to the fact that the current interpreter and the JIT backend are both
generated from the same specification, the behavior of Python code should be
completely unchanged. In practice, observable differences that have been found
and fixed during testing have tended to be bugs in the existing micro-op
translation and optimization stages, rather than bugs in the copy-and-patch
step.

Debugging
---------

Tools that profile and debug Python code will continue to work fine. This
includes in-process tools that use Python-provided functionality (like
``sys.monitoring``, ``sys.settrace``, or  ``sys.setprofile``), as well as
out-of-process tools that walk Python frames from the interpreter state.

However, it appears that profilers and debuggers *for C code* are currently
unable to trace back through JIT frames. Working with leaf frames is possible
(this is how the JIT itself is debugged), though it is of limited utility due to
the absence of proper debugging information for JIT frames.

Since the code templates emitted by the JIT are compiled by Clang, it *may* be
possible to allow JIT frames to be traced through by simply modifying the
compiler flags to use frame pointers more carefully. It may also be possible to
harvest and emit the debugging information produced by Clang. Neither of these
ideas have been explored very deeply.

While this is an issue that *should* be fixed, fixing it is not a particularly
high priority at this time. This is probably a problem best explored by somebody
with more domain expertise in collaboration with those maintaining the JIT, who
have little experience with the inner workings of these tools.

Security Implications
=====================

This JIT, like any JIT, produces large amounts of executable data at runtime.
This introduces a potential new attack surface to CPython, since a malicious
actor capable of influencing the contents of this data is therefore capable of
executing arbitrary code. This is a `well-known vulnerability
<https://en.wikipedia.org/wiki/Just-in-time_compilation#Security>`__ of JIT
compilers.

In order to mitigate this risk, the JIT has been written with best practices in
mind. In particular, the data in question is not exposed by the JIT compiler to
other parts of the program while it remains writable, and at *no* point is the
data both |wx|_.

.. Apparently this how you hack together a formatted link:

.. |wx| replace:: writable *and* executable
.. _wx: https://en.wikipedia.org/wiki/W%5EX

The nature of template-based JITs also seriously limits the kinds of code that
can be generated, further reducing the likelihood of a successful exploit. As an
additional precaution, the templates themselves are stored in static, read-only
memory.

However, it would be naive to assume that no possible vulnerabilities exist in
the JIT, especially at this early stage. The author is not a security expert,
but is available to join or work closely with the Python Security Response Team
to triage and fix security issues as they arise.

Apple Silicon
--------------

Though difficult to test without actually signing and packaging a macOS release,
it *appears* that macOS releases should `enable the JIT Entitlement for the
Hardened Runtime
<https://developer.apple.com/documentation/apple-silicon/porting-just-in-time-compilers-to-apple-silicon#Enable-the-JIT-Entitlement-for-the-Hardened-Runtime>`__.

This shouldn't make *installing* Python any harder, but may add additional steps
for release managers to perform.

How to Teach This
=================

Choose the sections that best describe you:

- **If you are a Python programmer or end user...**

  - ...nothing changes for you. Nobody should be distributing JIT-enabled
    CPython interpreters to you while it is still an experimental feature. Once
    it is non-experimental, you will probably notice slightly better performance
    and slightly higher memory usage. You shouldn't be able to observe any other
    changes.

- **If you maintain third-party packages...**

  - ...nothing changes for you. There are no API or ABI changes, and the JIT is
    not exposed to third-party code. You shouldn't need to change your CI
    matrix, and you shouldn't be able to observe differences in the way your
    packages work when the JIT is enabled.

- **If you profile or debug Python code...**

  - ...nothing changes for you. All Python profiling and tracing functionality
    remains.

- **If you profile or debug C code...**

  - ...currently, the ability to trace *through* JIT frames is limited. This may
    cause issues if you need to observe the entire C call stack, rather than
    just "leaf" frames. See the `Debugging`_ section above for more information.

- **If you compile your own Python interpreter....**

  - ...if you don't wish to build the JIT, you can simply ignore it. Otherwise,
    you will need to `install a compatible version of LLVM
    <https://github.com/python/cpython/blob/main/Tools/jit/README.md>`__, and
    pass the appropriate flag to the build scripts. Your build may take up to a
    minute longer. Note that the JIT should *not* be distributed to end users or
    used in production while it is still in the experimental phase.

- **If you're a maintainer of CPython (or a fork of CPython)...**

  - **...and you change the bytecode definitions or the main interpreter
    loop...**

    - ...in general, the JIT shouldn't be much of an inconvenience to you
      (depending on what you're trying to do). The micro-op interpreter isn't
      going anywhere, and still offers a debugging experience similar to what
      the main bytecode interpreter provides today. There is moderate likelihood
      that larger changes to the interpreter (such as adding new local
      variables, changing error handling and deoptimization logic, or changing
      the micro-op format) will require changes to the C template used to
      generate the JIT, which is meant to mimic the main interpreter loop. You
      may also occasionally just get unlucky and break JIT code generation,
      which will require you to either modify the Python build scripts yourself,
      or solicit the help of somebody more familiar with them (see below).

  - **...and you work on the JIT itself...**

    - ...you hopefully already have a decent idea of what you're getting
      yourself into. You will be regularly modifying the Python build scripts,
      the C template used to generate the JIT, and the C code that actually
      makes up the runtime portion of the JIT. You will also be dealing with
      all sorts of crashes, stepping over machine code in a debugger, staring at
      COFF/ELF/Mach-O dumps, developing on a wide range of platforms, and
      generally being the point of contact for the people changing the bytecode
      when CI starts failing on their PRs (see above). Ideally, you're at least
      *familiar* with assembly, have taken a couple of courses with "compilers"
      in their name, and have read a blog post or two about linkers.

  - **...and you maintain other parts of CPython...**

    - ...nothing changes for you. You shouldn't need to develop locally with JIT
      builds. If you choose to do so (for example, to help reproduce and triage
      JIT issues), your builds may take up to a minute longer each time the
      relevant files are modified.

Reference Implementation
========================

Key parts of the implementation include:

- |readme|_: Instructions for how to build the JIT.

- |jit|_: The entire runtime portion of the JIT compiler.

- |jit_stencils|_: An example of the JIT's generated templates.

- |template|_: The code which is compiled to produce the JIT's templates.

- |targets|_: The code to compile and parse the templates at build time.

.. |readme| replace:: ``Tools/jit/README.md``
.. _readme: https://github.com/python/cpython/blob/main/Tools/jit/README.md

.. |jit| replace:: ``Python/jit.c``
.. _jit: https://github.com/python/cpython/blob/main/Python/jit.c

.. |jit_stencils| replace:: ``jit_stencils.h``
.. _jit_stencils: https://gist.github.com/brandtbucher/9d3cc396dcb15d13f7e971175e987f3a

.. |template| replace:: ``Tools/jit/template.c``
.. _template: https://github.com/python/cpython/blob/main/Tools/jit/template.c

.. |targets| replace:: ``Tools/jit/_targets.py``
.. _targets: https://github.com/python/cpython/blob/main/Tools/jit/_targets.py

Rejected Ideas
==============

Maintain it outside of CPython
------------------------------

While it is *probably* possible to maintain the JIT outside of CPython, its
implementation is tied tightly enough to the rest of the interpreter that
keeping it up-to-date would probably be more difficult than actually developing
the JIT itself. Additionally, contributors working on the existing micro-op
definitions and optimizations would need to modify and build two separate
projects to measure the effects of their changes under the JIT (whereas today,
infrastructure exists to do this automatically for any proposed change).

Releases of the separate "JIT" project would probably also need to correspond to
specific CPython pre-releases and patch releases, depending on exactly what
changes are present. Individual CPython commits between releases likely wouldn't
have corresponding JIT releases at all, further complicating debugging efforts
(such as bisection to find breaking changes upstream).

Since the JIT is already quite stable, and the ultimate goal is for it to be a
non-experimental part of CPython, keeping it in ``main`` seems to be the best
path forward. With that said, the relevant code is organized in such a way that
the JIT can be easily "deleted" if it does not end up meeting its goals.

Turn it on by default
---------------------

On the other hand, some have suggested that the JIT should be enabled by default
in its current form.

Again, it is important to remember that a JIT is not a magic "go faster"
machine; currently, the JIT is about as fast as the existing specializing
interpreter. This may sound underwhelming, but it is actually a fairly
significant achievement, and it's the main reason why this approach was
considered viable enough to be merged into ``main`` for further development.

While the JIT provides significant gains over the existing micro-op interpreter,
it isn't yet a clear win when always enabled (especially considering its
increased memory consumption and additional build-time dependencies). That's the
purpose of this PEP: to clarify expectations about the objective criteria that
should be met in order to "flip the switch".

At least for now, having this in ``main``, but off by default, seems to be a
good compromise between always turning it on and not having it available at all.

Support multiple compiler toolchains
------------------------------------

Clang is specifically needed because it's the only C compiler with support for
guaranteed tail calls (|musttail|_), which are required by CPython's
`continuation-passing-style
<https://en.wikipedia.org/wiki/Continuation-passing_style#Tail_calls>`__ approach
to JIT compilation. Without it, the tail-recursive calls between templates could
result in unbounded C stack growth (and eventual overflow).

.. |musttail| replace:: ``musttail``
.. _musttail: https://clang.llvm.org/docs/AttributeReference.html#musttail

Since LLVM also includes other functionalities required by the JIT build process
(namely, utilities for object file parsing and disassembly), and additional
toolchains introduce additional testing and maintenance burden, it's convenient
to only support one major version of one toolchain at this time.

Compile the base interpreter's bytecode
---------------------------------------

Most of the prior art for copy-and-patch uses it as a fast baseline JIT, whereas
CPython's JIT is using the technique to compile optimized micro-op traces.

In practice, the new JIT currently sits somewhere between the "baseline" and
"optimizing" compiler tiers of other dynamic language runtimes. This is because
CPython uses its specializing adaptive interpreter to collect runtime profiling
information, which is used to detect and optimize "hot" paths through the code.
This step is carried out using self-modifying code, a technique which is much
more difficult to implement with a JIT compiler.

While it's *possible* to compile normal bytecode using copy-and-patch (in fact,
early prototypes predated the micro-op interpreter and did exactly this), it
just doesn't seem to provide enough optimization potential as the more granular
micro-op format.

Add GPU support
---------------

The JIT is currently CPU-only. It does not, for example, offload NumPy array
computations to CUDA GPUs, as JITs like `Numba
<https://numba.pydata.org/numba-doc/latest/cuda/overview.html>`__ do.

There is already a rich ecosystem of tools for accelerating these sorts of
specialized tasks, and CPython's JIT is not intended to replace them. Instead,
it is meant to improve the performance of general-purpose Python code, which is
less likely to benefit from deeper GPU integration.

Open Issues
===========

Speed
-----

Currently, the JIT is `about as fast as the existing specializing interpreter
<https://github.com/faster-cpython/benchmarking-public/blob/main/configs.svg>`__
on most platforms. Improving this is obviously a top priority at this point,
since providing a significant performance gain is the entire motivation for
having a JIT at all. A number of proposed improvements are already underway, and
this ongoing work is being tracked in `GH-115802
<https://github.com/python/cpython/issues/115802>`__.

Memory
------

Because it allocates additional memory for executable machine code, the JIT does
use more memory than the existing interpreter at runtime. According to the
official benchmarks, the JIT currently uses about `10-20% more memory than the
base interpreter
<https://github.com/faster-cpython/benchmarking-public/blob/main/memory_configs.svg>`__.
The upper end of this range is due to ``aarch64-apple-darwin``, which has larger
page sizes (and thus, a larger minimum allocation granularity).

However, these numbers should be taken with a grain of salt, as the benchmarks
themselves don't actually have a very high baseline of memory usage. Since they
have a higher ratio of code to data, the JIT's memory overhead is more
pronounced than it would be in a typical workload where memory pressure is more
likely to be a real concern.

Not much effort has been put into optimizing the JIT's memory usage yet, so
these numbers likely represent a maximum that will be reduced over time.
Improving this is a medium priority, and is being tracked in `GH-116017
<https://github.com/python/cpython/issues/116017>`__. We may consider
exposing configurable parameters for limiting memory consumption in the
future, but no official APIs will be exposed until the JIT meets the
requirements to be considered non-experimental.

Earlier versions of the JIT had a more complicated memory allocation scheme
which imposed a number of fragile limitations on the size and layout of the
emitted code, and significantly bloated the memory footprint of Python
executable. These issues are no longer present in the current design.

Dependencies
------------

At the time of writing, the JIT has a build-time dependency on LLVM. LLVM
is used to compile individual micro-op instructions into blobs of machine code,
which are then linked together to form the JIT's templates. These templates are
used to build CPython itself. The JIT has no runtime dependency on LLVM and is
therefore not at all exposed as a dependency to end users.

Building the JIT adds between 3 and 60 seconds to the build process, depending
on platform. It is only rebuilt whenever the generated files become out-of-date,
so only those who are actively developing the main interpreter loop will be
rebuilding it with any frequency.

Unlike many other generated files in CPython, the JIT's generated files are not
tracked by Git. This is because they contain compiled binary code templates
specific to not only the host platform, but also the current build configuration
for that platform. As such, hosting them would require a significant engineering
effort in order to build and host dozens of large binary files for each commit
that changes the generated code. While perhaps feasible, this is not a priority,
since installing the required tools is not prohibitively difficult for most
people building CPython, and the build step is not particularly time-consuming.

Since some still remain interested in this possibility, discussion is being
tracked in `GH-115869 <https://github.com/python/cpython/issues/115869>`__.

Footnotes
=========

.. [#untested] Due to lack of available hardware, the JIT is built, but not
   tested, for this platform.

.. [#emulated] Due to lack of available hardware, the JIT is built using
   cross-compilation and tested using hardware emulation for this platform. Some
   tests are skipped because emulation causes them to fail. However, the JIT has
   been successfully built and tested for this platform on non-emulated
   hardware.

Copyright
=========

This document is placed in the public domain or under the CC0-1.0-Universal
license, whichever is more permissive.
