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The code for Tier 2 is now only compiled when configured with `--enable-experimental-jit[=yes|interpreter]`. We drop support for `PYTHON_UOPS` and -`Xuops`, but you can disable the interpreter or JIT at runtime by setting `PYTHON_JIT=0`. You can also build it without enabling it by default using `--enable-experimental-jit=yes-off`; enable with `PYTHON_JIT=1`. On Windows, the `build.bat` script supports `--experimental-jit`, `--experimental-jit-off`, `--experimental-interpreter`. In the C code, `_Py_JIT` is defined as before when the JIT is enabled; the new variable `_Py_TIER2` is defined when the JIT *or* the interpreter is enabled. It is actually a bitmask: 1: JIT; 2: default-off; 4: interpreter.
50 lines
2.5 KiB
Markdown
50 lines
2.5 KiB
Markdown
The JIT Compiler
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This version of CPython can be built with an experimental just-in-time compiler. While most everything you already know about building and using CPython is unchanged, you will probably need to install a compatible version of LLVM first.
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## Installing LLVM
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The JIT compiler does not require end users to install any third-party dependencies, but part of it must be *built* using LLVM[^why-llvm]. You are *not* required to build the rest of CPython using LLVM, or even the same version of LLVM (in fact, this is uncommon).
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LLVM version 18 is required. Both `clang` and `llvm-readobj` need to be installed and discoverable (version suffixes, like `clang-18`, are okay). It's highly recommended that you also have `llvm-objdump` available, since this allows the build script to dump human-readable assembly for the generated code.
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It's easy to install all of the required tools:
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### Linux
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Install LLVM 18 on Ubuntu/Debian:
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```sh
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wget https://apt.llvm.org/llvm.sh
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chmod +x llvm.sh
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sudo ./llvm.sh 18
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```
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### macOS
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Install LLVM 18 with [Homebrew](https://brew.sh):
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```sh
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brew install llvm@18
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```
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Homebrew won't add any of the tools to your `$PATH`. That's okay; the build script knows how to find them.
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### Windows
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Install LLVM 18 [by searching for it on LLVM's GitHub releases page](https://github.com/llvm/llvm-project/releases?q=18), clicking on "Assets", downloading the appropriate Windows installer for your platform (likely the file ending with `-win64.exe`), and running it. **When installing, be sure to select the option labeled "Add LLVM to the system PATH".**
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### Dev Containers
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If you are working CPython in a [Codespaces instance](https://devguide.python.org/getting-started/setup-building/#using-codespaces), there's no need to install LLVM as the Fedora 40 base image includes LLVM 18 out of the box.
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## Building
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For `PCbuild`-based builds, pass the new `--experimental-jit` option to `build.bat`.
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For all other builds, pass the new `--enable-experimental-jit` option to `configure`.
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Otherwise, just configure and build as you normally would. Cross-compiling "just works", since the JIT is built for the host platform.
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[^why-llvm]: 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 approach to JIT compilation. Since LLVM also includes other functionalities we need (namely, object file parsing and disassembly), it's convenient to only support one toolchain at this time.
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