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47 lines
2.3 KiB
Markdown
47 lines
2.3 KiB
Markdown
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The JIT Compiler
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================
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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 16 is required. Both `clang` and `llvm-readobj` need to be installed and discoverable (version suffixes, like `clang-16`, 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 16 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 16
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```
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### macOS
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Install LLVM 16 with [Homebrew](https://brew.sh):
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```sh
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brew install llvm@16
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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 16 [by searching for it on LLVM's GitHub releases page](https://github.com/llvm/llvm-project/releases?q=16), 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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## 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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