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Add note about PYTHON_JIT environment variable to JIT README (GH-121635)

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Savannah Ostrowski 2024-07-17 15:17:47 -07:00 committed by GitHub
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@ -1,7 +1,7 @@
The JIT Compiler The JIT Compiler
================ ================
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. This version of CPython can be built with an experimental just-in-time compiler[^pep-744]. 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.
## Installing LLVM ## Installing LLVM
@ -57,6 +57,10 @@ For `PCbuild`-based builds, pass the new `--experimental-jit` option to `build.b
For all other builds, pass the new `--enable-experimental-jit` option to `configure`. For all other builds, pass the new `--enable-experimental-jit` option to `configure`.
Otherwise, just configure and build as you normally would. Cross-compiling "just works", since the JIT is built for the host platform. Otherwise, just configure and build as you normally would. Cross-compiling "just works", since the JIT is built for the host platform.
The JIT can also be enabled or disabled using the `PYTHON_JIT` environment variable, even on builds where it is enabled or disabled by default. More details about configuring CPython with the JIT and optional values for `--enable-experimental-jit` can be found [here](https://docs.python.org/dev/whatsnew/3.13.html#experimental-jit-compiler).
[^pep-744]: [PEP 744](https://peps.python.org/pep-0744/)
[^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. [^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.