GH-136895: Update JIT builds to use LLVM 21

Bump the JIT toolchain from LLVM 20 to LLVM 21 (21.1.8). Updated across CI workflows, build scripts, documentation, and the externals tag.
5477f5b3e6063aa8e265608ade7950eaa3cce7cf
commit 5477f5b3e6063aa8e265608ade7950eaa3cce7cf
@@ -21,7 +21,7 @@ concurrency:
 
 env:
   FORCE_COLOR: 1
-  LLVM_VERSION: 20
+  LLVM_VERSION: 21
 
 jobs:
   interpreter:

@@ -20,7 +20,7 @@ concurrency:
 
 env:
   FORCE_COLOR: 1
-  LLVM_VERSION: 20
+  LLVM_VERSION: 21
 
 jobs:
   windows:

diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-10-19-10-32-28.gh-issue-136895.HfsEh0.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-10-19-10-32-28.gh-issue-136895.HfsEh0.rst
@@ -1 +1 @@
-Update JIT compilation to use LLVM 20 at build time.
+Update JIT compilation to use LLVM 21 at build time.
@@ -82,7 +82,7 @@ if NOT "%IncludeLibffi%"=="false"  set binaries=%binaries% libffi-3.4.4
 if NOT "%IncludeSSL%"=="false"     set binaries=%binaries% openssl-bin-3.0.19
 if NOT "%IncludeTkinter%"=="false" set binaries=%binaries% tcltk-8.6.15.0
 if NOT "%IncludeSSLSrc%"=="false"  set binaries=%binaries% nasm-2.11.06
-if NOT "%IncludeLLVM%"=="false"    set binaries=%binaries% llvm-20.1.8.0
+if NOT "%IncludeLLVM%"=="false"    set binaries=%binaries% llvm-21.1.4.0
 
 for %%b in (%binaries%) do (
     if exist "%EXTERNALS_DIR%\%%b" (

@@ -428,7 +428,7 @@ void patch_x86_64_trampoline(unsigned char *location, int ordinal, jit_state *st
     #define DATA_ALIGN 8
 #elif defined(__x86_64__) || defined(_M_X64)
     // x86_64 trampolines: 14 bytes (jmp *(%rip) + 8-byte addr) + 2 bytes padding.
-    // Currently used on macOS where LLVM 20 GOT entries may exceed ±2GB
+    // Currently used on macOS where LLVM 21 GOT entries may exceed ±2GB
     // PC-relative range, but enabled on all x86_64 platforms defensively.
     #define TRAMPOLINE_SIZE 16
     #define DATA_ALIGN 8

@@ -9,32 +9,32 @@ Python 3.11 or newer is required to build the JIT.
 
 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).
 
-LLVM version 20 is the officially supported version. You can modify if needed using the `LLVM_VERSION` env var during configure. Both `clang` and `llvm-readobj` need to be installed and discoverable (version suffixes, like `clang-19`, 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.
+LLVM version 21 is the officially supported version. You can modify if needed using the `LLVM_VERSION` env var during configure. Both `clang` and `llvm-readobj` need to be installed and discoverable (version suffixes, like `clang-21`, 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.
 
 It's easy to install all of the required tools:
 
 ### Linux
 
-Install LLVM 20 on Ubuntu/Debian:
+Install LLVM 21 on Ubuntu/Debian:
 
 ```sh
 wget https://apt.llvm.org/llvm.sh
 chmod +x llvm.sh
-sudo ./llvm.sh 20
+sudo ./llvm.sh 21
 ```
 
-Install LLVM 20 on Fedora Linux 40 or newer:
+Install LLVM 21 on Fedora Linux 40 or newer:
 
 ```sh
-sudo dnf install 'clang(major) = 20' 'llvm(major) = 20'
+sudo dnf install 'clang(major) = 21' 'llvm(major) = 21'
 ```
 
 ### macOS
 
-Install LLVM 20 with [Homebrew](https://brew.sh):
+Install LLVM 21 with [Homebrew](https://brew.sh):
 
 ```sh
-brew install llvm@20
+brew install llvm@21
 ```
 
 Homebrew won't add any of the tools to your `$PATH`. That's okay; the build script knows how to find them.
@@ -43,18 +43,18 @@ Homebrew won't add any of the tools to your `$PATH`. That's okay; the build scri
 
 LLVM is downloaded automatically (along with other external binary dependencies) by `PCbuild\build.bat`.
 
-Otherwise, you can install LLVM 20 [by searching for it on LLVM's GitHub releases page](https://github.com/llvm/llvm-project/releases?q=20), 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".**
+Otherwise, you can install LLVM 21 [by searching for it on LLVM's GitHub releases page](https://github.com/llvm/llvm-project/releases?q=21), 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".**
 
 Alternatively, you can use [chocolatey](https://chocolatey.org):
 
 ```sh
-choco install llvm --version=20.1.8
+choco install llvm --version=21.1.8
 ```
 
 ### Dev Containers
 
 If you are working on 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 42 base image includes LLVM 20 out of the box.
+need to install LLVM as the Fedora 42 base image includes LLVM 21 out of the box.
 
 ## Building
 

@@ -10,9 +10,9 @@
 
 import _targets
 
-_LLVM_VERSION = 20
+_LLVM_VERSION = 21
 _LLVM_VERSION_PATTERN = re.compile(rf"version\s+{_LLVM_VERSION}\.\d+\.\d+\S*\s+")
-_EXTERNALS_LLVM_TAG = "llvm-20.1.8.0"
+_EXTERNALS_LLVM_TAG = "llvm-21.1.4.0"
 
 _P = typing.ParamSpec("_P")
 _R = typing.TypeVar("_R")
Original file line number	Diff line number	Diff line change
		@@ -1 +1 @@
		Update JIT compilation to use LLVM 20 at build time.
		Update JIT compilation to use LLVM 21 at build time.