@@ -21,7 +21,7 @@ concurrency:
 
 env:
   FORCE_COLOR: 1
-  LLVM_VERSION: 19
+  LLVM_VERSION: 21
 
 jobs:
   interpreter:
@@ -168,7 +168,6 @@ jobs:
       fail-fast: false
       matrix:
         include:
-
           - name: JIT without optimizations (Debug)
             configure_flags: --enable-experimental-jit --with-pydebug
             test_env: "PYTHON_UOPS_OPTIMIZE=0"

@@ -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
@@ -0,0 +1 @@
+Update JIT compilation to use LLVM 21 at build time.
@@ -3,6 +3,7 @@
 import argparse
 import os
 import pathlib
+import shutil
 import sys
 import time
 import urllib.error
@@ -22,15 +23,13 @@ def retrieve_with_retries(download_location, output_path, reporthook,
             )
         except (urllib.error.URLError, ConnectionError) as ex:
             if attempt == max_retries:
-                msg = f"Download from {download_location} failed."
-                raise OSError(msg) from ex
+                raise OSError(f'Download from {download_location} failed.') from ex
             time.sleep(2.25**attempt)
         else:
             return resp
 
-
 def fetch_zip(commit_hash, zip_dir, *, org='python', binary=False, verbose):
-    repo = f'cpython-{"bin" if binary else "source"}-deps'
+    repo = 'cpython-bin-deps' if binary else 'cpython-source-deps'
     url = f'https://github.com/{org}/{repo}/archive/{commit_hash}.zip'
     reporthook = None
     if verbose:
@@ -44,6 +43,29 @@ def fetch_zip(commit_hash, zip_dir, *, org='python', binary=False, verbose):
     return filename
 
 
+def fetch_release(tag, tarball_dir, *, org='python', verbose=False):
+    url = f'https://github.com/{org}/cpython-bin-deps/releases/download/{tag}/{tag}.tar.xz'
+    reporthook = None
+    if verbose:
+        reporthook = print
+    tarball_dir.mkdir(parents=True, exist_ok=True)
+    output_path = tarball_dir / f'{tag}.tar.xz'
+    retrieve_with_retries(url, output_path, reporthook)
+    return output_path
+
+
+def extract_tarball(externals_dir, tarball_path, tag):
+    output_path = externals_dir / tag
+    try:
+        shutil.unpack_archive(os.fspath(tarball_path), os.fspath(output_path))
+    except Exception as ex:
+        raise OSError(
+            f'Failed to extract {tarball_path}. The archive may be '
+            f'corrupted; try deleting it and re-running.'
+        ) from ex
+    return output_path
+
+
 def extract_zip(externals_dir, zip_path):
     with zipfile.ZipFile(os.fspath(zip_path)) as zf:
         zf.extractall(os.fspath(externals_dir))
@@ -55,6 +77,8 @@ def parse_args():
     p.add_argument('-v', '--verbose', action='store_true')
     p.add_argument('-b', '--binary', action='store_true',
                    help='Is the dependency in the binary repo?')
+    p.add_argument('-r', '--release', action='store_true',
+                   help='Download from GitHub release assets instead of branch')
     p.add_argument('-O', '--organization',
                    help='Organization owning the deps repos', default='python')
     p.add_argument('-e', '--externals-dir', type=pathlib.Path,
@@ -67,15 +91,36 @@ def parse_args():
 
 def main():
     args = parse_args()
-    zip_path = fetch_zip(
-        args.tag,
-        args.externals_dir / 'zips',
-        org=args.organization,
-        binary=args.binary,
-        verbose=args.verbose,
-    )
     final_name = args.externals_dir / args.tag
-    extracted = extract_zip(args.externals_dir, zip_path)
+
+    # Check if the dependency already exists in externals/ directory
+    # (either already downloaded/extracted, or checked into the git tree)
+    if final_name.exists():
+        if args.verbose:
+            print(f'{args.tag} already exists at {final_name}, skipping download.')
+        return
+
+    # Determine download method: release artifacts for large deps (like LLVM),
+    # otherwise zip download from GitHub branches
+    if args.release:
+        tarball_path = fetch_release(
+            args.tag,
+            args.externals_dir / 'tarballs',
+            org=args.organization,
+            verbose=args.verbose,
+        )
+        extracted = extract_tarball(args.externals_dir, tarball_path, args.tag)
+    else:
+        # Use zip download from GitHub branches
+        # (cpython-bin-deps if --binary, cpython-source-deps otherwise)
+        zip_path = fetch_zip(
+            args.tag,
+            args.externals_dir / 'zips',
+            org=args.organization,
+            binary=args.binary,
+            verbose=args.verbose,
+        )
+        extracted = extract_zip(args.externals_dir, zip_path)
     for wait in [1, 2, 3, 5, 8, 0]:
         try:
             extracted.replace(final_name)

@@ -1,5 +1,5 @@
 @echo off
-setlocal
+setlocal EnableDelayedExpansion
 rem Simple script to fetch source for external libraries
 
 if NOT DEFINED PCBUILD (set PCBUILD=%~dp0)
@@ -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-19.1.7.0
+if NOT "%IncludeLLVM%"=="false"    set binaries=%binaries% llvm-21.1.4.0
 
 for %%b in (%binaries%) do (
     if exist "%EXTERNALS_DIR%\%%b" (
@@ -92,7 +92,9 @@ for %%b in (%binaries%) do (
         git clone --depth 1 https://github.com/%ORG%/cpython-bin-deps --branch %%b "%EXTERNALS_DIR%\%%b"
     ) else (
         echo.Fetching %%b...
-        %PYTHON% -E "%PCBUILD%\get_external.py" -b -O %ORG% -e "%EXTERNALS_DIR%" %%b
+        set _fetch_args=--binary
+        echo %%b | findstr /B "llvm-" >nul && set _fetch_args=--release
+        %PYTHON% -E "%PCBUILD%\get_external.py" !_fetch_args! --organization %ORG% --externals-dir "%EXTERNALS_DIR%" %%b
     )
 )
 

@@ -419,15 +419,43 @@ patch_x86_64_32rx(unsigned char *location, uint64_t value)
 }
 
 void patch_aarch64_trampoline(unsigned char *location, int ordinal, jit_state *state);
+void patch_x86_64_trampoline(unsigned char *location, int ordinal, jit_state *state);
 
 #include "jit_stencils.h"
 
 #if defined(__aarch64__) || defined(_M_ARM64)
     #define TRAMPOLINE_SIZE 16
+    #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 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
 #else
     #define TRAMPOLINE_SIZE 0
+    #define DATA_ALIGN 1
 #endif
 
+// Get the trampoline memory location for a given symbol ordinal.
+static unsigned char *
+get_trampoline_slot(int ordinal, jit_state *state)
+{
+    const uint32_t symbol_mask = 1 << (ordinal % 32);
+    const uint32_t trampoline_mask = state->trampolines.mask[ordinal / 32];
+    assert(symbol_mask & trampoline_mask);
+
+    // Count the number of set bits in the trampoline mask lower than ordinal.
+    int index = _Py_popcount32(trampoline_mask & (symbol_mask - 1));
+    for (int i = 0; i < ordinal / 32; i++) {
+        index += _Py_popcount32(state->trampolines.mask[i]);
+    }
+
+    unsigned char *trampoline = state->trampolines.mem + index * TRAMPOLINE_SIZE;
+    assert((size_t)(index + 1) * TRAMPOLINE_SIZE <= state->trampolines.size);
+    return trampoline;
+}
+
 // Generate and patch AArch64 trampolines. The symbols to jump to are stored
 // in the jit_stencils.h in the symbols_map.
 void
@@ -444,20 +472,8 @@ patch_aarch64_trampoline(unsigned char *location, int ordinal, jit_state *state)
         return;
     }
 
-    // Masking is done modulo 32 as the mask is stored as an array of uint32_t
-    const uint32_t symbol_mask = 1 << (ordinal % 32);
-    const uint32_t trampoline_mask = state->trampolines.mask[ordinal / 32];
-    assert(symbol_mask & trampoline_mask);
-
-    // Count the number of set bits in the trampoline mask lower than ordinal,
-    // this gives the index into the array of trampolines.
-    int index = _Py_popcount32(trampoline_mask & (symbol_mask - 1));
-    for (int i = 0; i < ordinal / 32; i++) {
-        index += _Py_popcount32(state->trampolines.mask[i]);
-    }
-
-    uint32_t *p = (uint32_t*)(state->trampolines.mem + index * TRAMPOLINE_SIZE);
-    assert((size_t)(index + 1) * TRAMPOLINE_SIZE <= state->trampolines.size);
+    // Out of range - need a trampoline
+    uint32_t *p = (uint32_t *)get_trampoline_slot(ordinal, state);
 
 
     /* Generate the trampoline
@@ -474,6 +490,37 @@ patch_aarch64_trampoline(unsigned char *location, int ordinal, jit_state *state)
     patch_aarch64_26r(location, (uintptr_t)p);
 }
 
+// Generate and patch x86_64 trampolines.
+void
+patch_x86_64_trampoline(unsigned char *location, int ordinal, jit_state *state)
+{
+    uint64_t value = (uintptr_t)symbols_map[ordinal];
+    int64_t range = (int64_t)value - 4 - (int64_t)location;
+
+    // If we are in range of 32 signed bits, we can patch directly
+    if (range >= -(1LL << 31) && range < (1LL << 31)) {
+        patch_32r(location, value - 4);
+        return;
+    }
+
+    // Out of range - need a trampoline
+    unsigned char *trampoline = get_trampoline_slot(ordinal, state);
+
+    /* Generate the trampoline (14 bytes, padded to 16):
+       0: ff 25 00 00 00 00    jmp *(%rip)
+       6: XX XX XX XX XX XX XX XX   (64-bit target address)
+
+       Reference: https://wiki.osdev.org/X86-64_Instruction_Encoding#FF (JMP r/m64)
+    */
+    trampoline[0] = 0xFF;
+    trampoline[1] = 0x25;
+    memset(trampoline + 2, 0, 4);
+    memcpy(trampoline + 6, &value, 8);
+
+    // Patch the call site to call the trampoline instead
+    patch_32r(location, (uintptr_t)trampoline - 4);
+}
+
 static void
 combine_symbol_mask(const symbol_mask src, symbol_mask dest)
 {
@@ -515,8 +562,13 @@ _PyJIT_Compile(_PyExecutorObject *executor, const _PyUOpInstruction trace[], siz
     // Round up to the nearest page:
     size_t page_size = get_page_size();
     assert((page_size & (page_size - 1)) == 0);
-    size_t padding = page_size - ((code_size + state.trampolines.size + data_size) & (page_size - 1));
-    size_t total_size = code_size + state.trampolines.size + data_size  + padding;
+    size_t code_padding =
+        DATA_ALIGN - ((code_size + state.trampolines.size) & (DATA_ALIGN - 1));
+    size_t padding = page_size -
+        ((code_size + state.trampolines.size + code_padding + data_size) &
+         (page_size - 1));
+    size_t total_size =
+        code_size + state.trampolines.size + code_padding + data_size + padding;
     unsigned char *memory = jit_alloc(total_size);
     if (memory == NULL) {
         return -1;
@@ -535,7 +587,7 @@ _PyJIT_Compile(_PyExecutorObject *executor, const _PyUOpInstruction trace[], siz
     // Loop again to emit the code:
     unsigned char *code = memory;
     state.trampolines.mem = memory + code_size;
-    unsigned char *data = memory + code_size + state.trampolines.size;
+    unsigned char *data = memory + code_size + state.trampolines.size + code_padding;
     // Compile the shim, which handles converting between the native
     // calling convention and the calling convention used by jitted code
     // (which may be different for efficiency reasons).
@@ -557,7 +609,9 @@ _PyJIT_Compile(_PyExecutorObject *executor, const _PyUOpInstruction trace[], siz
     code += group->code_size;
     data += group->data_size;
     assert(code == memory + code_size);
-    assert(data == memory + code_size + state.trampolines.size + data_size);
+    assert(
+        data ==
+        memory + code_size + state.trampolines.size + code_padding + data_size);
     if (mark_executable(memory, total_size)) {
         jit_free(memory, total_size);
         return -1;

@@ -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 19 is required. 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 19 on Ubuntu/Debian:
+Install LLVM 21 on Ubuntu/Debian:
 
 ```sh
 wget https://apt.llvm.org/llvm.sh
 chmod +x llvm.sh
-sudo ./llvm.sh 19
+sudo ./llvm.sh 21
 ```
 
-Install LLVM 19 on Fedora Linux 40 or newer:
+Install LLVM 21 on Fedora Linux 40 or newer:
 
 ```sh
-sudo dnf install 'clang(major) = 19' 'llvm(major) = 19'
+sudo dnf install 'clang(major) = 21' 'llvm(major) = 21'
 ```
 
 ### macOS
 
-Install LLVM 19 with [Homebrew](https://brew.sh):
+Install LLVM 21 with [Homebrew](https://brew.sh):
 
 ```sh
-brew install llvm@19
+brew install llvm@21
 ```
 
 Homebrew won't add any of the tools to your `$PATH`. That's okay; the build script knows how to find them.
@@ -43,14 +43,18 @@ Homebrew won't add any of the tools to your `$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 19 [by searching for it on LLVM's GitHub releases page](https://github.com/llvm/llvm-project/releases?q=19), 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=19.1.0
+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 21 out of the box.
 
 ## Building
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		Update JIT compilation to use LLVM 21 at build time.