JIT: harden LLVM 20 upgrade with defensive improvements

- Enable x86_64 trampoline support on all x86_64 platforms, not just
  macOS. While currently only macOS emits X86_64_RELOC_BRANCH
  relocations, this ensures the trampoline infrastructure is ready if
  future LLVM changes produce out-of-range calls on other x86_64
  platforms.

- Add error handling to extract_tarball() in get_external.py so that
  corrupted or incomplete downloads produce a clear error message
  instead of an opaque traceback.

- Replace hardcoded LLVM version check in get_externals.bat with a
  prefix-based 'llvm-' match using findstr, so future LLVM version
  bumps only require changing the version string in one place.

Author: mcepl

PCbuild/get_external.py

@@ -56,7 +56,13 @@ def fetch_release(tag, tarball_dir, *, org='python', verbose=False):
 
 def extract_tarball(externals_dir, tarball_path, tag):
     output_path = externals_dir / tag
-    shutil.unpack_archive(os.fspath(tarball_path), os.fspath(output_path))
+    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
 
 

PCbuild/get_externals.bat

@@ -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)
@@ -92,11 +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...
-        if "%%b"=="llvm-20.1.8.0" (
-            %PYTHON% -E "%PCBUILD%\get_external.py" --release --organization %ORG% --externals-dir "%EXTERNALS_DIR%" %%b
-        ) else (
-            %PYTHON% -E "%PCBUILD%\get_external.py" --binary --organization %ORG% --externals-dir "%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
     )
 )
 

Python/jit.c

@@ -426,10 +426,11 @@ void patch_x86_64_trampoline(unsigned char *location, int ordinal, jit_state *st
 #if defined(__aarch64__) || defined(_M_ARM64)
     #define TRAMPOLINE_SIZE 16
     #define DATA_ALIGN 8
-#elif defined(__x86_64__) && defined(__APPLE__)
-    // LLVM 20 on macOS x86_64 debug builds: GOT entries may exceed ±2GB PC-relative
-    // range.
-    #define TRAMPOLINE_SIZE 16  // 14 bytes + 2 bytes padding for alignment
+#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
+    // PC-relative range, but enabled on all x86_64 platforms defensively.
+    #define TRAMPOLINE_SIZE 16
     #define DATA_ALIGN 8
 #else
     #define TRAMPOLINE_SIZE 0