JIT build fails on macOS 26: due to .cfi directives

[DarkaMaul]

Bug report

Bug description:

How to reproduce

CPython Commit: 4af46b4 (April 15)

./configure --enable-experimental-jit --with-pydebug
make

The following command fails with the following errors:

❯ make -j1
python3.15 ./Tools/jit/build.py aarch64-apple-darwin25.4.0 --output-dir . --pyconfig-dir . --cflags="" --llvm-version="" --debug

===========================================================
JIT support for aarch64-apple-darwin is still experimental!
 Please report any issues you encounter.
===========================================================

/var/folders/m9/4q7skk_n3vj6k6bvy2vc4tnr0000gn/T/_BINARY_OP_ADD_FLOAT_INPLACE_RIGHT_r13-0ccf97.s:879:1: error: starting new .cfi frame before finishing the previous one
; %bb.0:
^
/var/folders/m9/4q7skk_n3vj6k6bvy2vc4tnr0000gn/T/_BINARY_OP_ADD_FLOAT_INPLACE_RIGHT_r13-0ccf97.s:890:1: error: starting new .cfi frame before finishing the previous one
; %bb.0:
^
/var/folders/m9/4q7skk_n3vj6k6bvy2vc4tnr0000gn/T/_BINARY_OP_ADD_FLOAT_INPLACE_RIGHT_r13-0ccf97.s:901:1: error: starting new .cfi frame before finishing the previous one
; %bb.0:

...

^
 + Exception Group Traceback (most recent call last):
 | File "/Users/dm/Projects/cpython-upstream/Tools/jit/_targets.py", line 213, in _build_stencils
 | async with asyncio.TaskGroup() as group:
 | ~~~~~~~~~~~~~~~~~^^
 | File "/Users/dm/.local/share/uv/python/cpython-3.15.0a7-macos-aarch64-none/lib/python3.15/asyncio/taskgroups.py", line 72, in __aexit__
 | return await self._aexit(et, exc)
 | ^^^^^^^^^^^^^^^^^^^^^^^^^^
 | File "/Users/dm/.local/share/uv/python/cpython-3.15.0a7-macos-aarch64-none/lib/python3.15/asyncio/taskgroups.py", line 174, in _aexit
 | raise BaseExceptionGroup(
 | ...<2 lines>...
 | ) from None
 | ExceptionGroup: unhandled errors in a TaskGroup (1 sub-exception)
 +-+---------------- 1 ----------------
 | Traceback (most recent call last):
 | File "/Users/dm/Projects/cpython-upstream/Tools/jit/_targets.py", line 198, in _compile
 | await _llvm.run(
 | "clang", args_o, echo=self.verbose, llvm_version=self.llvm_version
 | )
 | File "/Users/dm/Projects/cpython-upstream/Tools/jit/_llvm.py", line 132, in run
 | output = await maybe_run(tool, args, echo=echo, llvm_version=llvm_version)
 | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 | File "/Users/dm/Projects/cpython-upstream/Tools/jit/_llvm.py", line 121, in maybe_run
 | return path and await _run(path, args, echo=echo)
 | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 | File "/Users/dm/Projects/cpython-upstream/Tools/jit/_llvm.py", line 63, in _run
 | raise RuntimeError(f"{tool} exited with return code {process.returncode}")
 | RuntimeError: clang exited with return code 1
 +------------------------------------

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
 File "/Users/dm/Projects/cpython-upstream/./Tools/jit/build.py", line 55, in <module>
 target.build(
 ~~~~~~~~~~~~^
 comment=comment,
 ^^^^^^^^^^^^^^^^
 force=args.force,
 ^^^^^^^^^^^^^^^^^
 jit_stencils=args.output_dir / f"jit_stencils-{target.triple}.h",
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 )
 ^
 File "/Users/dm/Projects/cpython-upstream/Tools/jit/_targets.py", line 256, in build
 stencil_groups = ASYNCIO_RUNNER.run(self._build_stencils())
 File "/Users/dm/.local/share/uv/python/cpython-3.15.0a7-macos-aarch64-none/lib/python3.15/asyncio/runners.py", line 127, in run
 return self._loop.run_until_complete(task)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
 File "/Users/dm/.local/share/uv/python/cpython-3.15.0a7-macos-aarch64-none/lib/python3.15/asyncio/base_events.py", line 719, in run_until_complete
 return future.result()
 ~~~~~~~~~~~~~^^
 File "/Users/dm/Projects/cpython-upstream/Tools/jit/_targets.py", line 211, in _build_stencils
 with tempfile.TemporaryDirectory() as tempdir:
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
 File "/Users/dm/.local/share/uv/python/cpython-3.15.0a7-macos-aarch64-none/lib/python3.15/tempfile.py", line 971, in __exit__
 self.cleanup()
 ~~~~~~~~~~~~^^
 File "/Users/dm/.local/share/uv/python/cpython-3.15.0a7-macos-aarch64-none/lib/python3.15/tempfile.py", line 975, in cleanup
 self._rmtree(self.name, ignore_errors=self._ignore_cleanup_errors)
 ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 File "/Users/dm/.local/share/uv/python/cpython-3.15.0a7-macos-aarch64-none/lib/python3.15/tempfile.py", line 955, in _rmtree
 _shutil.rmtree(name, onexc=onexc)
 ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^
 File "/Users/dm/.local/share/uv/python/cpython-3.15.0a7-macos-aarch64-none/lib/python3.15/shutil.py", line 852, in rmtree
 _rmtree_impl(path, dir_fd, onexc)
 ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^
 File "/Users/dm/.local/share/uv/python/cpython-3.15.0a7-macos-aarch64-none/lib/python3.15/shutil.py", line 721, in _rmtree_safe_fd
 _rmtree_safe_fd_step(stack, onexc)
 ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^
 File "/Users/dm/.local/share/uv/python/cpython-3.15.0a7-macos-aarch64-none/lib/python3.15/shutil.py", line 802, in _rmtree_safe_fd_step
 onexc(func, path, err)
 ~~~~~^^^^^^^^^^^^^^^^^
 File "/Users/dm/.local/share/uv/python/cpython-3.15.0a7-macos-aarch64-none/lib/python3.15/shutil.py", line 753, in _rmtree_safe_fd_step
 os.rmdir(name, dir_fd=dirfd)
 ~~~~~~~~^^^^^^^^^^^^^^^^^^^^
OSError: [Errno 66] Directory not empty: '/var/folders/m9/4q7skk_n3vj6k6bvy2vc4tnr0000gn/T/tmpjlgkd9jf'
make: *** [jit_stencils-aarch64-apple-darwin.h] Error 1

Root cause

Warning: this analysis was performed by Claude, and seemed plausible, but that's not my area of expertise.

LLVM 21's machine outliner (enabled by -Os) generates _OUTLINED_FUNCTION_* thunks at the end of the assembly output. Each thunk has .cfi_startproc but no matching .cfi_endproc:

_OUTLINED_FUNCTION_1: ; @OUTLINED_FUNCTION_1 Thunk
	.cfi_startproc
; %bb.0:
	...
	b	___assert_rtn
_OUTLINED_FUNCTION_2: ; @OUTLINED_FUNCTION_2 Thunk
	.cfi_startproc ; <-- ERROR: previous .cfi frame never ended
; %bb.0:
	...

The Xcode 26 assembler (Apple clang 21.0.0 / clang-2100) is stricter about nested .cfi frames than previous versions. CPython CI passes because it runs on macOS 15 with an older Xcode.

Note that -fno-asynchronous-unwind-tables is already passed during compilation, so the .cfi directives in these outlined thunks serve no purpose — the outliner just emits them unconditionally.

Environnement

• macOS 26.4.1 (Darwin 25.4.0), aarch64
• Xcode 26.4
• LLVM 21.1.8 via Homebrew (brew install llvm@21)

Patch

Applying the following (Claude-generated) patch worked on my computer:

--- a/Tools/jit/_targets.py
+++ b/Tools/jit/_targets.py
@@ -167,6 +167,9 @@
 # nicer debugging experience... but that needs a lot more research:
 "-fno-asynchronous-unwind-tables",
+ # The machine outliner can generate outlined thunks with
+ # .cfi_startproc but no .cfi_endproc, causing assembler errors
+ # on newer toolchains (e.g. Xcode 26 / macOS 26):
+ "-mno-outline",
 # Don't call built-in functions that we can't find or patch:
 "-fno-builtin",

Relevant issues

• CI: move Apple Silicon macOS workflows to macos-26 (Tahoe) #145098

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• GH-148560: Don't emit unwind info for JIT stencils #148703

[savannahostrowski]

Thanks for this report! This is sort of a duplicate of #146475 - we have a couple PRs in flight with approaches to fix this #146478 or #148364

[DarkaMaul]

🤦 I did try to search for existing issues, but I focused on the error message in my research.

[savannahostrowski]

No worries at all. After chatting with some folks, we've aligned that there are two issues here:

• gh-146475: Block Apple LLVM 21 from passing JIT tool version check #146478 - Apple clang is stricter than other assemblers so the JIT complains about these directives. However, we've decided that the right solution is to move forward with blocking Apple clang, since it ships as an incomplete toolchain for our purposes as we need llvm-readobj as well.
• We have unbalanced CFI directives, which honestly just shouldn't be there anyway, since they are unused. As a result, we do want to either not emit these or strip them out (like in GH-146475: Strip CFI directives from JIT stencils to allow using Apple LLVM 21 #148364).

I'm going to keep this issue open to track the latter and self-assign. Thanks again for reporting this!

[diegorusso]

This has been fixed as part of #148560