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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"JIT build fails on macOS 26: due to .cfi directives","articleBody":"# Bug report\n\n### Bug description:\n\n## How to reproduce\n\nCPython Commit: 4af46b4ab5af49d8df034320a9a70fcbb062f7cf (April 15)\n\n```bash\n./configure --enable-experimental-jit --with-pydebug\nmake\n``` \n\nThe following command fails with the following errors:\n\n```\n❯ make -j1\npython3.15 ./Tools/jit/build.py aarch64-apple-darwin25.4.0 --output-dir . --pyconfig-dir . --cflags=\"\" --llvm-version=\"\" --debug\n\n===========================================================\nJIT support for aarch64-apple-darwin is still experimental!\n          Please report any issues you encounter.\n===========================================================\n\n/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\n; %bb.0:\n^\n/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\n; %bb.0:\n^\n/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\n; %bb.0:\n\n...\n\n^\n  + Exception Group Traceback (most recent call last):\n  |   File \"/Users/dm/Projects/cpython-upstream/Tools/jit/_targets.py\", line 213, in _build_stencils\n  |     async with asyncio.TaskGroup() as group:\n  |                ~~~~~~~~~~~~~~~~~^^\n  |   File \"/Users/dm/.local/share/uv/python/cpython-3.15.0a7-macos-aarch64-none/lib/python3.15/asyncio/taskgroups.py\", line 72, in __aexit__\n  |     return await self._aexit(et, exc)\n  |            ^^^^^^^^^^^^^^^^^^^^^^^^^^\n  |   File \"/Users/dm/.local/share/uv/python/cpython-3.15.0a7-macos-aarch64-none/lib/python3.15/asyncio/taskgroups.py\", line 174, in _aexit\n  |     raise BaseExceptionGroup(\n  |     ...\u003c2 lines>...\n  |     ) from None\n  | ExceptionGroup: unhandled errors in a TaskGroup (1 sub-exception)\n  +-+---------------- 1 ----------------\n    | Traceback (most recent call last):\n    |   File \"/Users/dm/Projects/cpython-upstream/Tools/jit/_targets.py\", line 198, in _compile\n    |     await _llvm.run(\n    |         \"clang\", args_o, echo=self.verbose, llvm_version=self.llvm_version\n    |     )\n    |   File \"/Users/dm/Projects/cpython-upstream/Tools/jit/_llvm.py\", line 132, in run\n    |     output = await maybe_run(tool, args, echo=echo, llvm_version=llvm_version)\n    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n    |   File \"/Users/dm/Projects/cpython-upstream/Tools/jit/_llvm.py\", line 121, in maybe_run\n    |     return path and await _run(path, args, echo=echo)\n    |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n    |   File \"/Users/dm/Projects/cpython-upstream/Tools/jit/_llvm.py\", line 63, in _run\n    |     raise RuntimeError(f\"{tool} exited with return code {process.returncode}\")\n    | RuntimeError: clang exited with return code 1\n    +------------------------------------\n\nDuring handling of the above exception, another exception occurred:\n\nTraceback (most recent call last):\n  File \"/Users/dm/Projects/cpython-upstream/./Tools/jit/build.py\", line 55, in \u003cmodule>\n    target.build(\n    ~~~~~~~~~~~~^\n        comment=comment,\n        ^^^^^^^^^^^^^^^^\n        force=args.force,\n        ^^^^^^^^^^^^^^^^^\n        jit_stencils=args.output_dir / f\"jit_stencils-{target.triple}.h\",\n        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n    )\n    ^\n  File \"/Users/dm/Projects/cpython-upstream/Tools/jit/_targets.py\", line 256, in build\n    stencil_groups = ASYNCIO_RUNNER.run(self._build_stencils())\n  File \"/Users/dm/.local/share/uv/python/cpython-3.15.0a7-macos-aarch64-none/lib/python3.15/asyncio/runners.py\", line 127, in run\n    return self._loop.run_until_complete(task)\n           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^\n  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\n    return future.result()\n           ~~~~~~~~~~~~~^^\n  File \"/Users/dm/Projects/cpython-upstream/Tools/jit/_targets.py\", line 211, in _build_stencils\n    with tempfile.TemporaryDirectory() as tempdir:\n         ~~~~~~~~~~~~~~~~~~~~~~~~~~~^^\n  File \"/Users/dm/.local/share/uv/python/cpython-3.15.0a7-macos-aarch64-none/lib/python3.15/tempfile.py\", line 971, in __exit__\n    self.cleanup()\n    ~~~~~~~~~~~~^^\n  File \"/Users/dm/.local/share/uv/python/cpython-3.15.0a7-macos-aarch64-none/lib/python3.15/tempfile.py\", line 975, in cleanup\n    self._rmtree(self.name, ignore_errors=self._ignore_cleanup_errors)\n    ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File \"/Users/dm/.local/share/uv/python/cpython-3.15.0a7-macos-aarch64-none/lib/python3.15/tempfile.py\", line 955, in _rmtree\n    _shutil.rmtree(name, onexc=onexc)\n    ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^\n  File \"/Users/dm/.local/share/uv/python/cpython-3.15.0a7-macos-aarch64-none/lib/python3.15/shutil.py\", line 852, in rmtree\n    _rmtree_impl(path, dir_fd, onexc)\n    ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^\n  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\n    _rmtree_safe_fd_step(stack, onexc)\n    ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^\n  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\n    onexc(func, path, err)\n    ~~~~~^^^^^^^^^^^^^^^^^\n  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\n    os.rmdir(name, dir_fd=dirfd)\n    ~~~~~~~~^^^^^^^^^^^^^^^^^^^^\nOSError: [Errno 66] Directory not empty: '/var/folders/m9/4q7skk_n3vj6k6bvy2vc4tnr0000gn/T/tmpjlgkd9jf'\nmake: *** [jit_stencils-aarch64-apple-darwin.h] Error 1\n```\n\n\n## Root cause\n\n\u003cdetails>\n\u003csummary>\nWarning: this analysis was performed by Claude, and seemed plausible, but that's not my area of expertise.\n\u003c/summary>\n\nLLVM 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`:\n\n```asm\n_OUTLINED_FUNCTION_1:                   ; @OUTLINED_FUNCTION_1 Thunk\n\t.cfi_startproc\n; %bb.0:\n\t...\n\tb\t___assert_rtn\n_OUTLINED_FUNCTION_2:                   ; @OUTLINED_FUNCTION_2 Thunk\n\t.cfi_startproc          ; \u003c-- ERROR: previous .cfi frame never ended\n; %bb.0:\n\t...\n```\n\nThe 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.\n\nNote 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.\n\n\u003c/details>\n\n## Environnement\n\n- macOS 26.4.1 (Darwin 25.4.0), aarch64\n- Xcode 26.4\n- LLVM 21.1.8 via Homebrew (`brew install llvm@21`)\n\n## Patch\n\nApplying the following (Claude-generated) patch worked on my computer:\n\n```diff\n--- a/Tools/jit/_targets.py\n+++ b/Tools/jit/_targets.py\n@@ -167,6 +167,9 @@\n             # nicer debugging experience... but that needs a lot more research:\n             \"-fno-asynchronous-unwind-tables\",\n+            # The machine outliner can generate outlined thunks with\n+            # .cfi_startproc but no .cfi_endproc, causing assembler errors\n+            # on newer toolchains (e.g. Xcode 26 / macOS 26):\n+            \"-mno-outline\",\n             # Don't call built-in functions that we can't find or patch:\n             \"-fno-builtin\",\n```\n\n## Relevant issues\n\n- https://github.com/python/cpython/issues/145098\n\n### CPython versions tested on:\n\nCPython main branch\n\n### Operating systems tested on:\n\nmacOS\n\n\u003c!-- gh-linked-prs -->\n### Linked PRs\n* gh-148703\n\u003c!-- /gh-linked-prs -->\n","author":{"url":"https://github.com/DarkaMaul","@type":"Person","name":"DarkaMaul"},"datePublished":"2026-04-14T15:38:59Z","url":"https://github.com/python/cpython/issues/148560"}