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Title: GH-146475: Strip CFI directives from JIT stencils to allow using Apple LLVM 21 by savannahostrowski · Pull Request #148364 · python/cpython · GitHub

Open Graph Title: GH-146475: Strip CFI directives from JIT stencils to allow using Apple LLVM 21 by savannahostrowski · Pull Request #148364 · python/cpython

X Title: GH-146475: Strip CFI directives from JIT stencils to allow using Apple LLVM 21 by savannahostrowski · Pull Request #148364 · python/cpython

Description: After poking around at this a bit, it seems like Apple LLVM 21 is stricter about validating CFI frame nesting. In debug builds, .cfi_endproc directives can end up in blocks that the optimizer removes as unreachable, leaving unbalanced .cfi_startproc directives that the assembler rejects (and so you'll get something like this #146478 (comment) while building). Since JIT stencils are already compiled with -fno-asynchronous-unwind-tables, I believe the CFI directives are unused and so they can be stripped out. I also tried changing the flag to -fno-unwind-tables instead to just prevent us from emitting the directives in the first place but it seems like that's a bigger change as we'd have to handle SHT_X86_64_UNWIND sections on x86_64 Linux (https://github.com/python/cpython/actions/runs/24265500635/job/70859484777?pr=148364)? I'm not totally sure why that is. We could go that route but wanted to solicit opinions first before going down that road. This is an alternative to #146478 that tries to fix the root cause rather than blocking Apple LLVM 21. Issue: gh-146475

Open Graph Description: After poking around at this a bit, it seems like Apple LLVM 21 is stricter about validating CFI frame nesting. In debug builds, .cfi_endproc directives can end up in blocks that the optimizer remov...

X Description: After poking around at this a bit, it seems like Apple LLVM 21 is stricter about validating CFI frame nesting. In debug builds, .cfi_endproc directives can end up in blocks that the optimizer remov...

Opengraph URL: https://github.com/python/cpython/pull/148364

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6480297 Strip CFI directives in JIT stencils savannahostrowski Apr 10, 2026 https://github.com/python/cpython/pull/148364/commits/6480297f86369fcfd811af1f4b6ae6f4a74b24cf
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e29673e 📜🤖 Added by blurb_it. blurb-it[bot] Apr 10, 2026 https://github.com/python/cpython/pull/148364/commits/e29673e146f23c0244816a724a37b03f713edbd7
2a3ff7f Merge branch 'main' into fix-apple-llvm savannahostrowski Apr 10, 2026 https://github.com/python/cpython/pull/148364/commits/2a3ff7f5ade5e5333c48c62ecc81af65d5b40d30
00dfe93 Blurb savannahostrowski Apr 10, 2026 https://github.com/python/cpython/pull/148364/commits/00dfe93599bb542b8d2fa5e697709812d0f9043c
55069d8 oooo there is a flag savannahostrowski Apr 10, 2026 https://github.com/python/cpython/pull/148364/commits/55069d8339e8d5ae4179ea4ea50e85754b89f6d5
3e7cf60 Make comment more concise savannahostrowski Apr 10, 2026 https://github.com/python/cpython/pull/148364/commits/3e7cf60ac1be1b64c9505843637b29b3b7c7cae6
f110a95 Appease linter savannahostrowski Apr 10, 2026 https://github.com/python/cpython/pull/148364/commits/f110a958fa5eb6cdab558a75a447fc751846fad4
4e2a5d3 Just kidding, the flag breaks x86 linux savannahostrowski Apr 10, 2026 https://github.com/python/cpython/pull/148364/commits/4e2a5d315310d4f7ad0467c4aa186eeab4392d4c
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