| Issue |
178277
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| Summary |
Document current status of preserve_most/all/none cc
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| Labels |
new issue
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| Assignees |
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| Reporter |
jtstogel
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Documentation for `preserve_most` states that the feature is experimental and slated for use in Objective-C, but that note was added in 2016 (https://github.com/llvm/llvm-project/commit/35aa5cecf25162f4a88cdf1dbfe3f8fc3ed1a91c).
> This calling convention will be used by a future version of the Objective-C runtime and should therefore still be considered experimental at this time. Although this convention was created to optimize certain runtime calls to the Objective-C runtime, it is not limited to this runtime and might be used by other runtimes in the future too. The current implementation only supports X86-64, but the intention is to support more architectures in the future.
There are a few other places aspects in which documentation should be updated to reflect 2026, including:
* Updating the docs to reflect the current intended usage in Objective-C.
* Re-evaluating the current “experimental” status of `preserve_most` to ensure it’s up to date. I’m unsure when a feature in clang qualifies for promotion out of experimental. It’s been around since 2016, though it does have relatively recent bug fixes. I did some basic searching, and FWIW it’s used in the linux kernel for cold validation functions [1], some parts of v8 for error handling [2], tcmalloc for an initializer [3], and in Protobuf' μpb [4].
* Documenting the intended behavior for flags registers -- https://github.com/llvm/llvm-project/issues/64490.
* Addressing https://github.com/llvm/llvm-project/issues/105588 to some degree. I’ve sent a patch for glibc's PLT resolution [5], but it’s still pending viability. If it can't/won't be fixed in popular dynamic linkers, then we should at least note this pitfall in the documentation or provide warnings etc, as it seems like many folks rediscover this footgun. @fweimer-rh has good example documentation warning about PLT interactions in https://gitlab.com/x86-psABIs/x86-64-ABI/-/merge_requests/45.
Hi @swiftix, @ributzka, I see you were the original authors of the `preserve_most` docs (though now ~10y ago). I'm not seeing references in https://github.com/apple-oss-distributions/objc4. Do you know whether Objective-C runtime relies or intends to rely on `preserve_most`? Or could you help me find the right people? Thanks!
[1] https://github.com/torvalds/linux/commit/aebc7b0d8d91bbc69e976909963046bc48bca4fd
[2] https://github.com/v8/v8/blob/4d1e2794e00a1071e774361994ac5ac4b6d3f147/include/v8config.h#L394
[3] https://github.com/google/tcmalloc/blob/b4600aef594ece086808c6d7d9e720bdd51a4188/tcmalloc/static_vars.h#L214
[4] https://github.com/protocolbuffers/protobuf/blob/74a5ee1852602ab2bf7bf78cdb11185f564209e7/upb/wire/decode_fast/cardinality.h#L523
[5] https://patchwork.sourceware.org/project/glibc/patch/[email protected]
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