[DRAFT] Make IsRelocatable use the Clang builtin __is_trivially_relocatable(T) #2159
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Clang 18's `__is_trivially_relocatable(T)` isn't yet compatible with Folly's use of the term; false negatives are fine, but Clang dangerously gives false positives. However, after llvm/llvm-project#84621 , the builtin will no longer give any (known) false positives, which means it will be safe for Folly to start using the builtin as of Clang 19+.
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Suggest closing this one in favor of #2216. |
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) Summary: Well, llvm/llvm-project#84621 isn't going anywhere in Clang, therefore #2159 will presumably never be mergeable (in the foreseeable future, anyway). So here's an alternative approach, for your consideration: If [P1144's feature-test macro](https://open-std.org/jtc1/sc22/wg21/docs/papers/2024/p1144r10.html#wording) is set, then use `std::is_trivially_relocatable<T>` in place of `std::is_trivially_copyable<T>`. This will produce improved codegen for e.g. `fbvector<std::vector<int>>::erase`, when compiled on a compiler supporting P1144 semantics. https://godbolt.org/z/s6sa313h3 Compare https://github.com/charles-salvia/std_error/blob/3abc272/all_in_one.hpp#L180-L196 Pull Request resolved: #2216 Reviewed By: Gownta Differential Revision: D58147899 Pulled By: Orvid fbshipit-source-id: 671ed21eeb6080f583e6966e776d243c99c9c17f
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Summary: Well, llvm/llvm-project#84621 isn't going anywhere in Clang, therefore facebook/folly#2159 will presumably never be mergeable (in the foreseeable future, anyway). So here's an alternative approach, for your consideration: If [P1144's feature-test macro](https://open-std.org/jtc1/sc22/wg21/docs/papers/2024/p1144r10.html#wording) is set, then use `std::is_trivially_relocatable<T>` in place of `std::is_trivially_copyable<T>`. This will produce improved codegen for e.g. `fbvector<std::vector<int>>::erase`, when compiled on a compiler supporting P1144 semantics. https://godbolt.org/z/s6sa313h3 Compare https://github.com/charles-salvia/std_error/blob/3abc272/all_in_one.hpp#L180-L196 X-link: facebook/folly#2216 Reviewed By: Gownta Differential Revision: D58147899 Pulled By: Orvid fbshipit-source-id: 671ed21eeb6080f583e6966e776d243c99c9c17f
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Clang 18's
__is_trivially_relocatable(T)isn't yet compatible with Folly's use of the term; false negatives are fine, but Clang dangerously gives false positives. However, after llvm/llvm-project#84621, the builtin will no longer give any (known) false positives, which means it will be safe for Folly to start using the builtin as of Clang 19+.I'm posting this here hoping to kickstart a Folly discussion — "is Folly in favor of the Clang patch? would Folly indeed start using the Clang builtin, if it were long-term supported with the Folly(/Abseil/BSL/Qt/P1144/etc) semantics?"
Please do not merge this patch until llvm/llvm-project#84621 is successfully merged upstream; that patch is a prerequisite for this one.