[cxx-interop] Itanium ABI C++ records should have address-only layout when they can't be passed in registers#66694
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hyp merged 4 commits intoswiftlang:release/5.9from Jun 16, 2023
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…an't be passed in registers This ensures that a C++ record with only ObjC ARC pointers with trivial other members is passed by value in SIL Fixes swiftlang#61929 (cherry picked from commit decd21f)
…ble in Swift (cherry picked from commit 2c4188b)
…t failure on macOS (cherry picked from commit 988f373)
(cherry picked from commit 7abd265)
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@swift-ci please test |
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@swift-ci please test source compatibility |
DougGregor
approved these changes
Jun 16, 2023
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This ensures that a C++ record with only ObjC ARC pointers with trivial other members is passed by value in SIL.
Fixes #61929
Additionally, mark C++ classes with trivial_abi attribute as unavailable in Swift.
Non-trivial C++ records become AddressOnly Swift structures, as that's the only way to guarantee the invocation of their copy and/or destroy operations from Swift. AddressOnly implies indirect parameter passing as well. However, under Itanium ABI, some C++ records that have only ARC pointers can be passed directly even though they're non-trivial types. This causes a runtime crash as we think such C++ records are passed indirectly instead from Swift.
This change modifies the way we decide which records become AddressOnly Swift structs on Itanium ABI, by using Clang's own mechanism that determines if the record can be passed in registers or not. This largely aligns the semantics of C++'s indirect + non-trivial copy/destroy with Swift's AddressOnly semantics, except for C++ records annotated with
trivial_abi. Such types are now marked as unavailable in Swift instead, as they can't be supported yet in the current implementation.On Windows for the MSVC target the situation is more complicated, and this patch leaves the current behavior instead. The problem is that the MSVC ABI allows non-trivially destroyed but trivially copyable C++ records to be passed in registers. This can't be supported without AddressOnly in Swift, as we wouldn't be able to destroy such C++ record correctly. Thus, for now, preserve the existing behavior. A follow-up future fix will need to teach Swift's code generator how to pass AddressOnly C++ records directly to fix both the MSVC ABI and to support
trivial_abifor Itanium as well.trivial_abirecords though. We are not aware of any adopters usingtrivial_abirecords yet, so that shouldn't be a concern at the moment.