Author: Richard Smith Date: 2019-12-02T14:41:27-08:00 New Revision: 711c669ae92658aecc6fabccc583594924bac6d7
URL: https://github.com/llvm/llvm-project/commit/711c669ae92658aecc6fabccc583594924bac6d7 DIFF: https://github.com/llvm/llvm-project/commit/711c669ae92658aecc6fabccc583594924bac6d7.diff LOG: Fix comment to more accurately describe C++ language requirements around tail padding. Summary: As of C++ core issue 43 (http://wg21.link/cwg43), which was voted into the C++ working draft in 1999, it is not permissible to memcpy a base class subobject, even if it's of POD type, so there is no problem with reusing the tail padding of a base class. That issue was voted into the standard in DR status, so it applies retroactively to C++98 (and is in any case part of C++03). So stop suggesting that AlwaysUseTailPadding mode is non-conforming. Reviewers: rjmccall Reviewed By: rjmccall Subscribers: cfe-commits Tags: #clang Differential Revision: https://reviews.llvm.org/D70923 Added: Modified: clang/include/clang/Basic/TargetCXXABI.h Removed: ################################################################################ diff --git a/clang/include/clang/Basic/TargetCXXABI.h b/clang/include/clang/Basic/TargetCXXABI.h index b1be40272513..60343fe99c1d 100644 --- a/clang/include/clang/Basic/TargetCXXABI.h +++ b/clang/include/clang/Basic/TargetCXXABI.h @@ -277,27 +277,18 @@ class TargetCXXABI { /// padding of a base class? /// /// This decision cannot be changed without breaking platform ABI - /// compatibility, and yet it is tied to language guarantees which - /// the committee has so far seen fit to strengthen no less than - /// three separate times: - /// - originally, there were no restrictions at all; - /// - C++98 declared that objects could not be allocated in the - /// tail padding of a POD type; - /// - C++03 extended the definition of POD to include classes - /// containing member pointers; and - /// - C++11 greatly broadened the definition of POD to include - /// all trivial standard-layout classes. - /// Each of these changes technically took several existing - /// platforms and made them permanently non-conformant. + /// compatibility. In ISO C++98, tail padding reuse was only permitted for + /// non-POD base classes, but that restriction was removed retroactively by + /// DR 43, and tail padding reuse is always permitted in all de facto C++ + /// language modes. However, many platforms use a variant of the old C++98 + /// rule for compatibility. enum TailPaddingUseRules { /// The tail-padding of a base class is always theoretically - /// available, even if it's POD. This is not strictly conforming - /// in any language mode. + /// available, even if it's POD. AlwaysUseTailPadding, /// Only allocate objects in the tail padding of a base class if /// the base class is not POD according to the rules of C++ TR1. - /// This is non-strictly conforming in C++11 mode. UseTailPaddingUnlessPOD03, /// Only allocate objects in the tail padding of a base class if _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits