en Copyright 2015 Java http://172.16.0.5:8080/history/llvm-project-15.0.7/clang/test/SemaCXX/ms-empty_bases.cpp#7eca8a3d Address post-commit review feedback to r270457Add two tests which show our error handling behavior for invalidparameters in the layout_version and empty_bases attributes.Amend our documentation to make it more clear that__declspec(empty_bases) and __declspec(layout_version) can only apply toclasses, structs, and unions.llvm-svn: 270461 List of files: /llvm-project-15.0.7/clang/test/SemaCXX/ms-empty_bases.cpp Mon, 23 May 2016 17:32:35 +0000 David Majnemer <david.majnemer@gmail.com> http://172.16.0.5:8080/history/llvm-project-15.0.7/clang/test/SemaCXX/ms-empty_bases.cpp#cd3ebfe2 [MS ABI] Implement __declspec(empty_bases) and __declspec(layout_version)The layout_version attribute is pretty straightforward: use the layoutrules from version XYZ of MSVC when used likestruct __declspec(layout_version(XYZ)) S {};The empty_bases attribute is more interesting. It tries to get the C++empty base optimization to fire more often by tweaking the MSVC ABIrules in subtle ways:1. Disable the leading and trailing zero-sized object flags if a class is marked __declspec(empty_bases) and is empty. This means that given: struct __declspec(empty_bases) A {}; struct __declspec(empty_bases) B {}; struct C : A, B {}; 'C' will have size 1 and nvsize 0 despite not being annotated __declspec(empty_bases).2. When laying out virtual or non-virtual bases, disable the injection of padding between classes if the most derived class is marked __declspec(empty_bases). This means that given: struct A {}; struct B {}; struct __declspec(empty_bases) C : A, B {}; 'C' will have size 1 and nvsize 0.3. When calculating the offset of a non-virtual base, choose offset zero if the most derived class is marked __declspec(empty_bases) and the base is empty _and_ has an nvsize of 0. Because of the ABI rules, this does not mean that empty bases reliably get placed at offset 0! For example: struct A {}; struct B {}; struct __declspec(empty_bases) C : A, B { virtual ~C(); }; 'C' will be pointer sized to account for the vfptr at offset 0. 'A' and 'B' will _not_ be at offset 0 despite being empty! Instead, they will be located right after the vfptr. This occurs due to the interaction betweeen non-virtual base layout and virtual function pointer injection: injection occurs after the nv-bases and shifts them down by the size of a pointer.llvm-svn: 270457 List of files: /llvm-project-15.0.7/clang/test/SemaCXX/ms-empty_bases.cpp Mon, 23 May 2016 17:16:12 +0000 David Majnemer <david.majnemer@gmail.com>