ms-empty_bases.cpp - OpenGrok history log for /llvm-project/clang/test/SemaCXX/ms-empty

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# 7eca8a3d	23-May-2016	David Majnemer <david.majnemer@gmail.com>	Address post-commit review feedback to r270457 Add two tests which show our error handling behavior for invalid parameters in the layout_version and empty_bases attributes. Amend our documentation Address post-commit review feedback to r270457 Add two tests which show our error handling behavior for invalid parameters 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 to classes, structs, and unions. llvm-svn: 270461 show more ...
# cd3ebfe2	23-May-2016	David Majnemer <david.majnemer@gmail.com>	[MS ABI] Implement __declspec(empty_bases) and __declspec(layout_version) The layout_version attribute is pretty straightforward: use the layout rules from version XYZ of MSVC when used like struct [MS ABI] Implement __declspec(empty_bases) and __declspec(layout_version) The layout_version attribute is pretty straightforward: use the layout rules from version XYZ of MSVC when used like struct __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 ABI rules 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 show more ...