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llvm-project/clang/test/SemaCXX/ms-empty_bases.cpp

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[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
2016-05-24 01:16:12 +08:00
// RUN: %clang_cc1 -triple i386-pc-win32 %s -fsyntax-only -verify -fms-extensions -Wno-microsoft -std=c++11
struct __declspec(empty_bases) S {};
enum __declspec(empty_bases) E {}; // expected-warning{{'empty_bases' attribute only applies to classes}}
int __declspec(empty_bases) I; // expected-warning{{'empty_bases' attribute only applies to classes}}
typedef struct T __declspec(empty_bases) U; // expected-warning{{'empty_bases' attribute only applies to classes}}
auto z = []() __declspec(empty_bases) { return nullptr; }; // expected-warning{{'empty_bases' attribute only applies to classes}}
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
2016-05-24 01:32:35 +08:00
struct __declspec(empty_bases(1)) X {}; // expected-error{{'empty_bases' attribute takes no arguments}}