https://github.com/Endilll created https://github.com/llvm/llvm-project/pull/84313
This patch implements [CWG2586](https://cplusplus.github.io/CWG/issues/2583.html) "Common initial sequence should consider over-alignment". Note that alignment of union members doesn't have to match, as layout compatibility of unions is not defined in terms of common initial sequence (http://eel.is/c++draft/class.mem.general#25). >From 491fc16c777aff8b22893da1cdeb8d137cf28871 Mon Sep 17 00:00:00 2001 From: Vlad Serebrennikov <serebrennikov.vladis...@gmail.com> Date: Thu, 7 Mar 2024 15:16:35 +0300 Subject: [PATCH] [clang] Respect field alignment when evaluating layout compatiblity of structs This patch implements [CWG2586](https://cplusplus.github.io/CWG/issues/2583.html) "Common initial sequence should consider over-alignment ". Note that alignment of union members doesn't have to match, as layout compatibility of unions is not defined in terms of common initial sequence (http://eel.is/c++draft/class.mem.general#25). --- clang/docs/ReleaseNotes.rst | 4 ++++ clang/lib/Sema/SemaChecking.cpp | 9 +++++++-- clang/test/CXX/drs/dr25xx.cpp | 26 ++++++++++++++++++++++++++ clang/test/SemaCXX/type-traits.cpp | 13 ++++++++++++- clang/www/cxx_dr_status.html | 2 +- 5 files changed, 50 insertions(+), 4 deletions(-) diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst index 1b901a27fd19d1..62cc6aae4ee58b 100644 --- a/clang/docs/ReleaseNotes.rst +++ b/clang/docs/ReleaseNotes.rst @@ -115,6 +115,10 @@ Resolutions to C++ Defect Reports of two types. (`CWG1719: Layout compatibility and cv-qualification revisited <https://cplusplus.github.io/CWG/issues/1719.html>`_). +- Alignment of members is now respected when evaluating layout compatibility + of structs. + (`CWG2583: Common initial sequence should consider over-alignment <https://cplusplus.github.io/CWG/issues/2583.html>`_). + - ``[[no_unique_address]]`` is now respected when evaluating layout compatibility of two types. (`CWG2759: [[no_unique_address] and common initial sequence <https://cplusplus.github.io/CWG/issues/2759.html>`_). diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp index 3597f93a017136..5f607608cf7a53 100644 --- a/clang/lib/Sema/SemaChecking.cpp +++ b/clang/lib/Sema/SemaChecking.cpp @@ -19185,7 +19185,8 @@ static bool isLayoutCompatible(ASTContext &C, EnumDecl *ED1, EnumDecl *ED2) { /// Check if two fields are layout-compatible. static bool isLayoutCompatible(ASTContext &C, FieldDecl *Field1, - FieldDecl *Field2) { + FieldDecl *Field2, + bool IgnoreAlignment = false) { if (!isLayoutCompatible(C, Field1->getType(), Field2->getType())) return false; @@ -19204,6 +19205,10 @@ static bool isLayoutCompatible(ASTContext &C, FieldDecl *Field1, if (Field1->hasAttr<clang::NoUniqueAddressAttr>() || Field2->hasAttr<clang::NoUniqueAddressAttr>()) return false; + + if (!IgnoreAlignment && + Field1->getMaxAlignment() != Field2->getMaxAlignment()) + return false; return true; } @@ -19265,7 +19270,7 @@ static bool isLayoutCompatibleUnion(ASTContext &C, RecordDecl *RD1, E = UnmatchedFields.end(); for ( ; I != E; ++I) { - if (isLayoutCompatible(C, Field1, *I)) { + if (isLayoutCompatible(C, Field1, *I, /*IgnoreAlignment=*/true)) { bool Result = UnmatchedFields.erase(*I); (void) Result; assert(Result); diff --git a/clang/test/CXX/drs/dr25xx.cpp b/clang/test/CXX/drs/dr25xx.cpp index 9fc7cf59485caa..46532486e50e53 100644 --- a/clang/test/CXX/drs/dr25xx.cpp +++ b/clang/test/CXX/drs/dr25xx.cpp @@ -211,6 +211,32 @@ namespace dr2565 { // dr2565: 16 open 2023-06-07 #endif } +namespace dr2583 { // dr2583: 19 +#if __cplusplus >= 201103L +struct A { + int i; + char c; +}; + +struct B { + int i; + alignas(8) char c; +}; + +union U { + A a; + B b; +}; + +union V { + A a; + alignas(64) B b; +}; + +static_assert(!__is_layout_compatible(A, B), ""); +static_assert(__is_layout_compatible(U, V), ""); +#endif +} // namespace dr2583 namespace dr2598 { // dr2598: 18 #if __cplusplus >= 201103L diff --git a/clang/test/SemaCXX/type-traits.cpp b/clang/test/SemaCXX/type-traits.cpp index 23c339ebdf0826..831de2589dcb9e 100644 --- a/clang/test/SemaCXX/type-traits.cpp +++ b/clang/test/SemaCXX/type-traits.cpp @@ -1681,6 +1681,16 @@ union UnionLayout3 { [[no_unique_address]] CEmptyStruct d; }; +union UnionNoOveralignedMembers { + int a; + double b; +}; + +union UnionWithOveralignedMembers { + int a; + alignas(16) double b; +}; + struct StructWithAnonUnion { union { int a; @@ -1771,7 +1781,8 @@ void is_layout_compatible(int n) static_assert(__is_layout_compatible(CStruct, CStructNoUniqueAddress) != bool(__has_cpp_attribute(no_unique_address)), ""); static_assert(__is_layout_compatible(CStructNoUniqueAddress, CStructNoUniqueAddress2) != bool(__has_cpp_attribute(no_unique_address)), ""); static_assert(__is_layout_compatible(CStruct, CStructAlignment), ""); - static_assert(__is_layout_compatible(CStruct, CStructAlignedMembers), ""); // FIXME: alignment of members impact common initial sequence + static_assert(!__is_layout_compatible(CStruct, CStructAlignedMembers), ""); + static_assert(__is_layout_compatible(UnionNoOveralignedMembers, UnionWithOveralignedMembers), ""); static_assert(__is_layout_compatible(CStructWithBitfelds, CStructWithBitfelds), ""); static_assert(__is_layout_compatible(CStructWithBitfelds, CStructWithBitfelds2), ""); static_assert(!__is_layout_compatible(CStructWithBitfelds, CStructWithBitfelds3), ""); diff --git a/clang/www/cxx_dr_status.html b/clang/www/cxx_dr_status.html index 774c71bc1cb6b7..494c0e1e9007bd 100755 --- a/clang/www/cxx_dr_status.html +++ b/clang/www/cxx_dr_status.html @@ -15306,7 +15306,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2> <td><a href="https://cplusplus.github.io/CWG/issues/2583.html";>2583</a></td> <td>C++23</td> <td>Common initial sequence should consider over-alignment</td> - <td class="unknown" align="center">Unknown</td> + <td class="unreleased" align="center">Clang 19</td> </tr> <tr class="open" id="2584"> <td><a href="https://cplusplus.github.io/CWG/issues/2584.html";>2584</a></td> _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
