https://github.com/AaronBallman updated https://github.com/llvm/llvm-project/pull/115478
>From 5a9ac5588527cf76b1933a3efbc2285c7670126b Mon Sep 17 00:00:00 2001 From: Aaron Ballman <aa...@aaronballman.com> Date: Fri, 8 Nov 2024 08:18:18 -0500 Subject: [PATCH 1/2] [C2y] Add test coverage and documentation for WG14 N3341 This paper made empty structures and unions implementation-defined. We have always supported this as a GNU extension, so now we're documenting our behavior and removing the extension warning in C2y mode. --- clang/docs/LanguageExtensions.rst | 9 +++++++++ clang/docs/ReleaseNotes.rst | 6 ++++++ clang/lib/Sema/SemaDecl.cpp | 11 ++++++----- clang/test/C/C2y/n3341.c | 16 ++++++++++++++++ 4 files changed, 37 insertions(+), 5 deletions(-) create mode 100644 clang/test/C/C2y/n3341.c diff --git a/clang/docs/LanguageExtensions.rst b/clang/docs/LanguageExtensions.rst index f7285352b9deb9..4dea35d74ced07 100644 --- a/clang/docs/LanguageExtensions.rst +++ b/clang/docs/LanguageExtensions.rst @@ -5974,3 +5974,12 @@ Clang guarantees the following behaviors: padding bits are initialized to zero. Currently, the above extension only applies to C source code, not C++. + +Empty Objects in C +================== +The declaration of a structure or union type which has no named members is +undefined behavior (C23 and earlier) or implementation-defined behavior (C2y). +Clang allows the declaration of a structure or union type with no named members +in all C language modes. `sizeof` for such a type returns `0`, which is +different behavior than in C++ (where the size of such an object is typically +`1`). diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst index d0c43ff11f7bae..e41b9f0fab1035 100644 --- a/clang/docs/ReleaseNotes.rst +++ b/clang/docs/ReleaseNotes.rst @@ -298,6 +298,12 @@ C2y Feature Support paper adopts Clang's existing practice, so there were no changes to compiler behavior. +- Implemented support for `N3341 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3341.pdf>`_ + which makes empty structure and union objects implementation-defined in C. + ``-Wgnu-empty-struct`` will be emitted in C23 and earlier modes because the + behavior is a conforming GNU extension in those modes, but will no longer + have an effect in C2y mode. + C23 Feature Support ^^^^^^^^^^^^^^^^^^^ diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp index 6b0b4840a1eb2c..61c29e320d5c73 100644 --- a/clang/lib/Sema/SemaDecl.cpp +++ b/clang/lib/Sema/SemaDecl.cpp @@ -19365,11 +19365,12 @@ void Sema::ActOnFields(Scope *S, SourceLocation RecLoc, Decl *EnclosingDecl, } // Structs without named members are extension in C (C99 6.7.2.1p7), - // but are accepted by GCC. - if (NonBitFields == 0 && !getLangOpts().CPlusPlus) { - Diag(RecLoc, IsEmpty ? diag::ext_empty_struct_union : - diag::ext_no_named_members_in_struct_union) - << Record->isUnion(); + // but are accepted by GCC. In C2y, this became implementation-defined + // (C2y 6.7.3.2p10). + if (NonBitFields == 0 && !getLangOpts().CPlusPlus && !getLangOpts().C2y) { + Diag(RecLoc, IsEmpty ? diag::ext_empty_struct_union + : diag::ext_no_named_members_in_struct_union) + << Record->isUnion(); } } } else { diff --git a/clang/test/C/C2y/n3341.c b/clang/test/C/C2y/n3341.c new file mode 100644 index 00000000000000..523c3dd945ac1d --- /dev/null +++ b/clang/test/C/C2y/n3341.c @@ -0,0 +1,16 @@ +// RUN: %clang_cc1 -verify -std=c2y -Wall -pedantic %s +// RUN: %clang_cc1 -verify=gnu -Wall -pedantic %s + +/* WG14 N3341: Yes + * Slay Some Earthly Demons III + * + * Empty structure and union objects are now implementation-defined. + */ + +// expected-no-diagnostics + +struct R {}; // gnu-warning {{empty struct is a GNU extension}} +struct S { struct { }; }; // gnu-warning {{empty struct is a GNU extension}} +struct T { int : 0; }; // gnu-warning {{struct without named members is a GNU extension}} +union U {}; // gnu-warning {{empty union is a GNU extension}} + >From 3f207565e98c25ba83d4f9279d2bfa90cd649f53 Mon Sep 17 00:00:00 2001 From: Aaron Ballman <aa...@aaronballman.com> Date: Fri, 8 Nov 2024 08:20:18 -0500 Subject: [PATCH 2/2] Updating the website as well --- clang/www/c_status.html | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/clang/www/c_status.html b/clang/www/c_status.html index 989a572ae70fb6..809b4dd1bfee0e 100644 --- a/clang/www/c_status.html +++ b/clang/www/c_status.html @@ -191,7 +191,7 @@ <h2 id="c2y">C2y implementation status</h2> <tr> <td>Slay Some Earthly Demons III</td> <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3341.pdf";>N3341</a></td> - <td class="unknown" align="center">Unknown</td> + <td class="full" align="center">Yes</td> </tr> <tr> <td>Slay Some Earthly Demons IV</td> _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
