Allow weak/selectany external definitions in header units.

[akrieger]

weak and selectany are mechanisms for allowing the linker to resolve ODR violations. [module.import/6] states

A header unit shall not contain a definition of a non-inline function or variable whose name has external linkage.

But this prevents compiling any headers with such weak symbols defined. These occur in eg. some Windows SDK headers like DirectXMath.h.

#ifndef XMGLOBALCONST
#if defined(__GNUC__) && !defined(__MINGW32__)
#define XMGLOBALCONST extern const __attribute__((weak))
#else
#define XMGLOBALCONST extern const __declspec(selectany)
#endif
#endif

    XMGLOBALCONST XMVECTORF32 g_XMSinCoefficients0 = { { { -0.16666667f, +0.0083333310f, -0.00019840874f, +2.7525562e-06f } } };

Proposed solution: Do not emit diag::err_extern_def_in_header_unit if the FD or VDecl have either SelectAnyAttr or WeakAttr.

[github-actions]

Thank you for submitting a Pull Request (PR) to the LLVM Project!

This PR will be automatically labeled and the relevant teams will be notified.

If you wish to, you can add reviewers by using the "Reviewers" section on this page.

If this is not working for you, it is probably because you do not have write permissions for the repository. In which case you can instead tag reviewers by name in a comment by using @ followed by their GitHub username.

If you have received no comments on your PR for a week, you can request a review by "ping"ing the PR by adding a comment “Ping”. The common courtesy "ping" rate is once a week. Please remember that you are asking for valuable time from other developers.

If you have further questions, they may be answered by the LLVM GitHub User Guide.

You can also ask questions in a comment on this PR, on the LLVM Discord or on the forums.

[llvmbot]

@llvm/pr-subscribers-clang-modules

@llvm/pr-subscribers-clang

Author: None (akrieger)

Changes

weak and selectany are mechanisms for allowing the linker to resolve ODR violations. [module.import/6] states

> A header unit shall not contain a definition of a non-inline function or variable whose name has external linkage.

But this prevents compiling any headers with such weak symbols defined. These occur in eg. some Windows SDK headers like DirectXMath.h.

#ifndef XMGLOBALCONST
#if defined(__GNUC__) && !defined(__MINGW32__)
#define XMGLOBALCONST extern const __attribute__((weak))
#else
#define XMGLOBALCONST extern const __declspec(selectany)
#endif
#endif

    XMGLOBALCONST XMVECTORF32 g_XMSinCoefficients0 = { { { -0.16666667f, +0.0083333310f, -0.00019840874f, +2.7525562e-06f } } };

Proposed solution: Do not emit diag::err_extern_def_in_header_unit if the FD or VDecl have either SelectAnyAttr or WeakAttr.

---

Full diff: https://github.com/llvm/llvm-project/pull/162713.diff

2 Files Affected:

• (modified) clang/lib/Sema/SemaDecl.cpp (+4-2)
• (modified) clang/test/CXX/module/module.import/p6.cpp (+13)

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 6eaf7b9435491..0063bd51aa612 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -13822,7 +13822,8 @@ void Sema::AddInitializerToDecl(Decl *RealDecl, Expr *Init, bool DirectInit) {
       !VDecl->isInvalidDecl() && VDecl->isThisDeclarationADefinition() &&
       VDecl->getFormalLinkage() == Linkage::External && !VDecl->isInline() &&
       !VDecl->isTemplated() && !isa<VarTemplateSpecializationDecl>(VDecl) &&
-      !VDecl->getInstantiatedFromStaticDataMember()) {
+      !VDecl->getInstantiatedFromStaticDataMember() &&
+      !(VDecl->hasAttr<SelectAnyAttr>() || VDecl->hasAttr<WeakAttr>())) {
     Diag(VDecl->getLocation(), diag::err_extern_def_in_header_unit);
     VDecl->setInvalidDecl();
   }
@@ -16162,7 +16163,8 @@ Decl *Sema::ActOnStartOfFunctionDef(Scope *FnBodyScope, Decl *D,
       !FD->isInvalidDecl() && !FD->isInlined() &&
       BodyKind != FnBodyKind::Delete && BodyKind != FnBodyKind::Default &&
       FD->getFormalLinkage() == Linkage::External && !FD->isTemplated() &&
-      !FD->isTemplateInstantiation()) {
+      !FD->isTemplateInstantiation() &&
+      !(FD->hasAttr<SelectAnyAttr>() || FD->hasAttr<WeakAttr>())) {
     assert(FD->isThisDeclarationADefinition());
     Diag(FD->getLocation(), diag::err_extern_def_in_header_unit);
     FD->setInvalidDecl();
diff --git a/clang/test/CXX/module/module.import/p6.cpp b/clang/test/CXX/module/module.import/p6.cpp
index cb2d799e5b565..9e378a5fe7759 100644
--- a/clang/test/CXX/module/module.import/p6.cpp
+++ b/clang/test/CXX/module/module.import/p6.cpp
@@ -3,6 +3,9 @@
 
 // RUN: %clang_cc1 -std=c++20 -x c++-header %t/bad-header-unit.h \
 // RUN:  -emit-header-unit -o %t/bad-header-unit.pcm -verify
+// RUN: %clang_cc1 -std=c++20 -x c++-header %t/bad-header-unit-declspec.h \
+// RUN:  -emit-header-unit -o %t/bad-header-unit.pcm -verify \
+// RUN:  -fdeclspec
 
 //--- bad-header-unit.h
 
@@ -77,3 +80,13 @@ template <typename T> bool b() {
 }
 
 inline bool B = b<int>();
+
+__attribute__((weak)) int weak_fun_definition() { return 42; }
+
+__attribute__((weak)) int weak_var_definition = 42;
+
+//--- bad-header-unit-declspec.h
+
+/* The cases below should compile without diagnostics.  */
+
+__declspec(selectany) int selectany_var_definition = 42; // expected-no-diagnostics

[github-actions]

⚠️ We detected that you are using a GitHub private e-mail address to contribute to the repo.
Please turn off Keep my email addresses private setting in your account.
See LLVM Developer Policy and LLVM Discourse for more information.

[ChuanqiXu9]

Looks not bad. Let's add a comment for it.