[clang] [clang] Avoid caching linkage before redeclaration merging (PR #221476)
Vedran Miletić via cfe-commits
cfe-commits at lists.llvm.org
Sat Sep 5 11:18:42 PDT 2026
https://github.com/vedranmiletic created https://github.com/llvm/llvm-project/pull/221476
Linkage queries made while a declaration is still being formed can cache a result based only on a previous declaration. Compute linkage without caching in these paths, and defer applying #pragma redefine_extname until after redeclaration merging.
Fixes #112737
Fixes #173464
Assisted-by: Codex (OpenAI ChatGPT)
>From 8c0bc0d5867814b85f676152fb6e70ee75283950 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Vedran=20Mileti=C4=87?= <vedran.miletic at mpcdf.mpg.de>
Date: Sat, 5 Sep 2026 19:00:16 +0200
Subject: [PATCH] [clang] Avoid caching linkage before redeclaration merging
Linkage queries made while a declaration is still being formed can cache
a result based only on a previous declaration. Compute linkage without
caching in these paths, and defer applying #pragma redefine_extname
until after redeclaration merging.
Fixes #112737
Fixes #173464
Assisted-by: Codex (OpenAI ChatGPT)
---
clang/include/clang/AST/Decl.h | 3 ++
clang/lib/AST/Decl.cpp | 6 +++
clang/lib/Sema/SemaDecl.cpp | 37 ++++++++++++-------
clang/lib/Sema/SemaDeclAttr.cpp | 3 +-
.../SemaCXX/redecl-linkage-before-merge.cpp | 13 +++++++
5 files changed, 48 insertions(+), 14 deletions(-)
create mode 100644 clang/test/SemaCXX/redecl-linkage-before-merge.cpp
diff --git a/clang/include/clang/AST/Decl.h b/clang/include/clang/AST/Decl.h
index eafeeecac7794..ccae943ed4b6e 100644
--- a/clang/include/clang/AST/Decl.h
+++ b/clang/include/clang/AST/Decl.h
@@ -422,6 +422,9 @@ class NamedDecl : public Decl {
/// those.
Linkage getLinkageInternal() const;
+ /// Compute this entity's linkage without caching it.
+ Linkage computeLinkageInternal() const;
+
/// Get the linkage from a semantic point of view. Entities in
/// anonymous namespaces are external (in c++98).
Linkage getFormalLinkage() const;
diff --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
index a620e9f211ca6..78225f877dca3 100644
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -1188,6 +1188,12 @@ Linkage NamedDecl::getLinkageInternal() const {
.getLinkage();
}
+Linkage NamedDecl::computeLinkageInternal() const {
+ return LinkageComputer{}
+ .computeLVForDecl(this, LVComputationKind::forLinkageOnly())
+ .getLinkage();
+}
+
static bool isExportedFromModuleInterfaceUnit(const NamedDecl *D) {
// FIXME: Handle isModulePrivate.
switch (D->getModuleOwnershipKind()) {
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 2451baaf94703..b12c2c8858409 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -10670,19 +10670,6 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
StringLiteral *SE = cast<StringLiteral>(E);
NewFD->addAttr(
AsmLabelAttr::Create(Context, SE->getString(), SE->getStrTokenLoc(0)));
- } else if (!ExtnameUndeclaredIdentifiers.empty()) {
- llvm::MapVector<IdentifierInfo *, AsmLabelAttr *>::iterator I =
- ExtnameUndeclaredIdentifiers.find(NewFD->getIdentifier());
- if (I != ExtnameUndeclaredIdentifiers.end()) {
- if (isDeclExternC(NewFD)) {
- NewFD->addAttr(I->second);
- ExtnameUndeclaredIdentifiers.erase(I);
- } else if (NewFD->getDeclContext()
- ->getRedeclContext()
- ->isTranslationUnit())
- Diag(NewFD->getLocation(), diag::warn_redefine_extname_not_applied)
- << /*Variable*/0 << NewFD;
- }
}
// Copy the parameter declarations from the declarator D to the function
@@ -10850,6 +10837,26 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
}
}
+ // This linkage query must happen after merging previous declarations.
+ auto ApplyPragmaRedefineExtname = [&] {
+ if (D.getAsmLabel() || ExtnameUndeclaredIdentifiers.empty())
+ return;
+
+ auto I = ExtnameUndeclaredIdentifiers.find(NewFD->getIdentifier());
+ if (I == ExtnameUndeclaredIdentifiers.end())
+ return;
+
+ if (isDeclExternC(NewFD)) {
+ NewFD->addAttr(I->second);
+ ExtnameUndeclaredIdentifiers.erase(I);
+ } else if (NewFD->getDeclContext()
+ ->getRedeclContext()
+ ->isTranslationUnit()) {
+ Diag(NewFD->getLocation(), diag::warn_redefine_extname_not_applied)
+ << /*Variable*/ 0 << NewFD;
+ }
+ };
+
if (!getLangOpts().CPlusPlus) {
// Perform semantic checking on the function declaration.
if (!NewFD->isInvalidDecl() && NewFD->isMain())
@@ -10869,6 +10876,8 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
Previous.getResultKind() != LookupResultKind::FoundOverloaded) &&
"previous declaration set still overloaded");
+ ApplyPragmaRedefineExtname();
+
// Diagnose no-prototype function declarations with calling conventions that
// don't support variadic calls. Only do this in C and do it after merging
// possibly prototyped redeclarations.
@@ -11038,6 +11047,8 @@ Sema::ActOnFunctionDeclarator(Scope *S, Declarator &D, DeclContext *DC,
Previous.getResultKind() != LookupResultKind::FoundOverloaded) &&
"previous declaration set still overloaded");
+ ApplyPragmaRedefineExtname();
+
NamedDecl *PrincipalDecl = (FunctionTemplate
? cast<NamedDecl>(FunctionTemplate)
: NewFD);
diff --git a/clang/lib/Sema/SemaDeclAttr.cpp b/clang/lib/Sema/SemaDeclAttr.cpp
index 106605962171b..c096c1a467cb4 100644
--- a/clang/lib/Sema/SemaDeclAttr.cpp
+++ b/clang/lib/Sema/SemaDeclAttr.cpp
@@ -1809,7 +1809,8 @@ static void handleAliasAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
}
} else {
const auto *VD = cast<VarDecl>(D);
- if (VD->isThisDeclarationADefinition() && VD->isExternallyVisible()) {
+ if (VD->isThisDeclarationADefinition() &&
+ isExternallyVisible(VD->computeLinkageInternal())) {
S.Diag(AL.getLoc(), diag::err_alias_is_definition) << VD << 0;
return;
}
diff --git a/clang/test/SemaCXX/redecl-linkage-before-merge.cpp b/clang/test/SemaCXX/redecl-linkage-before-merge.cpp
new file mode 100644
index 0000000000000..b9d8d05f124ca
--- /dev/null
+++ b/clang/test/SemaCXX/redecl-linkage-before-merge.cpp
@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s
+
+// https://github.com/llvm/llvm-project/issues/173464
+// A linkage query before redeclaration merging must not cache a result that
+// depends on a previous declaration.
+typedef float v4f __attribute__((vector_size(16)));
+extern const v4f kAlias;
+const v4f kAlias __attribute__((alias("kOne")));
+
+// https://github.com/llvm/llvm-project/issues/112737
+#pragma redefine_extname foo_cpp bar_cpp
+static int foo_cpp(); // expected-warning {{#pragma redefine_extname is applicable to external C declarations only; not applied to function 'foo_cpp'}}
+extern int foo_cpp() { return 1; } // expected-warning {{#pragma redefine_extname is applicable to external C declarations only; not applied to function 'foo_cpp'}}
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