[clang] [Sema] Mark alias/ifunc targets used and consider mangled names (PR #87130)

[Fangrui Song via cfe-commits]

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@@ -1980,6 +1981,36 @@ static void handleWeakRefAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   D->addAttr(::new (S.Context) WeakRefAttr(S.Context, AL));
 }
 
+// Mark alias/ifunc target as used. Due to name mangling, we look up the
+// demangled name ignoring parameters. This should handle the majority of use
+// cases while leaving namespace scope names unmarked.
+static void markUsedForAliasOrIfunc(Sema &S, Decl *D, const ParsedAttr &AL,
+                                    StringRef Str) {
+  char *Demangled = nullptr;
+  if (S.getASTContext().getCXXABIKind() != TargetCXXABI::Microsoft)
+    Demangled = llvm::itaniumDemangle(Str, /*ParseParams=*/false);
+  std::unique_ptr<MangleContext> MC(S.Context.createMangleContext());
+  SmallString<256> Name;
+
+  const DeclarationNameInfo Target(
+      &S.Context.Idents.get(Demangled ? Demangled : Str), AL.getLoc());
+  LookupResult LR(S, Target, Sema::LookupOrdinaryName);
+  if (S.LookupName(LR, S.TUScope)) {
+    for (NamedDecl *ND : LR) {
+      if (MC->shouldMangleDeclName(ND)) {
+        llvm::raw_svector_ostream Out(Name);
+        Name.clear();
+        MC->mangleName(GlobalDecl(ND), Out);
+      } else {
+        Name = ND->getIdentifier()->getName();
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MaskRay wrote:

The `if (Name == Str)` comparison below is to mark the correct overload while leaving others unmarked.

The previous code just doesn't bother with overload checking, leading to overly marked functions when extended to C++ / C `__attribute__((overload))`

https://github.com/llvm/llvm-project/pull/87130