Fix PR6473.

Clang's support for weakref is now better than llvm-gcc's :-)

We don't introduce a new symbol and we correctly mark undefined references weak only if there is no
definition or regular undefined references in the same file.

llvm-svn: 97733

clang/lib/CodeGen/CodeGenModule.cpp

@@ -619,9 +619,41 @@ bool CodeGenModule::MayDeferGeneration(const ValueDecl *Global) {
   return false;
 }
 
+llvm::Constant *CodeGenModule::GetWeakRefReference(const ValueDecl *VD) {
+  const AliasAttr *AA = VD->getAttr<AliasAttr>();
+  assert(AA && "No alias?");
+
+  const llvm::Type *DeclTy = getTypes().ConvertTypeForMem(VD->getType());
+
+  // Unique the name through the identifier table.
+  const char *AliaseeName =
+    getContext().Idents.get(AA->getAliasee()).getNameStart();
+
+  // See if there is already something with the target's name in the module.
+  llvm::GlobalValue *Entry = GlobalDeclMap[AliaseeName];
+
+  llvm::Constant *Aliasee;
+  if (isa<llvm::FunctionType>(DeclTy))
+    Aliasee = GetOrCreateLLVMFunction(AliaseeName, DeclTy, GlobalDecl());
+  else
+    Aliasee = GetOrCreateLLVMGlobal(AliaseeName,
+                                    llvm::PointerType::getUnqual(DeclTy), 0);
+  if (!Entry) {
+    llvm::GlobalValue* F = cast<llvm::GlobalValue>(Aliasee);
+    F->setLinkage(llvm::Function::ExternalWeakLinkage);    
+    WeakRefReferences.insert(F);
+  }
+
+  return Aliasee;
+}
+
 void CodeGenModule::EmitGlobal(GlobalDecl GD) {
   const ValueDecl *Global = cast<ValueDecl>(GD.getDecl());
 
+  // Weak references don't produce any output by themselves.
+  if (Global->hasAttr<WeakRefAttr>())
+    return;
+
   // If this is an alias definition (which otherwise looks like a declaration)
   // emit it now.
   if (Global->hasAttr<AliasAttr>())
@@ -708,6 +740,14 @@ llvm::Constant *CodeGenModule::GetOrCreateLLVMFunction(const char *MangledName,
   // Lookup the entry, lazily creating it if necessary.
   llvm::GlobalValue *&Entry = GlobalDeclMap[MangledName];
   if (Entry) {
+    if (WeakRefReferences.count(Entry)) {
+      const FunctionDecl *FD = cast_or_null<FunctionDecl>(D.getDecl());
+      if (FD && !FD->hasAttr<WeakAttr>())
+	Entry->setLinkage(llvm::Function::ExternalLinkage);
+
+      WeakRefReferences.erase(Entry);
+    }
+
     if (Entry->getType()->getElementType() == Ty)
       return Entry;
 
@@ -817,6 +857,13 @@ llvm::Constant *CodeGenModule::GetOrCreateLLVMGlobal(const char *MangledName,
   // Lookup the entry, lazily creating it if necessary.
   llvm::GlobalValue *&Entry = GlobalDeclMap[MangledName];
   if (Entry) {
+    if (WeakRefReferences.count(Entry)) {
+      if (D && !D->hasAttr<WeakAttr>())
+	Entry->setLinkage(llvm::Function::ExternalLinkage);
+
+      WeakRefReferences.erase(Entry);
+    }
+
     if (Entry->getType() == Ty)
       return Entry;