Alias must point to a definition

Reapplying the patch after modifying the test case.

Inlining the destructor caused the compiler to generate bad IR which failed the Verifier in the backend.
https://llvm.org/bugs/show_bug.cgi?id=30341

This patch disables alias to available_externally definitions.

Reviewers: eugenis, rsmith

Differential Revision: https://reviews.llvm.org/D24682

llvm-svn: 283063

clang/lib/CodeGen/CGCXX.cpp

@@ -134,6 +134,11 @@ bool CodeGenModule::TryEmitDefinitionAsAlias(GlobalDecl AliasDecl,
   llvm::GlobalValue::LinkageTypes TargetLinkage =
       getFunctionLinkage(TargetDecl);
 
+  // available_externally definitions aren't real definitions, so we cannot
+  // create an alias to one.
+  if (TargetLinkage == llvm::GlobalValue::AvailableExternallyLinkage)
+    return true;
+
   // Check if we have it already.
   StringRef MangledName = getMangledName(AliasDecl);
   llvm::GlobalValue *Entry = GetGlobalValue(MangledName);
@@ -156,14 +161,7 @@ bool CodeGenModule::TryEmitDefinitionAsAlias(GlobalDecl AliasDecl,
 
   // Instead of creating as alias to a linkonce_odr, replace all of the uses
   // of the aliasee.
-  if (llvm::GlobalValue::isDiscardableIfUnused(Linkage) &&
-     (TargetLinkage != llvm::GlobalValue::AvailableExternallyLinkage ||
-      !TargetDecl.getDecl()->hasAttr<AlwaysInlineAttr>())) {
-    // FIXME: An extern template instantiation will create functions with
-    // linkage "AvailableExternally". In libc++, some classes also define
-    // members with attribute "AlwaysInline" and expect no reference to
-    // be generated. It is desirable to reenable this optimisation after
-    // corresponding LLVM changes.
+  if (llvm::GlobalValue::isDiscardableIfUnused(Linkage)) {
     addReplacement(MangledName, Aliasee);
     return false;
   }

clang/test/CodeGenCXX/alias-available-externally.cpp

@@ -0,0 +1,20 @@
+// RUN: %clang_cc1 -O1 -std=c++11 -emit-llvm -disable-llvm-passes -o - %s | FileCheck %s
+// Clang should not generate alias to available_externally definitions.
+// Check that the destructor of Foo is defined.
+// The destructors have different return type for different targets.
+// CHECK: define linkonce_odr {{.*}} @_ZN3FooD2Ev
+template <class CharT>
+struct String {
+  String() {}
+  ~String();
+};
+
+template <class CharT>
+inline __attribute__((visibility("hidden"), always_inline))
+String<CharT>::~String() {}
+
+extern template struct String<char>;
+
+struct Foo : public String<char> { Foo() { String<char> s; } };
+
+Foo f;