Use aliases for more constructors and destructors.

With this patch we produce alias for cases like

template<typename T>
struct foobar {
  foobar() {
  }
};
template struct foobar<void>;

It is safe to use aliases to weak symbols, as long and the alias itself is also
weak.

llvm-svn: 192300

clang/lib/CodeGen/CGCXX.cpp

@@ -108,32 +108,17 @@ bool CodeGenModule::TryEmitDefinitionAsAlias(GlobalDecl AliasDecl,
   // support aliases with that linkage, fail.
   llvm::GlobalValue::LinkageTypes Linkage = getFunctionLinkage(AliasDecl);
 
-  switch (Linkage) {
-  // We can definitely emit aliases to definitions with external linkage.
-  case llvm::GlobalValue::ExternalLinkage:
-  case llvm::GlobalValue::ExternalWeakLinkage:
-    break;
-
-  // Same with local linkage.
-  case llvm::GlobalValue::InternalLinkage:
-  case llvm::GlobalValue::PrivateLinkage:
-  case llvm::GlobalValue::LinkerPrivateLinkage:
-    break;
-
-  // We should try to support linkonce linkages.
-  case llvm::GlobalValue::LinkOnceAnyLinkage:
-  case llvm::GlobalValue::LinkOnceODRLinkage:
+  // We can't use an alias if the linkage is not valid for one.
+  if (!llvm::GlobalAlias::isValidLinkage(Linkage))
     return true;
 
-  // Other linkages will probably never be supported.
-  default:
-    return true;
-  }
-
   llvm::GlobalValue::LinkageTypes TargetLinkage
     = getFunctionLinkage(TargetDecl);
 
-  if (llvm::GlobalValue::isWeakForLinker(TargetLinkage))
+  // Don't create an strong alias to a linker weak symbol. If the linker
+  // decides to drop the symbol, the alias would become undefined.
+  if (llvm::GlobalValue::isWeakForLinker(TargetLinkage) &&
+      !llvm::GlobalValue::isWeakForLinker(Linkage))
     return true;
 
   // Derive the type for the alias.