Disable this-return optimizations when targeting iOS 5 and earlier.

Clang implements the part of the ARM ABI saying that certain functions
(e.g., constructors and destructors) return "this", but Apple's version of
gcc and llvm-gcc did not. The libstdc++ dylib on iOS 5 was built with
llvm-gcc, which means that clang cannot safely assume that code from the C++
runtime will correctly follow the ABI. It is also possible to run into this
problem when linking with other libraries built with gcc or llvm-gcc. Even
though there is no way to reliably detect that situation, it is most likely
to come up when targeting older versions of iOS. Disabling the optimization
for any code targeting iOS 5 solves the libstdc++ problem and has a reasonably
good chance of fixing the issue for other older libraries as well.
<rdar://problem/16377159>

llvm-svn: 205272

clang/lib/CodeGen/CodeGenModule.cpp

@@ -775,7 +775,12 @@ void CodeGenModule::SetFunctionAttributes(GlobalDecl GD,
   if (!IsIncompleteFunction)
     SetLLVMFunctionAttributes(FD, getTypes().arrangeGlobalDeclaration(GD), F);
 
-  if (getCXXABI().HasThisReturn(GD)) {
+  // Add the Returned attribute for "this", except for iOS 5 and earlier
+  // where substantial code, including the libstdc++ dylib, was compiled with
+  // GCC and does not actually return "this".
+  if (getCXXABI().HasThisReturn(GD) &&
+      !(getTarget().getTriple().isiOS() &&
+        getTarget().getTriple().isOSVersionLT(6))) {
     assert(!F->arg_empty() &&
            F->arg_begin()->getType()
              ->canLosslesslyBitCastTo(F->getReturnType()) &&