The Darwin kernel does not provide useful guard variable support.

Issue this as an IR-gen error;  it's not really worthwhile doing this
"right", i.e. in Sema, because IR gen knows a lot of tricks beyond
what the constant evaluator knows.

llvm-svn: 127854

clang/lib/Driver/Tools.cpp

@@ -1138,6 +1138,12 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   if (getToolChain().getTriple().getOS() != llvm::Triple::Darwin)
     CmdArgs.push_back("-mconstructor-aliases");
 
+  // Darwin's kernel doesn't support guard variables; just die if we
+  // try to use them.
+  if (KernelOrKext &&
+      getToolChain().getTriple().getOS() == llvm::Triple::Darwin)
+    CmdArgs.push_back("-fforbid-guard-variables");
+
   if (Args.hasArg(options::OPT_mms_bitfields)) {
     CmdArgs.push_back("-mms-bitfields");
   }