-Xllvm flags are compiled out of Release builds, since they impact startup time. -Xcc -Xclang will probably work because swiftc uses Clang to set up the LLVM IR module, but it's relying on undocumented behavior. (-Xclang flags are just as unstable as -Xfrontend flags for Swift.) What is stable are the -target-cpu and -target-variant driver flags, but I don't remember what they're hooked up to.