@Arjen: My understand was exactly the other way round. The gcc manual says: “Specifying -march=cpu-type implies -mtune=cpu-type" and "march=cpu-type allows GCC to generate code that may not run at all on processors other than the one indicated.”
The Intel manual says for -xcode: Do not use code values to create binaries that will execute on a processor that is not compatible with the targeted processor.