Right, the biggest blocker for a while was that LLVM did not support Xtensa as a backend at all. Now it does have it, but it's still experimental and doesn't seem to be complete yet, although there's active progress on it, see https://github.com/espressif/llvm-project/issues/4. The LLVM backend support alone is not enough, Clang will also need to gain support for Xtensa. Once that happens, there shouldn't be anything hard about supporting it in Swift.