Common: CallLambdaTrampoline can return a value As it is currently written, CallLambdaTrampoline does not return a value. However, some of the functions that are being wrapped may return a value that the JIT is expected to understand. A compiler *cough cough clang* may opt to alter %rax after the wrapped lambda returns, e.g. popping a previous value, which can clobber the return value. If we actually have a return value, then the compiler must not clobber it.