Question about Linux routing - ip_route_output_key vs fib_lookup

Hi,
I had a question about Linux routing functions, suppose I want to find
out what interface does a route to an IP address belong to - is it
better (in terms of performance) to use ip_route_output_key or
fib_lookup. My initial guess from looking at the source code is
fib_lookup since the routing cache wont be helpful since I wont have
any interface information (rather want to find out interface
information) but wanted to be sure. Thanks.

Megh