There's a proposal to add bit manipulation functions in C, specifically leading zeros is helpful to find highest bit set. See http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2827.htm#design-bit-leading.trailing.zeroes.ones
They are expected to be implemented as built-ins where possible, so sure it is an efficient way.
This is similar to what was recently added to C++ (std::countl_zero, etc).