clz pattern

[Lu, John]

Hi,

I'm trying to utilize the clz pattern:

  (define_insn "clzhi2"
    [(set (match_operand:HI 0 "register_operand" "=r")
        (clz:HI (match_operand:HI 1 "register_operand" "r")))]
    ""
    "cntlz %0 %1")

I can build a compiler successfully with this pattern, but I
can't find any C source that will utilize this pattern.  I was
wondering how GCC utilizes these patterns (and others like it),
which have a functionality that does not straightforwardly map to 
any C operator.  

Thanks,
John Lu