Fast multiple match function

Hi,

I know that one can find all occurrences of x in a vector v by doing

which(x == v).

However, if I need to do this again and again, where v is remaining the
same, then this is quite inefficient. In my particular case, I need to do
this millions of times, and length(v) = 100 million.

Does anyone have suggestion on how to go about it?
I know of a package called fmatch that does the above for the match
function. But they don't handle multiple matches.

Thanks