>use p->hashfraction when doing non-time-critical calculations, rather than
>using HASHFRACTION directly. in time-critical calculations, if HASHFRACTION
>is a power of two, check that p->hashfraction == HASHFRACTION and if so do
>the calculation with the compiled-in value so that the compiler can optimize
>out (potentially) expensive divisions. if p->hashfraction != HASHFRACTION,
>actually do the division. This has the result that on machines with slow
>division, the division can be optimized out of the common case, but that
>if HASHFRACTION changes from the compiled-in value (for whatever reason),
>profiling will still work. Changes suggested by Chris Torek.
