prefetch on ppc64

[Paul Mackerras]

Serge E. Hallyn writes:

> While investigating the inordinate performance impact one of my patches
> seemed to be having, we tracked it down to two hlist_for_each_entry
> loops, and finally to the prefetch instruction in the loop.

I would be interested to know what results you get if you leave the
loops using hlist_for_each_entry but change prefetch() and prefetchw()
to do the dcbt or dcbtst instruction only if the address is non-zero,
like this:

static inline void prefetch(const void *x)
{
	if (x)
	        __asm__ __volatile__ ("dcbt 0,%0" : : "r" (x));
}

static inline void prefetchw(const void *x)
{
	if (x)
	        __asm__ __volatile__ ("dcbtst 0,%0" : : "r" (x));
}

It seems that doing a prefetch on a NULL pointer, while it doesn't
cause a fault, does waste time looking for a translation of the zero
address.

Paul.