> What's the performance hit for doing a pref on a cache line that is > already pref'd? Does it turn into a nop, or do we get some horrible > degenerate case? Are 64 bit processors always at least 32 byte cache > line size? I don't really expect anyone to know the answers > right now. I > expect I'll need to time code to tell. This makes generating > them at run > time look better and better. As very general rules of thumb: - A pref to a line which is already in the cache take a cycle in the load/store unit and does not go back out to the memory subsystem. There are some possible ships-passing-in-the-night scenarios, but most processors do what you'd expect. - Most 64-bit processors are built for high-end applications, and most high-end processors most likely have at least 32-bit lines. One usually has smaller line sizes when the processor is intended for lower-end applications, or where the memory subsystem isn't all that good. /gmu --- Michael Uhler, Chief Technology Officer MIPS Technologies, Inc. Email: uhler at mips.com 1225 Charleston Road Mountain View, CA 94043
