On 6/2/06, Davis, Mark <[EMAIL PROTECTED]> wrote:
Question: does gcc now know the difference between prefetching to cache L1 via "lfetch", as opposed to prefetching only to level L2 via "lfetch.nt1"?
The ia64 backend knows the difference, see the prefetch pattern in ia64.md. But ia64 is the only backend that supports this kind of explicit locality parameter. And since no-one from the ia64 community cared much about gcc until recently, gcc's prefetching pass (which is limited anyway) does not generate lfetch.nt1 or other prefetches with explicit locality parameters.
For floating point data, the latter is the only interesting case because float loads only access the L2. Thus using "lfetch" for floating point arrays will unnecessarily wipe out > the contents of L1. (gcc 3.2.3 only seems to generate "lfetch", which is why I ask...)
You could experiment with this for ia64 by hacking issue_prefetch_ref in tree-ssa-loop-prefetch.c to issue a prefetch to L2 for floating point types. Gr. Steven
