[gclist] reference counting
Fri, 15 Sep 2000 00:30:01 -0400
Manoj Plakal wrote:
> What you seem to need is an "overwrite-cache-line"
> kind of store instruction which tells the cache that it
> does not need to fetch the line in case of a miss
> since the data is going to be over-written. Seems kind
> of tricky for the software since it has to guarantee
> that it will over-write a whole cache line's worth
> of data, and make sure that there are no parts of
> the cache line that are "left over" and which need
> to be fetched (I guess this is where sub-blocking
> would be useful).
Interestingly, the Compaq C compiler for Alpha appears to generate the
WH64 instruction in some situations, wrapped in a conditional to test
for overlap. Apparently the cost of possible branch misprediction is
outweighed by savings in memory latency. But at least its use is
transparent to the source code.
I wonder if allocating memory on exact cache boundaries would be
worthwhile for this sort of optimization.