[gclist] reference counting

[Jeff Sturm]

"Boehm, Hans" wrote:
> As far as I have been able to determine, there are very few machines that
> have the required cache property, notably one or two (now very obsolete) DEC
> machines that were used in most of the experiments.  The hardware I have
> worked with recently always seems to fetch the cache line when you write the
> first word to it.  (I'm not a hardware expert, but I suspect it's hard to
> maintain partially valid cache lines in a multiprocessor.)  If someone has
> more information, please post.

Alpha appears to have some support for write-allocate.  The WH64
instruction specifies that a 64-byte cache line containing a given
address may be overlaid with unspecified data.  Cache coherency is still
required for multiprocessors, so I assume WH64 has no effect on
addresses that are already resident in any cache.  A related
instruction, ECB, allows early eviction of cache lines.  Perhaps these
two could benefit a tracing collector?  I haven't experimented much with
them yet.

-Jeff