[gclist] write barrier

[boehm.PARC@xerox.com]

Nick Barnes wrote:

"A write barrier is a mechanism for performing some memory management action
when some forms of memory write take place".

Sounds good to me.

Paul wrote:

"The Appel-Ellis-Li GC uses a pagewise read barrier to implement a variant
of Henry Baker's incremental copying algorithm.  I believe there was/is an
incremental variant of Joel Bartlett's mostly-copying GC that used a read
barrier like this. (?)"

If I remember correctly, then the SRC Modula 3 collector does this on some
platforms.  If not, hopefully someone will correct me.

Hans