Re: Reference Counting (was Re: Searching Method for Incremental Garbage Collection)

[firth@sei.cmu.edu (Robert Firth)]

In article <hbakerD07221.1C0@netcom.com> hbaker@netcom.com (Henry G. Baker) writes:

>You should be prepared for huge overheads when updating reference
>counts on shared objects in a multiple-threaded (multiple-tasked)
>program.

Not necessarily.  The PE3200, for example, has an "add to memory"
insturction that seems tailor made for this problem.

	AM reg, mem

In practice, the constants +1 and -1 were so often used as the
addend that I simply had the compiler permanently allocate two
registers to them, so any reference counting became a single
atomic instruction.

Well, things are now riscier.  So here's another way: make the
reference counting a sequence of instructions:

	L reg, mem
	AI reg, 1
	S reg, mem

In the task context switch, check whether you interrupted that
sequence *and the memory address points into the shared heap*
(trivial on a risc machine with essentially one memory address
mode), and simply slave whatever didn't get done.  On some
machines, it may be cheaper always to slave rather than even
bother checking the address.