RFR (S) 8078438: Interpreter should support conditional card marks (UseCondCardMark)

[Andrew Haley]

On 04/29/2015 05:20 PM, Andrew Dinn wrote:
> But then neither is there anything to guarantee that the conditional
> test at T3_a sees the write of 0x1 at T3_b. We know T3_b > T1_a but that
> is all. The mutator and GC thread would have to employ some sort of
> ordering relationship between the write at T3_b and the read at T3_a to
> guarantee that. TSO won't do it. Nor will an stlr at T1_a.
> 
> Of course, if the mutator writes the card unconditionally at T3_a then
> there is no problem -- the GC will see the dirty card at the next dirty
> scan. The problem is that the condition test may be based on out of date
> information causing a necessary card mark to be omitted.

I wondered if a halfway house would work.  Something like

Mutator:

T2_a:
0.x = a
membar(store_store)
if card[o >> 9] != 0x0
  card[o >> 9] = 0x0

There is a release in write_ref_field() but I think it's only used by
mutator threads.  I think there is something vital we don't
understand.  I think the actual collection (and the write of the card
table entry by the collector) happens only at a safepoint.  GC people,
can you help?

Thanks,
Andrew.