Johan and I talked about this on IRC. The card table is actually irrelevant for newly allocated arrays, so in fact we can just not initialise it at all. Cheers, Simon On 10/03/2014 08:22, Johan Tibell wrote:
Hi,
For MutableArray#s, a card is supposed to be marked if the array is pointing to an object in a younger generation than itself resides in. Since the array always points to elements in the same or an older generation when it gets allocated, we should be fine with marking the array as clean upon allocation and setting all the cards to zero. Right now it's marked as *dirty* and all cards are (modulo a bug) marked as zero.
I think we should mark these arrays as clean to avoid some GC costs later. We could avoid writing the card table on allocation as well, as it's always safe to default to dirty. However, this will incur extra GC costs later (after the first write to the array) as the whole array will appear as dirty (due to garbage values in the card table as it wasn't initialized) even if only a single element was written.
I'm leaning towards 1) marking arrays as clean and 2) zeroing out the card table when the array is created. Thoughts?
-- Johan