RFR: 8234328: VectorSet::clear can cause fragmentation

[Claes Redestad]

Hi,

today, VectorSet::clear "reclaims" storage when the size is large.

However, since the backing array is allocated in a resource arena, this
is dubious since the currently retained memory is only actually freed
and made reusable if it's currently the last chunk of memory allocated
in the arena. This means a clear() is likely to just waste the allocated
memory until we exit the current resource scope

Instead, I propose a strategy where instead of "freeing" we keep track
of the currently allocated size of the VectorSet separately from the in-
use size. We can then defer the memset to reset/clear the memory to the
next time we need to grow, thus avoiding unnecessary reallocations and
memsets. This limits the memory waste.

Bug:    https://bugs.openjdk.java.net/browse/JDK-8234328
Webrev: http://cr.openjdk.java.net/~redestad/8234328/open.00/

Testing: tier1-3

Either of reset() or clear() could now be removed, which seems like a
straightforward follow-up RFE. With some convincing I could roll it into
this patch.

Thanks!

/Claes