C2 compare and swap fixes

[Andrew Haley]

On 06/06/16 09:01, Roland Westrelin wrote:
> What follows is my own interpretation: in case the value in memory is a
> from space reference and the expected value passed to weakCAS is a to
> space value, then the weakCAS would fail as long as the value in memory
> is not updated to the to space value which as I understand, could cause
> a thread to be stuck in a loop with a weakCAS until the GC threads
> finally update the reference and could be a long time. So I don't think
> we can simply let the weakCAS fail.

That's correct.  On x86 we don't have any mechanism to implement a
weakCAS, so we just go with a strong one.  On AArch64 we can use a
custom version which implements the correct semantics.

Andrew.