C++ already gives us a tool for pointer zapping

Performance is always going to be dependent on the implementation, as a tradeoff with power and area, and there are some quirks on Morello due to being an experimental prototype implementation, but in general it would be reasonable to expect them to perform somewhere between plain CAS(A)(L) and CAS(A)(L)P. On a high-performance implementation there will likely be a 128-bit data path already there (true of Morello) so they probably all perform about the same, but a lower-performance implementation could choose to have a narrower data path to reduce core area and take an extra cycle. But in terms of cache coherency traffic it's the same, it still fits in a single cache line, you're just accessing more of that cache line.