There's information that the M1 makes refcounting faster than before, i.e. on the Intel platforms. People reported that some benchmarks show the improvement of specifically refcount increment/decrement operations up to 5-6 times.
The short version of my question is: is
isKnownUniquelyReferenced() affected by this optimization? In other words, does CoW () benefit from M1 or not?
The long version: I'm not knowledgeable in how Apple's M1 could make atomic increment/decrements so fast, but my hypothesis is that, because the value of the reference counter is only ever needed after the atomic decrement operation, you can have atomic operations that bypass the caches and modify the counters directly in memory. I.e. if the caches contain the wrong value, that should be totally fine since nobody's interested in the value itself before decrementing it. This, as I understand it, allows to bypass normal memory ordering and maintain the refcounters in the app without causing CPU cache shake-ups.
So now there's
isKnownUniquelyReferenced() which seems to be the case that would require the actual value and therefore should issue a memory fence, i.e. be inefficient.
I may be totally wrong on any or all of the above, so I'd appreciate if someone can clarify this. Thanks!