Int128 and Interlocked operations

[BreyerW]

@tannergooding is it technically possible to implement atomic operations for 128bit int structs with current hardware? Im mostly interested in Increment and bitwise operations available in Interlocked class. If yes do you think such addition is worthwhile and warrant proposal to extend Interlocked class?

[tannergooding]

Not really feasible, IMO. You can certainly implement some form of "interlocked" data structures but due to the nature of the implementation, they would all (except sometimes CompareExchange) be implemented as expensive loops. Additionally, on 32-bit such operations get even worse and you can get into really odd behavior.

The most feasible thing is really to just expose Interlocked.CompareExchange that operates over 2x nint or 2x nuint. That can be atomic on both 32-bit and 64-bit and is more practically useful IMO.

[BreyerW]

Hmm based on your answer atomic Increment and bitwise operations wouldnt be possible on 2x nint either (since u mentioned only CompareExchange as possible candidate)? I guess i will stick to ulong its large enough for most cases in my project

[tannergooding]

They'd be possible, but equivalent to doing most of these operation as long or ulong on a 32-bit platform. That is, they'd be implemented with a more complex loop instead.

For example, Interlocked.Add is implemented on 32-bit as:

do
{
    Old = *Addend;
}
while (InterlockedCompareExchange64(Addend, Old + Value, Old) != Old);

Where InterlockedCompareExchange64 itself is generally a cmpxchg8b or similar instruction. That is, it does the equivalent of exchanging 2x nint or 2x nuint.