Reduce garbage allocation in SyncVar Deserialize when syncvar has no hook

[liamcary]

While profiling my project I found that deserializing sync vars causes small allocations on the client each time the value changes. In my case I had a double and an int that was changing frequently, and each time they changed there was a 40 bytes and 32 byte allocation in GeneratedSyncVarDeserialize. There's no explicit memory allocation in GeneratedSyncVarDeserialize, but removing the Action<T, T> OnChanged parameter fixes the allocation, even though the NetworkBehaviour's DeserializeSyncVar method passes in null. I assume its a boxing operation on the generic delegate when checking if its null.

To fix the allocation, I implemented a NoHook alternative for each of the 4 GeneratedSyncVarDeserialize methods, then updated the NetworkBehaviourProcessor to call the NoHook method for SyncVar's that don't have a hook. Its not the cleanest implementation, and ideally the delegate could be passed in without any allocation, to reduce memory allocations when deserializing syncvars that DO have a hook.

This is a before and after screenshot of the profiler with Deep Profile enabled:

[liamcary]

Out of curiousity, i tried an alternative approach of adding a boolean 'hasOnChange' parameter to see if the garbage allocation only happens when checking if OnChanged is null. Unfortunately it still allocates, so it seems like the presence of the Action<T, T> parameter causes allocations, whether its used or not, and whether its null or not.

[miwarnec]

interesting find, gonna check this soon