[Performance] Devirtualize IList Vectors

[jamescourtney]

IList<T> is implemented by many different classes:

• T[]
• List<T>
• FlatBufferVector<T>
• ReadOnlyCollection<T>
• Custom implementations

Some of these, such as T[] are special and can observe large performance gains when using the type directly, but suffer the virtual method penalty when exposed through the IList<T> interface.

Devirtualization would look like:

public void ListOp<T>(IList<T> list)
{
     if (list is T[] array)
     {
         {array-loop}
     }
     else if (list is List<T> realList)
     {
        {list-loop}
     }
     else
     {
         {ilist-loop}
     }
}

This issue tracks the work necessary to investigate:

• Which well known IList<T> implementations have substantial performance gains when the underlying type is used directly? Probably T[] and List<T>, but anything else?
• What is the penalty for devirtualization via the is operator? How expensive are these comparisons?
• Is there a length threshold at which this becomes (un)profitable?
• What is the code size impact? ie, does the increase in binary size come with an increase in performance?

[jamescourtney]

Published in version 5.2