Support devirtualizing/inlining Comparer<T>.Default.Compare

[stephentoub]

dotnet/coreclr#14125 made EqualityComparer<T>.Default an intrinsic recognized by the JIT, teaching the JIT what concrete type of comparer would be used for any given T and enabling EqualityComparer<T>.Default.Equals to be fully devirtualized and inlined. This in turn led to optimizations in managed code, where code that was making heavy use of EqualityComparer<T>.Default as a default implementation added specialized code paths that used EqualityComparer<T>.Default, e.g. in various collections like Dictionary<>.

Comparer<T>.Default could benefit from similar optimizations. Comparer<T>.Default.Compare doesn't show up quite as much as EqualityComparer<T>.Default.Equals, but it still shows up in a lot of places, and there are a variety of collections (e.g. the new PriorityQueue<> being added in .NET 6) that would similarly optimize for Comparer<T>.Default if Comparer<T>.Default.Compare could be devirtualized and inlined.

Is this feasible?

cc: @AndyAyersMS, @eiriktsarpalis

[stephentoub]

Just found another issue for this, so closing as a dup of #39873.

[AndyAyersMS]

Should be feasible, yes. Will leave further discussion to the linked issue.