Spreads.Unsafe

This library adds several methods to System.Runtime.CompilerServices.Unsafe package that are used in Spreads library to get the maximum performance. It could be compiled from a working repo of corefx if placed alogside with S.R.CS.U folder.

The added methods emit a constrained call to instance methods of known interfaces on instances of a generic type T without a type constraint where T : IKnownInterface<T>.

For example, calling the IComparable<T>.CompareTo method is implemented like this:

  .method public hidebysig static int32 CompareToConstrained<T>(!!T& left, !!T& right) cil managed aggressiveinlining
  {
        .custom instance void System.Runtime.Versioning.NonVersionableAttribute::.ctor() = ( 01 00 00 00 )
        .maxstack 8
        ldarg.0
        ldarg.1
        ldobj !!T
        constrained. !!T
        callvirt instance int32 class [System.Runtime]System.IComparable`1<!!T>::CompareTo(!0)
        ret 
  } // end of method Unsafe::CompareToConstrained

In addition to the IComparable<T> interface there are IEquatable<T> and the following custom ones:

public interface IInt64Diffable<T> : IComparable<T>
{
    T Add(long diff);
    long Diff(T other);
}

public interface IDelta<T>
{
    T AddDelta(T delta);
    T GetDelta(T other);
}

KeyComparer<T>

The main use case/sample is KeyComparer<T> implemented here. A benchmark shows that the unsafe CompareToConstrained method and the KeyComparer<T> that uses it are c.2x faster than the Comparer<T>.Default when called via the IComparer<T> interface and are c.1.6x faster when the default comparer is called directly as a class.

ComparerInterfaceAndCachedConstrainedComparer

Case	MOPS	Elapsed	GC0	GC1	GC2	Memory
Unsafe	403.23	248 ms	0.0	0.0	0.0	0.000 MB
KeyComparer*	396.83	252 ms	0.0	0.0	0.0	0.000 MB
Default	255.75	391 ms	0.0	0.0	0.0	0.000 MB
Interface	211.42	473 ms	0.0	0.0	0.0	0.000 MB

* KeyComparer<T> uses the JIT compile-time constant optimization for known types and falls back to the Unsafe.CompareToConstrained method for types that implement IComparable<T> interface. On .NET 4.6.1 there is no visible difference with and without the special cases: Unsafe.CompareToConstrained performs as fast as the if (typeof(T) == typeof(Int32)) { ... } pattern. See the discussion here and implementation with comments here explaining why the special cases could be needed on some platforms.

Unsafe methods could only be called on instances of a generic type T when the type implements a relevant interface. KeyComparer<T> has a static readonly field that (in theory) allows to use the same JIT optimization mentioned above:

private static readonly bool IsIComparable = typeof(IComparable<T>).GetTypeInfo().IsAssignableFrom(typeof(T));

public int Compare(T x, T y)
{
    ...
    if (IsIComparable) // JIT compile-time constant 
    {
    return Unsafe.CompareToConstrained(ref x, ref y);
    }
    ...
}

But even if such optimization breaks in this particular case (see the linked discussion) then checking a static bool field is still much cheaper than a virtual call, especially given that its value is constant for the lifetime of a program and branch prediction should be 100% effective.

FastDictionary

Another use case is FastDictionary<TKey,TValue> that uses unsafe methods via KeyEqualityComparer<T>, which is very similar to KeyComparer<T> above. FastDictionay is a rewrite of S.C.G.Dictionary<TKey,TValue> that avoids virtual calls to an equality comparer.

A benchmark for <int,int> types shows that FastDictionary<int,int> is c.70% faster than S.C.G.Dictionary<int,int>:

CompareSCGAndFastDictionaryWithInts

Case	MOPS	Elapsed	GC0	GC1	GC2	Memory
FastDictionary	120.48	415 ms	0.0	0.0	0.0	0.000 MB
Dictionary	71.63	698 ms	0.0	0.0	0.0	0.000 MB

Such implementation is much simpler than one with an additoinal generic parameter for a comparer, as recently discussed in this blog post. It is also more flexible than constraining TKey to where TKey : IEquatable<TKey> and gives the same performance.

Another benchmark with a key as a custom 16-bytes Symbol struct** shows c.50% performance gain:

CompareSCGAndFastDictionaryWithSymbol

Case	MOPS	Elapsed	GC0	GC1	GC2	Memory
FastDictionary	63.69	157 ms	0.0	0.0	0.0	0.000 MB
Dictionary	43.29	231 ms	0.0	0.0	0.0	0.000 MB

** Note that the Symbol struct also uses unsafe methods, but the used ones are copied from S.R.CS.Unsafe package and are not specific to this library. However, this library could be a replacement and due to different namespaces they will not cause any conflicts if used together.

Status and version

Verion 1.0 is available on NuGet as Spreads.Unsafe.

There is an interesting discussion about intrinsifying EqualityComparer<T>,
aiming to achieve a similar goal of inlining calls when possible. However, the last comment from a CoreCLR member says that:

I am not aware of anybody working on this right now, so it is pretty unlikely [that it has a chance to appear in .NET Core 2.0].

Future versions of .NET Core may have much faster comparers, but for existing code and platforms Spreads.Unsafe library gives the required performance right here and now.

License

Spreads.Unsafe is licensed under the MIT license.