Improve perf of Enumerable.Order{Descending} for primitives (#76733)

* Improve perf of Enumerable.Order{Descending} for primitives

For non-floating point primitive types, stable sorts are indistinguishable from unstable sorts.  They could be distinguishable if there are custom keys associated with each item, but Order{Descending} doesn't have separate keys from the elements themselves.  As such, we can avoid all of the overhead associated with the int[] map Order{Descending}By creates as part of implementing the stable sort, avoid always specifying a comparer, etc.  This PR does so for Order{Descending}(comparer) followed by ToArray, ToList, and foreach/GetEnumerator.

* Don't try to optimize custom comparers

src/libraries/System.Linq/src/System/Linq/OrderBy.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Collections.Generic;
+using System.Runtime.CompilerServices;
 
 namespace System.Linq
 {
@@ -23,7 +24,7 @@ public static partial class Enumerable
         /// This method compares elements by using the default comparer <see cref="Comparer{T}.Default"/>.
         /// </remarks>
         public static IOrderedEnumerable<T> Order<T>(this IEnumerable<T> source) =>
-            OrderBy(source, EnumerableSorter<T>.IdentityFunc);
+            Order(source, comparer: null);
 
         /// <summary>
         /// Sorts the elements of a sequence in ascending order.
@@ -42,7 +43,9 @@ public static IOrderedEnumerable<T> Order<T>(this IEnumerable<T> source) =>
         /// If comparer is <see langword="null"/>, the default comparer <see cref="Comparer{T}.Default"/> is used to compare elements.
         /// </remarks>
         public static IOrderedEnumerable<T> Order<T>(this IEnumerable<T> source, IComparer<T>? comparer) =>
-            OrderBy(source, EnumerableSorter<T>.IdentityFunc, comparer);
+            TypeIsImplicitlyStable<T>() && (comparer is null || comparer == Comparer<T>.Default) ?
+                new OrderedImplicitlyStableEnumerable<T>(source, descending: false) :
+                OrderBy(source, EnumerableSorter<T>.IdentityFunc, comparer);
 
         public static IOrderedEnumerable<TSource> OrderBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)
             => new OrderedEnumerable<TSource, TKey>(source, keySelector, null, false, null);
@@ -66,7 +69,7 @@ public static IOrderedEnumerable<TSource> OrderBy<TSource, TKey>(this IEnumerabl
         /// This method compares elements by using the default comparer <see cref="Comparer{T}.Default"/>.
         /// </remarks>
         public static IOrderedEnumerable<T> OrderDescending<T>(this IEnumerable<T> source) =>
-            OrderByDescending(source, EnumerableSorter<T>.IdentityFunc);
+            OrderDescending(source, comparer: null);
 
         /// <summary>
         /// Sorts the elements of a sequence in descending order.
@@ -85,7 +88,9 @@ public static IOrderedEnumerable<T> OrderDescending<T>(this IEnumerable<T> sourc
         /// If comparer is <see langword="null"/>, the default comparer <see cref="Comparer{T}.Default"/> is used to compare elements.
         /// </remarks>
         public static IOrderedEnumerable<T> OrderDescending<T>(this IEnumerable<T> source, IComparer<T>? comparer) =>
-            OrderByDescending(source, EnumerableSorter<T>.IdentityFunc, comparer);
+            TypeIsImplicitlyStable<T>() && (comparer is null || comparer == Comparer<T>.Default) ?
+                new OrderedImplicitlyStableEnumerable<T>(source, descending: true) :
+                OrderByDescending(source, EnumerableSorter<T>.IdentityFunc, comparer);
 
         public static IOrderedEnumerable<TSource> OrderByDescending<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector) =>
             new OrderedEnumerable<TSource, TKey>(source, keySelector, null, true, null);
@@ -132,6 +137,17 @@ public static IOrderedEnumerable<TSource> ThenByDescending<TSource, TKey>(this I
 
             return source.CreateOrderedEnumerable(keySelector, comparer, true);
         }
+
+        /// <summary>Gets whether the results of an unstable sort will be observably the same as a stable sort.</summary>
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static bool TypeIsImplicitlyStable<T>() =>
+            typeof(T) == typeof(sbyte) || typeof(T) == typeof(byte) ||
+            typeof(T) == typeof(int) || typeof(T) == typeof(uint) ||
+            typeof(T) == typeof(short) || typeof(T) == typeof(ushort) ||
+            typeof(T) == typeof(long) || typeof(T) == typeof(ulong) ||
+            typeof(T) == typeof(Int128) || typeof(T) == typeof(UInt128) ||
+            typeof(T) == typeof(nint) || typeof(T) == typeof(nuint) ||
+            typeof(T) == typeof(bool) || typeof(T) == typeof(char);
     }
 
     public interface IOrderedEnumerable<out TElement> : IEnumerable<TElement>

src/libraries/System.Linq/src/System/Linq/OrderedEnumerable.SpeedOpt.cs

@@ -3,13 +3,13 @@
 
 using System.Collections;
 using System.Collections.Generic;
-using System.Diagnostics.CodeAnalysis;
+using System.Runtime.InteropServices;
 
 namespace System.Linq
 {
     internal abstract partial class OrderedEnumerable<TElement> : IPartition<TElement>
     {
-        public TElement[] ToArray()
+        public virtual TElement[] ToArray()
         {
             Buffer<TElement> buffer = new Buffer<TElement>(_source);
 
@@ -29,7 +29,7 @@ public TElement[] ToArray()
             return array;
         }
 
-        public List<TElement> ToList()
+        public virtual List<TElement> ToList()
         {
             Buffer<TElement> buffer = new Buffer<TElement>(_source);
             int count = buffer._count;
@@ -247,4 +247,21 @@ private TElement Last(Buffer<TElement> buffer)
             return value;
         }
     }
+
+    internal sealed partial class OrderedImplicitlyStableEnumerable<TElement> : OrderedEnumerable<TElement>
+    {
+        public override TElement[] ToArray()
+        {
+            TElement[] array = _source.ToArray();
+            Sort(array, _descending);
+            return array;
+        }
+
+        public override List<TElement> ToList()
+        {
+            List<TElement> list = _source.ToList();
+            Sort(CollectionsMarshal.AsSpan(list), _descending);
+            return list;
+        }
+    }
 }

src/libraries/System.Linq/src/System/Linq/OrderedEnumerable.cs

@@ -18,7 +18,7 @@ internal abstract partial class OrderedEnumerable<TElement> : IOrderedEnumerable
         private int[] SortedMap(Buffer<TElement> buffer, int minIdx, int maxIdx) =>
             GetEnumerableSorter().Sort(buffer._items, buffer._count, minIdx, maxIdx);
 
-        public IEnumerator<TElement> GetEnumerator()
+        public virtual IEnumerator<TElement> GetEnumerator()
         {
             Buffer<TElement> buffer = new Buffer<TElement>(_source);
             if (buffer._count > 0)
@@ -62,9 +62,7 @@ internal IEnumerator<TElement> GetEnumerator(int minIdx, int maxIdx)
 
         internal abstract EnumerableSorter<TElement> GetEnumerableSorter(EnumerableSorter<TElement>? next);
 
-        private CachingComparer<TElement> GetComparer() => GetComparer(null);
-
-        internal abstract CachingComparer<TElement> GetComparer(CachingComparer<TElement>? childComparer);
+        internal abstract CachingComparer<TElement> GetComparer(CachingComparer<TElement>? childComparer = null);
 
         IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
 
@@ -159,6 +157,57 @@ internal override CachingComparer<TElement> GetComparer(CachingComparer<TElement
         }
     }
 
+    /// <summary>An ordered enumerable used by Order/OrderDescending for Ts that are bitwise indistinguishable for any considered equal.</summary>
+    internal sealed partial class OrderedImplicitlyStableEnumerable<TElement> : OrderedEnumerable<TElement>
+    {
+        private readonly bool _descending;
+
+        public OrderedImplicitlyStableEnumerable(IEnumerable<TElement> source, bool descending) : base(source)
+        {
+            Debug.Assert(Enumerable.TypeIsImplicitlyStable<TElement>());
+
+            if (source is null)
+            {
+                ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
+            }
+
+            _descending = descending;
+        }
+
+        internal override CachingComparer<TElement> GetComparer(CachingComparer<TElement>? childComparer) =>
+            childComparer == null ?
+                new CachingComparer<TElement, TElement>(EnumerableSorter<TElement>.IdentityFunc, Comparer<TElement>.Default, _descending) :
+                new CachingComparerWithChild<TElement, TElement>(EnumerableSorter<TElement>.IdentityFunc, Comparer<TElement>.Default, _descending, childComparer);
+
+        internal override EnumerableSorter<TElement> GetEnumerableSorter(EnumerableSorter<TElement>? next) =>
+            new EnumerableSorter<TElement, TElement>(EnumerableSorter<TElement>.IdentityFunc, Comparer<TElement>.Default, _descending, next);
+
+        public override IEnumerator<TElement> GetEnumerator()
+        {
+            var buffer = new Buffer<TElement>(_source);
+            if (buffer._count > 0)
+            {
+                Sort(buffer._items.AsSpan(0, buffer._count), _descending);
+                for (int i = 0; i < buffer._count; i++)
+                {
+                    yield return buffer._items[i];
+                }
+            }
+        }
+
+        private static void Sort(Span<TElement> span, bool descending)
+        {
+            if (descending)
+            {
+                span.Sort(static (a, b) => Comparer<TElement>.Default.Compare(b, a));
+            }
+            else
+            {
+                span.Sort();
+            }
+        }
+    }
+
     // A comparer that chains comparisons, and pushes through the last element found to be
     // lower or higher (depending on use), so as to represent the sort of comparisons
     // done by OrderBy().ThenBy() combinations.

src/libraries/System.Linq/tests/OrderDescendingTests.cs

@@ -103,6 +103,48 @@ public void SourceReverseOfResultNullPassedAsComparer()
             Assert.Equal(expected, source.OrderDescending(null));
         }
 
+        [Fact]
+        public void OrderedDescendingToArray()
+        {
+            var source = new[]
+            {
+                5, 9, 6, 7, 8, 5, 20
+            };
+            var expected = new[]
+            {
+                20, 9, 8, 7, 6, 5, 5
+            };
+
+            Assert.Equal(expected, source.OrderDescending().ToArray());
+        }
+
+        [Fact]
+        public void EmptyOrderedDescendingToArray()
+        {
+            Assert.Empty(Enumerable.Empty<int>().OrderDescending().ToArray());
+        }
+
+        [Fact]
+        public void OrderedDescendingToList()
+        {
+            var source = new[]
+            {
+                5, 9, 6, 7, 8, 5, 20
+            };
+            var expected = new[]
+            {
+                20, 9, 8, 7, 6, 5, 5
+            };
+
+            Assert.Equal(expected, source.OrderDescending().ToList());
+        }
+
+        [Fact]
+        public void EmptyOrderedDescendingToList()
+        {
+            Assert.Empty(Enumerable.Empty<int>().OrderDescending().ToList());
+        }
+
         [Fact]
         public void SameKeysVerifySortStable()
         {

src/libraries/System.Linq/tests/OrderTests.cs

@@ -485,5 +485,13 @@ public void CultureOrderElementAt()
                 }
             }
         }
+
+        [Fact]
+        public void StableSort_CustomComparerAlwaysReturns0()
+        {
+            byte[] values = new byte[] { 0x45, 0x7D, 0x4B, 0x61, 0x27 };
+            byte[] newValues = values.Order(Comparer<byte>.Create((a, b) => 0)).ToArray();
+            AssertExtensions.SequenceEqual(values, newValues);
+        }
     }
 }