wip

Author: MichalStrehovsky

src/libraries/System.Linq/src/System/Linq/AnyAll.cs

@@ -21,7 +21,7 @@ public static bool Any<TSource>(this IEnumerable<TSource> source)
  }
 
 #if !OPTIMIZE_FOR_SIZE
- if (source is Iterator<TSource> iterator)
+ if (!IsSizeOptimized && source is Iterator<TSource> iterator)
  {
  int count = iterator.GetCount(onlyIfCheap: true);
  if (count >= 0)

src/libraries/System.Linq/src/System/Linq/Count.cs

@@ -21,7 +21,7 @@ public static int Count<TSource>(this IEnumerable<TSource> source)
  }
 
 #if !OPTIMIZE_FOR_SIZE
- if (source is Iterator<TSource> iterator)
+ if (!IsSizeOptimized && source is Iterator<TSource> iterator)
  {
  return iterator.GetCount(onlyIfCheap: false);
  }
@@ -116,7 +116,7 @@ public static bool TryGetNonEnumeratedCount<TSource>(this IEnumerable<TSource> s
  }
 
 #if !OPTIMIZE_FOR_SIZE
- if (source is Iterator<TSource> iterator)
+ if (!IsSizeOptimized && source is Iterator<TSource> iterator)
  {
  int c = iterator.GetCount(onlyIfCheap: true);
  if (c >= 0)

src/libraries/System.Linq/src/System/Linq/ElementAt.cs

@@ -24,7 +24,7 @@ public static TSource ElementAt<TSource>(this IEnumerable<TSource> source, int i
  bool found;
  TSource? element =
 #if !OPTIMIZE_FOR_SIZE
- source is Iterator<TSource> iterator ? iterator.TryGetElementAt(index, out found) :
+ !IsSizeOptimized && source is Iterator<TSource> iterator ? iterator.TryGetElementAt(index, out found) :
 #endif
  TryGetElementAtNonIterator(source, index, out found);
 
@@ -124,7 +124,7 @@ public static TSource ElementAt<TSource>(this IEnumerable<TSource> source, Index
 
  return
 #if !OPTIMIZE_FOR_SIZE
- source is Iterator<TSource> iterator ? iterator.TryGetElementAt(index, out found) :
+ !IsSizeOptimized && source is Iterator<TSource> iterator ? iterator.TryGetElementAt(index, out found) :
 #endif
  TryGetElementAtNonIterator(source, index, out found);
  }

src/libraries/System.Linq/src/System/Linq/Enumerable.cs

@@ -2,13 +2,17 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Collections.Generic;
+using System.Diagnostics.CodeAnalysis;
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 
 namespace System.Linq
 {
  public static partial class Enumerable
  {
+ [FeatureSwitchDefinition("System.Linq.Enumerable.IsSizeOptimized")]
+ internal static bool IsSizeOptimized { get; } = AppContext.TryGetSwitch("System.Linq.Enumerable.IsSizeOptimized", out bool isEnabled) ? isEnabled : false;
+
  public static IEnumerable<TSource> AsEnumerable<TSource>(this IEnumerable<TSource> source) => source;
 
  /// <summary>Returns an empty <see cref="IEnumerable{TResult}"/>.</summary>

src/libraries/System.Linq/src/System/Linq/First.cs

@@ -71,7 +71,7 @@ public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source,
 
  return
 #if !OPTIMIZE_FOR_SIZE
- source is Iterator<TSource> iterator ? iterator.TryGetFirst(out found) :
+ !IsSizeOptimized && source is Iterator<TSource> iterator ? iterator.TryGetFirst(out found) :
 #endif
  TryGetFirstNonIterator(source, out found);
  }

src/libraries/System.Linq/src/System/Linq/Iterator.cs

@@ -91,7 +91,7 @@ public virtual IEnumerable<TResult> Select<TResult>(Func<TSource, TResult> selec
 #if OPTIMIZE_FOR_SIZE
  new IEnumerableSelectIterator<TSource, TResult>(this, selector);
 #else
- new IteratorSelectIterator<TSource, TResult>(this, selector);
+ !IsSizeOptimized ? new IteratorSelectIterator<TSource, TResult>(this, selector) : new IEnumerableSelectIterator<TSource, TResult>(this, selector);
 #endif
 
 

src/libraries/System.Linq/src/System/Linq/Last.cs

@@ -70,7 +70,7 @@ public static TSource LastOrDefault<TSource>(this IEnumerable<TSource> source, F
 
  return
 #if !OPTIMIZE_FOR_SIZE
- source is Iterator<TSource> iterator ? iterator.TryGetLast(out found) :
+ !IsSizeOptimized && source is Iterator<TSource> iterator ? iterator.TryGetLast(out found) :
 #endif
  TryGetLastNonIterator(source, out found);
  }

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

@@ -7,9 +7,28 @@ namespace System.Linq
 {
  public static partial class Enumerable
  {
- private static IEnumerable<TSource> SkipIterator<TSource>(IEnumerable<TSource> source, int count) =>
- source is IList<TSource> sourceList ?
- (IEnumerable<TSource>)new IListSkipTakeIterator<TSource>(sourceList, count, int.MaxValue) :
- new IEnumerableSkipTakeIterator<TSource>(source, count, -1);
+ private static IEnumerable<TSource> SkipIterator<TSource>(IEnumerable<TSource> source, int count)
+ {
+ if (IsSizeOptimized)
+ {
+ return SizeOptimizedSkipIterator(source, count);
+ }
+ else
+ {
+ return source is IList<TSource> sourceList ?
+ (IEnumerable<TSource>)new IListSkipTakeIterator<TSource>(sourceList, count, int.MaxValue) :
+ new IEnumerableSkipTakeIterator<TSource>(source, count, -1);
+ }
+ }
+
+ private static IEnumerable<TSource> SizeOptimizedSkipIterator<TSource>(IEnumerable<TSource> source, int count)
+ {
+ using IEnumerator<TSource> e = source.GetEnumerator();
+ while (count > 0 && e.MoveNext()) count--;
+ if (count <= 0)
+ {
+ while (e.MoveNext()) yield return e.Current;
+ }
+ }
  }
 }

src/libraries/System.Linq/src/System/Linq/Skip.cs

@@ -31,7 +31,7 @@ public static IEnumerable<TSource> Skip<TSource>(this IEnumerable<TSource> sourc
  count = 0;
  }
 #if !OPTIMIZE_FOR_SIZE
- else if (source is Iterator<TSource> iterator)
+ else if (!IsSizeOptimized && source is Iterator<TSource> iterator)
  {
  return iterator.Skip(count) ?? Empty<TSource>();
  }

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

@@ -13,6 +13,9 @@ private static IEnumerable<TSource> TakeIterator<TSource>(IEnumerable<TSource> s
  Debug.Assert(source is not null && !IsEmptyArray(source));
  Debug.Assert(count > 0);
 
+ if (IsSizeOptimized)
+ return SizeOptimizedTakeIterator(source, count);
+
  return
  source is Iterator<TSource> iterator ? (iterator.Take(count) ?? Empty<TSource>()) :
  source is IList<TSource> sourceList ? new IListSkipTakeIterator<TSource>(sourceList, 0, count - 1) :
@@ -24,6 +27,9 @@ private static IEnumerable<TSource> TakeRangeIterator<TSource>(IEnumerable<TSour
  Debug.Assert(source is not null && !IsEmptyArray(source));
  Debug.Assert(startIndex >= 0 && startIndex < endIndex);
 
+ if (IsSizeOptimized)
+ return SizeOptimizedTakeRangeIterator(source, startIndex, endIndex);
+
  return
  source is Iterator<TSource> iterator ? TakeIteratorRange(iterator, startIndex, endIndex) :
  source is IList<TSource> sourceList ? new IListSkipTakeIterator<TSource>(sourceList, startIndex, endIndex - 1) :
@@ -42,5 +48,41 @@ static IEnumerable<TSource> TakeIteratorRange(Iterator<TSource> iterator, int st
  return [];
  }
  }
+
+ private static IEnumerable<TSource> SizeOptimizedTakeIterator<TSource>(IEnumerable<TSource> source, int count)
+ {
+ Debug.Assert(count > 0);
+
+ foreach (TSource element in source)
+ {
+ yield return element;
+ if (--count == 0) break;
+ }
+ }
+
+ private static IEnumerable<TSource> SizeOptimizedTakeRangeIterator<TSource>(IEnumerable<TSource> source, int startIndex, int endIndex)
+ {
+ Debug.Assert(source is not null);
+ Debug.Assert(startIndex >= 0 && startIndex < endIndex);
+
+ using IEnumerator<TSource> e = source.GetEnumerator();
+
+ int index = 0;
+ while (index < startIndex && e.MoveNext())
+ {
+ ++index;
+ }
+
+ if (index < startIndex)
+ {
+ yield break;
+ }
+
+ while (index < endIndex && e.MoveNext())
+ {
+ yield return e.Current;
+ ++index;
+ }
+ }
  }
 }