Add fast path for linq count with predicate

src/libraries/System.Linq/src/System/Linq/Aggregate.cs

@@ -19,21 +19,37 @@ public static TSource Aggregate<TSource>(this IEnumerable<TSource> source, Func<
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.func);
             }
 
-            using (IEnumerator<TSource> e = source.GetEnumerator())
+            TSource result;
+            if (source.TryGetSpan(out ReadOnlySpan<TSource> span))
             {
+                if (span.IsEmpty)
+                {
+                    ThrowHelper.ThrowNoElementsException();
+                }
+
+                result = span[0];
+                for (int i = 1; i < span.Length; i++)
+                {
+                    result = func(result, span[i]);
+                }
+            }
+            else
+            {
+                using IEnumerator<TSource> e = source.GetEnumerator();
+
                 if (!e.MoveNext())
                 {
                     ThrowHelper.ThrowNoElementsException();
                 }
 
-                TSource result = e.Current;
+                result = e.Current;
                 while (e.MoveNext())
                 {
                     result = func(result, e.Current);
                 }
-
-                return result;
             }
+
+            return result;
         }
 
         public static TAccumulate Aggregate<TSource, TAccumulate>(this IEnumerable<TSource> source, TAccumulate seed, Func<TAccumulate, TSource, TAccumulate> func)
@@ -49,9 +65,19 @@ public static TAccumulate Aggregate<TSource, TAccumulate>(this IEnumerable<TSour
             }
 
             TAccumulate result = seed;
-            foreach (TSource element in source)
+            if (source.TryGetSpan(out ReadOnlySpan<TSource> span))
             {
-                result = func(result, element);
+                foreach (TSource element in span)
+                {
+                    result = func(result, element);
+                }
+            }
+            else
+            {
+                foreach (TSource element in source)
+                {
+                    result = func(result, element);
+                }
             }
 
             return result;
@@ -75,9 +101,19 @@ public static TResult Aggregate<TSource, TAccumulate, TResult>(this IEnumerable<
             }
 
             TAccumulate result = seed;
-            foreach (TSource element in source)
+            if (source.TryGetSpan(out ReadOnlySpan<TSource> span))
             {
-                result = func(result, element);
+                foreach (TSource element in span)
+                {
+                    result = func(result, element);
+                }
+            }
+            else
+            {
+                foreach (TSource element in source)
+                {
+                    result = func(result, element);
+                }
             }
 
             return resultSelector(result);

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

@@ -55,11 +55,24 @@ public static bool Any<TSource>(this IEnumerable<TSource> source, Func<TSource,
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.predicate);
             }
 
-            foreach (TSource element in source)
+            if (source.TryGetSpan(out ReadOnlySpan<TSource> span))
             {
-                if (predicate(element))
+                foreach (TSource element in span)
                 {
-                    return true;
+                    if (predicate(element))
+                    {
+                        return true;
+                    }
+                }
+            }
+            else
+            {
+                foreach (TSource element in source)
+                {
+                    if (predicate(element))
+                    {
+                        return true;
+                    }
                 }
             }
 
@@ -78,11 +91,24 @@ public static bool All<TSource>(this IEnumerable<TSource> source, Func<TSource,
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.predicate);
             }
 
-            foreach (TSource element in source)
+            if (source.TryGetSpan(out ReadOnlySpan<TSource> span))
+            {
+                foreach (TSource element in span)
+                {
+                    if (!predicate(element))
+                    {
+                        return false;
+                    }
+                }
+            }
+            else
             {
-                if (!predicate(element))
+                foreach (TSource element in source)
                 {
-                    return false;
+                    if (!predicate(element))
+                    {
+                        return false;
+                    }
                 }
             }
 

src/libraries/System.Linq/src/System/Linq/Contains.cs

@@ -20,9 +20,28 @@ public static bool Contains<TSource>(this IEnumerable<TSource> source, TSource v
 
             if (comparer is null)
             {
+                // While it's tempting, this must not delegate to ICollection<TSource>.Contains, as the historical semantics
+                // of a null comparer with this method are to use EqualityComparer<TSource>.Default, and that might differ
+                // from the semantics encoded in ICollection<TSource>.Contains.
+
+                // We don't bother special-casing spans here as explicitly providing a null comparer with a known collection type
+                // is relatively rare. If you don't care about the comparer, you use the other overload, and while it will delegate
+                // to this overload with a null comparer, it'll only do so for collections from which we can't extract a span.
+                // And if you do care about the comparer, you're generally passing in a non-null one.
+
                 foreach (TSource element in source)
                 {
-                    if (EqualityComparer<TSource>.Default.Equals(element, value)) // benefits from devirtualization and likely inlining
+                    if (EqualityComparer<TSource>.Default.Equals(element, value))
+                    {
+                        return true;
+                    }
+                }
+            }
+            else if (source.TryGetSpan(out ReadOnlySpan<TSource> span))
+            {
+                foreach (TSource element in span)
+                {
+                    if (comparer.Equals(element, value))
                     {
                         return true;
                     }

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

@@ -60,16 +60,26 @@ public static int Count<TSource>(this IEnumerable<TSource> source, Func<TSource,
             }
 
             int count = 0;
-            foreach (TSource element in source)
+            if (source.TryGetSpan(out ReadOnlySpan<TSource> span))
             {
-                checked
+                foreach (TSource element in span)
                 {
                     if (predicate(element))
                     {
                         count++;
                     }
                 }
             }
+            else
+            {
+                foreach (TSource element in source)
+                {
+                    if (predicate(element))
+                    {
+                        checked { count++; }
+                    }
+                }
+            }
 
             return count;
         }
@@ -136,15 +146,15 @@ public static long LongCount<TSource>(this IEnumerable<TSource> source)
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.source);
             }
 
+            // TryGetSpan isn't used here because if it's expected that there are more than int.MaxValue elements,
+            // the source can't possibly be something from which we can extract a span.
+
             long count = 0;
             using (IEnumerator<TSource> e = source.GetEnumerator())
             {
-                checked
+                while (e.MoveNext())
                 {
-                    while (e.MoveNext())
-                    {
-                        count++;
-                    }
+                    checked { count++; }
                 }
             }
 
@@ -163,15 +173,15 @@ public static long LongCount<TSource>(this IEnumerable<TSource> source, Func<TSo
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.predicate);
             }
 
+            // TryGetSpan isn't used here because if it's expected that there are more than int.MaxValue elements,
+            // the source can't possibly be something from which we can extract a span.
+
             long count = 0;
             foreach (TSource element in source)
             {
-                checked
+                if (predicate(element))
                 {
-                    if (predicate(element))
-                    {
-                        count++;
-                    }
+                    checked { count++; }
                 }
             }
 

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

@@ -114,12 +114,26 @@ public static TSource FirstOrDefault<TSource>(this IEnumerable<TSource> source,
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.predicate);
             }
 
-            foreach (TSource element in source)
+            if (source.TryGetSpan(out ReadOnlySpan<TSource> span))
             {
-                if (predicate(element))
+                foreach (TSource element in span)
                 {
-                    found = true;
-                    return element;
+                    if (predicate(element))
+                    {
+                        found = true;
+                        return element;
+                    }
+                }
+            }
+            else
+            {
+                foreach (TSource element in source)
+                {
+                    if (predicate(element))
+                    {
+                        found = true;
+                        return element;
+                    }
                 }
             }
 

src/libraries/System.Linq/src/System/Linq/Single.cs

@@ -117,8 +117,30 @@ public static TSource SingleOrDefault<TSource>(this IEnumerable<TSource> source,
                 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.predicate);
             }
 
-            using (IEnumerator<TSource> e = source.GetEnumerator())
+            if (source.TryGetSpan(out ReadOnlySpan<TSource> span))
             {
+                for (int i = 0; i < span.Length; i++)
+                {
+                    TSource result = span[i];
+                    if (predicate(result))
+                    {
+                        for (i++; (uint)i < (uint)span.Length; i++)
+                        {
+                            if (predicate(span[i]))
+                            {
+                                ThrowHelper.ThrowMoreThanOneMatchException();
+                            }
+                        }
+
+                        found = true;
+                        return result;
+                    }
+                }
+            }
+            else
+            {
+                using IEnumerator<TSource> e = source.GetEnumerator();
+
                 while (e.MoveNext())
                 {
                     TSource result = e.Current;