Faster List Add & Clear

src/mscorlib/src/System/Collections/Generic/List.cs

@@ -12,14 +12,16 @@
 **
 ** 
 ===========================================================*/
-namespace System.Collections.Generic {
 
+using System.Reflection;
+
+namespace System.Collections.Generic
+{
     using System;
-    using System.Runtime;
-    using System.Runtime.Versioning;
     using System.Diagnostics;
     using System.Diagnostics.Contracts;
     using System.Collections.ObjectModel;
+    using System.Runtime.CompilerServices;
 
     // Implements a variable-size List that uses an array of objects to store the
     // elements. A List has a capacity, which is the allocated length
@@ -208,9 +210,15 @@ public class List<T> : IList<T>, System.Collections.IList, IReadOnlyList<T>
         // increased by one. If required, the capacity of the list is doubled
         // before adding the new element.
         //
-        public void Add(T item) {
-            if (_size == _items.Length) EnsureCapacity(_size + 1);
-            _items[_size++] = item;
+        public void Add(T item)
+        {
+            var size = _size;
+            if (size == _items.Length)
+            {
+                EnsureCapacity(size + 1);
+            }
+            _size = size + 1;
+            _items[size] = item;
             _version++;
         }
 
@@ -291,10 +299,15 @@ public int BinarySearch(T item, IComparer<T> comparer)
 
 
         // Clears the contents of List.
-        public void Clear() {
+        public void Clear()
+        {
             if (_size > 0)
             {
-                Array.Clear(_items, 0, _size); // Don't need to doc this but we clear the elements so that the gc can reclaim the references.
+                if (JitHelpers.ContainsReferences<T>())
+                {
+                    Array.Clear(_items, 0, _size); // Clear the elements so that the gc can reclaim the references.
+                }
+
                 _size = 0;
             }
             _version++;
@@ -388,6 +401,7 @@ bool System.Collections.IList.Contains(Object item)
         // value. If the current capacity of the list is less than min, the
         // capacity is increased to twice the current capacity or to min,
         // whichever is larger.
+        [MethodImpl(MethodImplAttributes.NoInlining)]
         private void EnsureCapacity(int min) {
             if (_items.Length < min) {
                 int newCapacity = _items.Length == 0? _defaultCapacity : _items.Length * 2;