Faster List Add

dotnet · Feb 22, 2017 · 828225c · 828225c
1 parent 0b4e58a
commit 828225c
Showing 1 changed file with 27 additions and 3 deletions.
diff --git a/src/mscorlib/src/System/Collections/Generic/List.cs b/src/mscorlib/src/System/Collections/Generic/List.cs
@@ -208,10 +208,34 @@ 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)
+        {
+            if (_size < _items.Length)
+            {
+                AddWithoutResize(item);
+            }
+            else
+            {
+                AddWithResize(item);
+            }
+        }
+
+        private void AddWithoutResize(T item)
+        {
+            var size = _size;
+            _version++;
+            _size = size + 1;
+            _items[size] = item;
+        }
+
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        private void AddWithResize(T item)
+        {
+            var size = _size;
             _version++;
+            _size = size + 1;
+            EnsureCapacity(size + 1);
+            _items[size] = item;
         }
 
         int System.Collections.IList.Add(Object item)