improve InsertRange(Span<>) perf by reducing (#107683)

one array copy

similar to #90089

Co-authored-by: Stephen Toub <[email protected]>
Co-authored-by: Eirik Tsarpalis <[email protected]>

Author: 3 people

src/libraries/System.Private.CoreLib/src/System/Collections/Generic/CollectionExtensions.cs

@@ -136,14 +136,13 @@ public static void InsertRange<T>(this List<T> list, int index, params ReadOnlyS
             {
                 if (list._items.Length - list._size < source.Length)
                 {
-                    list.Grow(checked(list._size + source.Length));
+                    list.GrowForInsertion(index, source.Length);
                 }
-
-                // If the index at which to insert is less than the number of items in the list,
-                // shift all items past that location in the list down to the end, making room
-                // to copy in the new data.
-                if (index < list._size)
+                else if (index < list._size)
                 {
+                    // If the index at which to insert is less than the number of items in the list,
+                    // shift all items past that location in the list down to the end, making room
+                    // to copy in the new data.
                     Array.Copy(list._items, index, list._items, index + source.Length, list._size - index);
                 }
 

src/libraries/System.Private.CoreLib/src/System/Collections/Generic/List.cs

@@ -461,7 +461,7 @@ internal void Grow(int capacity)
         /// </summary>
         /// <param name="indexToInsert">Index of the first insertion.</param>
         /// <param name="insertionCount">How many elements will be inserted.</param>
-        private void GrowForInsertion(int indexToInsert, int insertionCount = 1)
+        internal void GrowForInsertion(int indexToInsert, int insertionCount = 1)
         {
             Debug.Assert(insertionCount > 0);