Reduce memory and CPU costs due to SegmentedList usage

src/Dependencies/Collections/SegmentedArray`1.cs

@@ -94,6 +94,39 @@ public SegmentedArray(int length)
             }
         }
 
+        public SegmentedArray<T> Resize(int newLength)
+        {
+            // For now, only allow growing resizes
+            if (newLength <= _length)
+                throw new ArgumentOutOfRangeException(nameof(newLength));
+
+            var newItems = new T[(newLength + SegmentSize - 1) >> SegmentShift][];
+            var lastPageSize = newLength - ((newItems.Length - 1) << SegmentShift);
+
+            // Copy over all old pages
+            for (var i = 0; i < _items.Length; i++)
+                newItems[i] = _items[i];
+
+            // If the previous last page is still the last page, resize it to lastPageSize.
+            // Otherwise, resize it to SegmentSize.
+            if (_items.Length > 0)
+            {
+                Array.Resize(
+                    ref newItems[_items.Length - 1],
+                    _items.Length == newItems.Length ? lastPageSize : SegmentSize);
+            }
+
+            // Create all new pages (except the last one which is done separately)
+            for (var i = _items.Length; i < newItems.Length - 1; i++)
+                newItems[i] = new T[SegmentSize];
+
+            // Create a new last page if necessary
+            if (_items.Length < newItems.Length)
+                newItems[newItems.Length - 1] = new T[lastPageSize];
+
+            return new SegmentedArray<T>(newLength, newItems);
+        }
+
         private SegmentedArray(int length, T[][] items)
         {
             _length = length;

src/Dependencies/Collections/SegmentedList`1.cs

@@ -135,12 +135,7 @@ public int Capacity
                 {
                     if (value > 0)
                     {
-                        var newItems = new SegmentedArray<T>(value);
-                        if (_size > 0)
-                        {
-                            SegmentedArray.Copy(_items, newItems, _size);
-                        }
-                        _items = newItems;
+                        _items = _items.Resize(value);
                     }
                     else
                     {
@@ -503,6 +498,14 @@ internal void Grow(int capacity)
             // Capacities exceeding Array.MaxLength will be surfaced as OutOfMemoryException by Array.Resize.
             if (newCapacity < capacity)
                 newCapacity = capacity;
+            else
+            {
+                var segmentSize = SegmentedArrayHelper.GetSegmentSize<T>();
+
+                // If caller didn't request a large capacity increase, limit the increase to a single page
+                if (newCapacity > segmentSize)
+                    newCapacity = (((capacity - 1) / segmentSize) + 1) * segmentSize;
+            }
 
             Capacity = newCapacity;
         }

src/Tools/IdeCoreBenchmarks/SegmentedListBenchmarks_InsertRange.cs

@@ -8,6 +8,7 @@
 
 namespace IdeCoreBenchmarks
 {
+    [MemoryDiagnoser]
     [DisassemblyDiagnoser]
     public class SegmentedListBenchmarks_InsertRange
     {
@@ -23,7 +24,7 @@ public class SegmentedListBenchmarks_InsertRange
         private Microsoft.CodeAnalysis.Collections.SegmentedList<object?> _segmentedValuesObject = null!;
         private SegmentedArray<object?> _segmentedInsertValuesObject;
 
-        [Params(100000)]
+        [Params(1_000, 10_000, 100_000, 1_000_000)]
         public int Count { get; set; }
 
         [GlobalSetup]
@@ -40,6 +41,17 @@ public void GlobalSetup()
             _segmentedInsertValuesObject = new SegmentedArray<object?>(100);
         }
 
+        [Benchmark(Description = "AddToSegmentedList<object>", Baseline = true)]
+        public void AddList()
+        {
+            var array = new Microsoft.CodeAnalysis.Collections.SegmentedList<object?>();
+            var iterations = Count;
+            for (var i = 0; i < iterations; i++)
+            {
+                array.Add(null);
+            }
+        }
+
         [Benchmark(Description = "List<int>", Baseline = true)]
         public void InsertRangeList()
         {