Reduce allocations in TransformArrayInitializers

DecodeArrayInitializer - Instead of relying on the Add method of a list to expand the underlying array when necessary, the code now allocates a big enough array to fit all the elements removing the need for the Add method to expand the array several times.

BlockFromInitializer now reuses a single instance of List<ILInstruction> instead of reallocating a new one every time and clears it when necessary. The same pre-allocation approach mentioned above has been implemented here too.

ICSharpCode.Decompiler/IL/Transforms/TransformArrayInitializers.cs

@@ -1,4 +1,4 @@
-// Copyright (c) 2015 Siegfried Pammer
+// Copyright (c) 2015 Siegfried Pammer
 // 
 // Permission is hereby granted, free of charge, to any person obtaining a copy of this
 // software and associated documentation files (the "Software"), to deal in the Software
@@ -620,16 +620,21 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			var block = new Block(BlockKind.ArrayInitializer);
 			block.Instructions.Add(new StLoc(v, new NewArr(elementType, arrayLength.Select(l => new LdcI4(l)).ToArray())));
 			int step = arrayLength.Length + 1;
+
+			var indices = new List<ILInstruction>();
 			for (int i = 0; i < values.Length / step; i++)
 			{
 				// values array is filled backwards
 				var value = values[step * i];
-				var indices = new List<ILInstruction>();
+
+				indices.EnsureCapacity(step - 1);
 				for (int j = step - 1; j >= 1; j--)
 				{
 					indices.Add(values[step * i + j]);
 				}
+
 				block.Instructions.Add(StElem(new LdLoc(v), indices.ToArray(), value, elementType));
+				indices.Clear();
 			}
 			block.FinalInstruction = new LdLoc(v);
 			return block;
@@ -774,6 +779,8 @@ namespace ICSharpCode.Decompiler.IL.Transforms
 			if (initialValue.RemainingBytes < (totalLength * elementSize))
 				return false;
 
+			output.EnsureCapacity(totalLength + totalLength * arrayLength.Length);
+
 			for (int i = 0; i < totalLength; i++)
 			{
 				output.Add(decoder(ref initialValue));

ICSharpCode.Decompiler/Util/CollectionExtensions.cs

@@ -371,6 +371,31 @@ namespace ICSharpCode.Decompiler.Util
 			return first;
 		}
 
+#if !NETCORE
+		public static int EnsureCapacity<T>(this List<T> list, int capacity)
+		{
+			if (capacity < 0)
+				throw new ArgumentOutOfRangeException(nameof(capacity));
+			if (list.Capacity < capacity)
+			{
+				const int DefaultCapacity = 4;
+				const int MaxLength = 0X7FFFFFC7;
+
+				int newcapacity = list.Capacity == 0 ? DefaultCapacity : 2 * list.Capacity;
+
+				if ((uint)newcapacity > MaxLength)
+					newcapacity = MaxLength;
+
+				if (newcapacity < capacity)
+					newcapacity = capacity;
+
+				list.Capacity = newcapacity;
+			}
+
+			return list.Capacity;
+		}
+#endif
+
 		#region Aliases/shortcuts for Enumerable extension methods
 		public static bool Any<T>(this ICollection<T> list) => list.Count > 0;
 		public static bool Any<T>(this T[] array, Predicate<T> match) => Array.Exists(array, match);