Make System.Net.Http.HPack.IntegerDecoder a struct. (#33563)

See also dotnet/runtime#54209.

Author: teo-tsirpanis

src/Servers/Kestrel/Core/src/Internal/Http3/QPack/DecoderStreamReader.cs

@@ -43,7 +43,7 @@ private enum State
         private const byte InsertCountIncrementPrefixMask = 0x3F;
         private const int InsertCountIncrementPrefix = 6;
 
-        private readonly IntegerDecoder _integerDecoder = new IntegerDecoder();
+        private IntegerDecoder _integerDecoder;
         private State _state;
 
         public DecoderStreamReader()

src/Servers/Kestrel/Core/src/Internal/Http3/QPack/EncoderStreamReader.cs

@@ -86,7 +86,7 @@ private enum State
         private int _stringIndex;
         private readonly DynamicTable _dynamicTable = new DynamicTable(1000); // TODO figure out architecture.
 
-        private readonly IntegerDecoder _integerDecoder = new IntegerDecoder();
+        private IntegerDecoder _integerDecoder;
         private State _state = State.Ready;
         private bool _huffman;
 

src/Servers/Kestrel/perf/Microbenchmarks/IntegerDecoderBenchmark.cs

@@ -13,27 +13,28 @@ public class IntegerDecoderBenchmark
         private int _prefixLength = 5; // Arbitrary prefix length
         private byte _singleByte = 0x1e; // 30
         private byte[] _multiByte = new byte[] { 0x1f, 0xe0, 0xff, 0xff, 0xff, 0x03}; // int32.MaxValue
-        private IntegerDecoder _integerDecoder = new IntegerDecoder();
 
         [Benchmark(Baseline = true, OperationsPerInvoke = Iterations)]
         public void DecodeSingleByteInteger()
         {
+            var integerDecoder = new IntegerDecoder();
             for (var i = 0; i < Iterations; i++)
             {
-                _integerDecoder.BeginTryDecode(_singleByte, _prefixLength, out _);
+                integerDecoder.BeginTryDecode(_singleByte, _prefixLength, out _);
             }
         }
 
         [Benchmark(OperationsPerInvoke = Iterations)]
         public void DecodeMultiByteInteger()
         {
+            var integerDecoder = new IntegerDecoder();
             for (var i = 0; i < Iterations; i++)
             {
-                _integerDecoder.BeginTryDecode(_multiByte[0], _prefixLength, out _);
+                integerDecoder.BeginTryDecode(_multiByte[0], _prefixLength, out _);
 
                 for (var j = 1; j < _multiByte.Length; j++)
                 {
-                    _integerDecoder.TryDecode(_multiByte[j], out _);
+                    integerDecoder.TryDecode(_multiByte[j], out _);
                 }
             }
         }

src/Shared/runtime/Http2/Hpack/HPackDecoder.cs

@@ -83,7 +83,7 @@ private enum State : byte
         private readonly int _maxDynamicTableSize;
         private readonly int _maxHeadersLength;
         private readonly DynamicTable _dynamicTable;
-        private readonly IntegerDecoder _integerDecoder = new IntegerDecoder();
+        private IntegerDecoder _integerDecoder;
         private byte[] _stringOctets;
         private byte[] _headerNameOctets;
         private byte[] _headerValueOctets;

src/Shared/runtime/Http2/Hpack/IntegerDecoder.cs

@@ -6,7 +6,7 @@
 
 namespace System.Net.Http.HPack
 {
-    internal sealed class IntegerDecoder
+    internal struct IntegerDecoder
     {
         private int _i;
         private int _m;

src/Shared/runtime/Http3/QPack/QPackDecoder.cs

@@ -123,7 +123,7 @@ private enum State
         private int _headerValueLength;
         private int _stringLength;
         private int _stringIndex;
-        private readonly IntegerDecoder _integerDecoder = new IntegerDecoder();
+        private IntegerDecoder _integerDecoder;
 
         private static ArrayPool<byte> Pool => ArrayPool<byte>.Shared;