Improve writing single characters with UTF8 text writer (#1814)

src/Microsoft.AspNetCore.SignalR.Common/Internal/Protocol/Utf8BufferTextWriter.cs

@@ -5,6 +5,7 @@ using System;
 using System.Buffers;
 using System.Diagnostics;
 using System.IO;
+using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 using System.Text;
 
@@ -86,21 +87,29 @@ namespace Microsoft.AspNetCore.SignalR.Internal.Protocol
 
         public override void Write(char value)
         {
-            var destination = GetBuffer();
-
             if (value <= 127)
             {
-                destination[0] = (byte)value;
+                EnsureBuffer();
+
+                // Only need to set one byte
+                // Avoid Memory<T>.Slice overhead for perf
+                _memory.Span[_memoryUsed] = (byte)value;
                 _memoryUsed++;
             }
             else
             {
+                WriteMultiByteChar(value);
+            }
+        }
+
+        private unsafe void WriteMultiByteChar(char value)
+        {
+            var destination = GetBuffer();
+
             // Json.NET only writes ASCII characters by themselves, e.g. {}[], etc
             // this should be an exceptional case
             var bytesUsed = 0;
             var charsUsed = 0;
-                unsafe
-                {
 #if NETCOREAPP2_1
             _encoder.Convert(new Span<char>(&value, 1), destination, false, out charsUsed, out bytesUsed, out _);
 #else
@@ -109,21 +118,18 @@ namespace Microsoft.AspNetCore.SignalR.Internal.Protocol
                 _encoder.Convert(&value, 1, destinationBytes, destination.Length, false, out charsUsed, out bytesUsed, out _);
             }
 #endif
-                }
+
             Debug.Assert(charsUsed == 1);
 
-                if (bytesUsed > 0)
-                {
             _memoryUsed += bytesUsed;
         }
-            }
-        }
 
         public override void Write(string value)
         {
             WriteInternal(value.AsSpan());
         }
 
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
         private Span<byte> GetBuffer()
         {
             EnsureBuffer();