Devirtualized default value comparison (#37674)

* Devirtualized default value comparison

* Fixed double comparison for ref types

* Fixed serialization

* Addressed issues

Author: YohDeadfall

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonConverterOfT.cs

@@ -2,7 +2,6 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-using System.Collections.Generic;
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.Runtime.CompilerServices;
@@ -76,8 +75,6 @@ internal override sealed JsonParameterInfo CreateJsonParameterInfo()
         /// </summary>
         internal bool IsInternalConverter { get; set; }
 
-        internal readonly EqualityComparer<T> _defaultComparer = EqualityComparer<T>.Default;
-
         // This non-generic API is sealed as it just forwards to the generic version.
         internal sealed override bool TryWriteAsObject(Utf8JsonWriter writer, object? value, JsonSerializerOptions options, ref WriteStack state)
         {

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/JsonPropertyInfoOfT.cs

@@ -2,9 +2,9 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.Collections.Generic;
 using System.Diagnostics;
 using System.Reflection;
-using System.Runtime.CompilerServices;
 using System.Text.Json.Serialization;
 
 namespace System.Text.Json
@@ -16,8 +16,6 @@ namespace System.Text.Json
     /// or a type's converter, if the current instance is a <see cref="JsonClassInfo.PropertyInfoForClassInfo"/>.
     internal sealed class JsonPropertyInfo<T> : JsonPropertyInfo
     {
-        private static readonly T s_defaultValue = default!;
-
         public Func<object, T>? Get { get; private set; }
         public Action<object, T>? Set { get; private set; }
 
@@ -97,44 +95,45 @@ public override JsonConverter ConverterBase
 
         public override bool GetMemberAndWriteJson(object obj, ref WriteStack state, Utf8JsonWriter writer)
         {
-            bool success;
             T value = Get!(obj);
 
+            // Since devirtualization only works in non-shared generics,
+            // the default comparer is uded only for value types for now.
+            // For reference types there is a quick check for null.
+            if (IgnoreDefaultValuesOnWrite && (
+                default(T) == null ? value == null : EqualityComparer<T>.Default.Equals(default, value)))
+            {
+                return true;
+            }
+
             if (value == null)
             {
-                Debug.Assert(s_defaultValue == null && Converter.CanBeNull);
+                Debug.Assert(Converter.CanBeNull);
 
-                success = true;
-                if (!IgnoreDefaultValuesOnWrite)
+                if (Converter.HandleNull)
                 {
-                    if (!Converter.HandleNull)
+                    // No object, collection, or re-entrancy converter handles null.
+                    Debug.Assert(Converter.ClassType == ClassType.Value);
+
+                    if (state.Current.PropertyState < StackFramePropertyState.Name)
                     {
-                        writer.WriteNullSection(EscapedNameSection);
+                        state.Current.PropertyState = StackFramePropertyState.Name;
+                        writer.WritePropertyNameSection(EscapedNameSection);
                     }
-                    else
+
+                    int originalDepth = writer.CurrentDepth;
+                    Converter.Write(writer, value, Options);
+                    if (originalDepth != writer.CurrentDepth)
                     {
-                        // No object, collection, or re-entrancy converter handles null.
-                        Debug.Assert(Converter.ClassType == ClassType.Value);
-
-                        if (state.Current.PropertyState < StackFramePropertyState.Name)
-                        {
-                            state.Current.PropertyState = StackFramePropertyState.Name;
-                            writer.WritePropertyNameSection(EscapedNameSection);
-                        }
-
-                        int originalDepth = writer.CurrentDepth;
-                        Converter.Write(writer, value, Options);
-                        if (originalDepth != writer.CurrentDepth)
-                        {
-                            ThrowHelper.ThrowJsonException_SerializationConverterWrite(Converter);
-                        }
+                        ThrowHelper.ThrowJsonException_SerializationConverterWrite(Converter);
                     }
                 }
-            }
-            else if (IgnoreDefaultValuesOnWrite && Converter._defaultComparer.Equals(s_defaultValue, value))
-            {
-                Debug.Assert(s_defaultValue != null && !Converter.CanBeNull);
-                success = true;
+                else
+                {
+                    writer.WriteNullSection(EscapedNameSection);
+                }
+
+                return true;
             }
             else
             {
@@ -144,10 +143,8 @@ public override bool GetMemberAndWriteJson(object obj, ref WriteStack state, Utf
                     writer.WritePropertyNameSection(EscapedNameSection);
                 }
 
-                success = Converter.TryWrite(writer, value, Options, ref state);
+                return Converter.TryWrite(writer, value, Options, ref state);
             }
-
-            return success;
         }
 
         public override bool GetMemberAndWriteJsonExtensionData(object obj, ref WriteStack state, Utf8JsonWriter writer)
@@ -179,7 +176,7 @@ public override bool ReadJsonAndSetMember(object obj, ref ReadStack state, ref U
                     ThrowHelper.ThrowJsonException_DeserializeUnableToConvertValue(Converter.TypeToConvert);
                 }
 
-                Debug.Assert(s_defaultValue == null);
+                Debug.Assert(default(T) == null);
 
                 if (!IgnoreDefaultValuesOnRead)
                 {