Optimize struct record serialization to not box

Author: Tarmil

src/FSharp.SystemTextJson/Record.Reflection.fs

@@ -6,7 +6,16 @@ open FSharp.Reflection
 open System.Reflection.Emit
 open System.Text.Json
 
-type internal Serializer = Action<Utf8JsonWriter, obj, JsonSerializerOptions>
+type internal RefobjFieldGetter<'Record, 'Field> = Func<'Record, 'Field>
+type internal StructFieldGetter<'Record, 'Field> = delegate of inref<'Record> -> 'Field
+
+type internal RefobjSerializer<'Record> = Action<Utf8JsonWriter, 'Record, JsonSerializerOptions>
+type internal StructSerializer<'Record> = delegate of Utf8JsonWriter * inref<'Record> * JsonSerializerOptions -> unit
+
+[<Struct>]
+type internal Serializer<'Record> =
+    | SStruct of s: StructSerializer<'Record>
+    | SRefobj of n: RefobjSerializer<'Record>
 
 type internal RefobjFieldSetter<'Record, 'Field> = Action<'Record, 'Field>
 type internal StructFieldSetter<'Record, 'Field> = delegate of byref<'Record> * 'Field -> unit
@@ -24,7 +33,7 @@ type internal RecordField<'Record> =
         Name: string
         Type: Type
         Ignore: bool
-        Serialize: Serializer
+        Serialize: Serializer<'Record>
         Deserialize: Deserializer<'Record>
     }
 
@@ -74,27 +83,39 @@ module internal RecordReflection =
                 setter.Invoke(record, value))
             |> DRefobj
 
-    let private serializer<'Field> (f: FieldInfo) =
+    let private serializer<'Record, 'Field> (f: FieldInfo) =
         let getter =
             let dynMethod =
                 new DynamicMethod(
                     f.Name,
                     f.FieldType,
-                    [| typeof<obj> |],
+                    [|
+                        (if f.DeclaringType.IsValueType
+                            then typeof<'Record>.MakeByRefType()
+                            else typeof<'Record>)
+                    |],
                     typedefof<RecordField<_>>.Module,
                     skipVisibility = true
                 )
             let gen = dynMethod.GetILGenerator()
             gen.Emit(OpCodes.Ldarg_0)
-            if f.DeclaringType.IsValueType then
-                gen.Emit(OpCodes.Unbox, f.DeclaringType)
             gen.Emit(OpCodes.Ldfld, f)
             gen.Emit(OpCodes.Ret)
-            dynMethod.CreateDelegate(typeof<Func<obj, 'Field>>) :?> Func<obj, 'Field>
-        Serializer(fun writer record options ->
-            let v = getter.Invoke(record)
-            JsonSerializer.Serialize<'Field>(writer, v, options)
-        )
+            dynMethod
+        if f.DeclaringType.IsValueType then
+            let getter = getter.CreateDelegate(typeof<StructFieldGetter<'Record, 'Field>>) :?> StructFieldGetter<'Record, 'Field>
+            StructSerializer<'Record>(fun writer record options ->
+                let v = getter.Invoke(&record)
+                JsonSerializer.Serialize<'Field>(writer, v, options)
+            )
+            |> SStruct
+        else
+            let getter = getter.CreateDelegate(typeof<RefobjFieldGetter<'Record, 'Field>>) :?> RefobjFieldGetter<'Record, 'Field>
+            RefobjSerializer<'Record>(fun writer record options ->
+                let v = getter.Invoke(record)
+                JsonSerializer.Serialize<'Field>(writer, v, options)
+            )
+            |> SRefobj
 
     let private thisModule = typedefof<RecordField<_>>.Assembly.GetType("System.Text.Json.Serialization.RecordReflection")
 
@@ -106,9 +127,9 @@ module internal RecordReflection =
         ||> Array.map2 (fun f p ->
             let serializer =
                 thisModule.GetMethod("serializer", BindingFlags.Static ||| BindingFlags.NonPublic)
-                    .MakeGenericMethod(p.PropertyType)
+                    .MakeGenericMethod(recordTy, p.PropertyType)
                     .Invoke(null, [|f|])
-                    :?> Serializer
+                    :?> Serializer<'Record>
             let deserializer =
                 thisModule.GetMethod("deserializer", BindingFlags.Static ||| BindingFlags.NonPublic)
                     .MakeGenericMethod(recordTy, p.PropertyType)

src/FSharp.SystemTextJson/Record.fs

@@ -70,7 +70,9 @@ type JsonRecordConverter<'T>() =
         for p in fields do
             if not p.Ignore then
                 writer.WritePropertyName(p.Name)
-                p.Serialize.Invoke(writer, value, options)
+                match p.Serialize with
+                | SStruct p -> p.Invoke(writer, &value, options)
+                | SRefobj p -> p.Invoke(writer, value, options)
         writer.WriteEndObject()
 
 type JsonRecordConverter() =