Fix to #35239 - EF9: SaveChanges() is significantly slower in .NET9 v…

…s. .NET8 when using .ToJson() Mapping vs. PostgreSQL Legacy POCO mapping

Problem was that as part of AOT refactoring we changed way that we build comparers. Specifically, comparers of collections - ListOfValueTypesComparer, ListOfNullableValueTypesComparer and ListOfReferenceTypesComparer. Before those list comparer Compare, Hashcode and Snapshot methods would take as argument element comparer, which was responsible for comparing elements. We need to be able to express these in code for AOT but we are not able to generate constant of type ValueComparer (or ValueComparer) that was needed. As a solution, each comparer now stores expression describing how it can be constructed, so we use that instead (as we are perfectly capable to expressing that in code form). Problem is that now every time compare, snapshot or hashcode method is called for array type, we construct new ValueComparer for the element type. As a result in the reported case we would generate 1000s of comparers which all have to be compiled and that causes huge overhead.

Fix is to pass relevant func from the element comparer to the outer comparer. We only passed the element comparer object to the outer Compare/Hashcode/Snapshot function to call that relevant func. This way we avoid constructing redundant comparers.

For ListOfReferenceTypesComparer it's a bit trickier - TElement of the outer comparer doesn't always match TElement of the element comparer. If we detect this case, we see if the lambdas (Equals, Hashcode, Snapshot) are compatible between what we expect (in the list comparer method) and what we get from the element comparer. If they are compatible we just pass the element lambda to the list method. If they are not compatible we rewrite the element lambda so that they match.

Fixes #35239

src/EFCore.Cosmos/ChangeTracking/Internal/StringDictionaryComparer.cs

@@ -15,12 +15,21 @@ namespace Microsoft.EntityFrameworkCore.Cosmos.ChangeTracking.Internal;
 public sealed class StringDictionaryComparer<TDictionary, TElement> : ValueComparer<object>, IInfrastructure<ValueComparer>
 {
     private static readonly MethodInfo CompareMethod = typeof(StringDictionaryComparer<TDictionary, TElement>).GetMethod(
+        nameof(Compare), BindingFlags.Static | BindingFlags.NonPublic, [typeof(object), typeof(object), typeof(Func<TElement, TElement, bool>)])!;
+
+    private static readonly MethodInfo LegacyCompareMethod = typeof(StringDictionaryComparer<TDictionary, TElement>).GetMethod(
         nameof(Compare), BindingFlags.Static | BindingFlags.NonPublic, [typeof(object), typeof(object), typeof(ValueComparer)])!;
 
     private static readonly MethodInfo GetHashCodeMethod = typeof(StringDictionaryComparer<TDictionary, TElement>).GetMethod(
+        nameof(GetHashCode), BindingFlags.Static | BindingFlags.NonPublic, [typeof(IEnumerable), typeof(Func<TElement, int>)])!;
+
+    private static readonly MethodInfo LegacyGetHashCodeMethod = typeof(StringDictionaryComparer<TDictionary, TElement>).GetMethod(
         nameof(GetHashCode), BindingFlags.Static | BindingFlags.NonPublic, [typeof(IEnumerable), typeof(ValueComparer)])!;
 
     private static readonly MethodInfo SnapshotMethod = typeof(StringDictionaryComparer<TDictionary, TElement>).GetMethod(
+        nameof(Snapshot), BindingFlags.Static | BindingFlags.NonPublic, [typeof(object), typeof(Func<TElement, TElement>)])!;
+
+    private static readonly MethodInfo LegacySnapshotMethod = typeof(StringDictionaryComparer<TDictionary, TElement>).GetMethod(
         nameof(Snapshot), BindingFlags.Static | BindingFlags.NonPublic, [typeof(object), typeof(ValueComparer)])!;
 
     /// <summary>
@@ -52,46 +61,134 @@ ValueComparer IInfrastructure<ValueComparer>.Instance
         var prm1 = Expression.Parameter(typeof(object), "a");
         var prm2 = Expression.Parameter(typeof(object), "b");
 
-        return Expression.Lambda<Func<object?, object?, bool>>(
-            Expression.Call(
-                CompareMethod,
+        if (elementComparer is ValueComparer<TElement>)
+        {
+            // (a, b) => Compare(a, b, elementComparer.Equals)
+            return Expression.Lambda<Func<object?, object?, bool>>(
+                Expression.Call(
+                    CompareMethod,
+                    prm1,
+                    prm2,
+                    elementComparer.EqualsExpression),
                 prm1,
-                prm2,
+                prm2);
+        }
+        else
+        {
+            // (a, b) => Compare(a, b, new Comparer(...))
+            return Expression.Lambda<Func<object?, object?, bool>>(
+                Expression.Call(
+                    LegacyCompareMethod,
+                    prm1,
+                    prm2,
 #pragma warning disable EF9100
-                elementComparer.ConstructorExpression),
+                    elementComparer.ConstructorExpression),
 #pragma warning restore EF9100
-            prm1,
-            prm2);
+                prm1,
+                prm2);
+        }
     }
 
     private static Expression<Func<object, int>> GetHashCodeLambda(ValueComparer elementComparer)
     {
         var prm = Expression.Parameter(typeof(object), "o");
 
-        return Expression.Lambda<Func<object, int>>(
-            Expression.Call(
-                GetHashCodeMethod,
-                Expression.Convert(
-                    prm,
-                    typeof(IEnumerable)),
+        if (elementComparer is ValueComparer<TElement>)
+        {
+            // o => GetHashCode((IEnumerable)o, elementComparer.GetHashCode)
+            return Expression.Lambda<Func<object, int>>(
+                Expression.Call(
+                    GetHashCodeMethod,
+                    Expression.Convert(
+                        prm,
+                        typeof(IEnumerable)),
+                        elementComparer.HashCodeExpression),
+                prm);
+        }
+        else
+        {
+            // o => GetHashCode((IEnumerable)o, new Comparer(...))
+            return Expression.Lambda<Func<object, int>>(
+                Expression.Call(
+                    LegacyGetHashCodeMethod,
+                    Expression.Convert(
+                        prm,
+                        typeof(IEnumerable)),
 #pragma warning disable EF9100
-                elementComparer.ConstructorExpression),
+                    elementComparer.ConstructorExpression),
 #pragma warning restore EF9100
-            prm);
+                prm);
+        }
     }
 
     private static Expression<Func<object, object>> SnapshotLambda(ValueComparer elementComparer)
     {
         var prm = Expression.Parameter(typeof(object), "source");
 
-        return Expression.Lambda<Func<object, object>>(
-            Expression.Call(
-                SnapshotMethod,
-                prm,
+        if (elementComparer is ValueComparer<TElement>)
+        {
+            // source => Snapshot(source, elementComparer.Snapshot)
+            return Expression.Lambda<Func<object, object>>(
+                Expression.Call(
+                    SnapshotMethod,
+                    prm,
+                    elementComparer.SnapshotExpression),
+                prm);
+        }
+        else
+        {
+            // source => Snapshot(source, new Comparer(..))
+            return Expression.Lambda<Func<object, object>>(
+                Expression.Call(
+                    LegacySnapshotMethod,
+                    prm,
 #pragma warning disable EF9100
-                elementComparer.ConstructorExpression),
+                    elementComparer.ConstructorExpression),
 #pragma warning restore EF9100
-            prm);
+                prm);
+        }
+    }
+
+    private static bool Compare(object? a, object? b, Func<TElement?, TElement?, bool> elementCompare)
+    {
+        if (ReferenceEquals(a, b))
+        {
+            return true;
+        }
+
+        if (a is null)
+        {
+            return b is null;
+        }
+
+        if (b is null)
+        {
+            return false;
+        }
+
+        if (a is IReadOnlyDictionary<string, TElement?> aDictionary && b is IReadOnlyDictionary<string, TElement?> bDictionary)
+        {
+            if (aDictionary.Count != bDictionary.Count)
+            {
+                return false;
+            }
+
+            foreach (var pair in aDictionary)
+            {
+                if (!bDictionary.TryGetValue(pair.Key, out var bValue)
+                    || !elementCompare(pair.Value, bValue))
+                {
+                    return false;
+                }
+            }
+
+            return true;
+        }
+
+        throw new InvalidOperationException(
+            CosmosStrings.BadDictionaryType(
+                (a is IDictionary<string, TElement?> ? b : a).GetType().ShortDisplayName(),
+                typeof(IDictionary<,>).MakeGenericType(typeof(string), typeof(TElement)).ShortDisplayName()));
     }
 
     private static bool Compare(object? a, object? b, ValueComparer elementComparer)
@@ -136,6 +233,27 @@ private static bool Compare(object? a, object? b, ValueComparer elementComparer)
                 typeof(IDictionary<,>).MakeGenericType(typeof(string), elementComparer.Type).ShortDisplayName()));
     }
 
+    private static int GetHashCode(IEnumerable source, Func<TElement?, int> elementGetHashCode)
+    {
+        if (source is not IReadOnlyDictionary<string, TElement?> sourceDictionary)
+        {
+            throw new InvalidOperationException(
+                CosmosStrings.BadDictionaryType(
+                    source.GetType().ShortDisplayName(),
+                    typeof(IList<>).MakeGenericType(typeof(TElement)).ShortDisplayName()));
+        }
+
+        var hash = new HashCode();
+
+        foreach (var pair in sourceDictionary)
+        {
+            hash.Add(pair.Key);
+            hash.Add(pair.Value == null ? 0 : elementGetHashCode(pair.Value));
+        }
+
+        return hash.ToHashCode();
+    }
+
     private static int GetHashCode(IEnumerable source, ValueComparer elementComparer)
     {
         if (source is not IReadOnlyDictionary<string, TElement?> sourceDictionary)
@@ -157,6 +275,25 @@ private static int GetHashCode(IEnumerable source, ValueComparer elementComparer
         return hash.ToHashCode();
     }
 
+    private static IReadOnlyDictionary<string, TElement?> Snapshot(object source, Func<TElement?, TElement?> elementSnapshot)
+    {
+        if (source is not IReadOnlyDictionary<string, TElement?> sourceDictionary)
+        {
+            throw new InvalidOperationException(
+                CosmosStrings.BadDictionaryType(
+                    source.GetType().ShortDisplayName(),
+                    typeof(IDictionary<,>).MakeGenericType(typeof(string), typeof(TElement)).ShortDisplayName()));
+        }
+
+        var snapshot = new Dictionary<string, TElement?>();
+        foreach (var pair in sourceDictionary)
+        {
+            snapshot[pair.Key] = pair.Value == null ? default : (TElement?)elementSnapshot(pair.Value);
+        }
+
+        return snapshot;
+    }
+
     private static IReadOnlyDictionary<string, TElement?> Snapshot(object source, ValueComparer elementComparer)
     {
         if (source is not IReadOnlyDictionary<string, TElement?> sourceDictionary)

src/EFCore/ChangeTracking/ListOfNullableValueTypesComparer.cs

@@ -33,15 +33,15 @@ public sealed class ListOfNullableValueTypesComparer<TConcreteList, TElement> :
 
     private static readonly MethodInfo CompareMethod = typeof(ListOfNullableValueTypesComparer<TConcreteList, TElement>).GetMethod(
         nameof(Compare), BindingFlags.Static | BindingFlags.NonPublic,
-        [typeof(IEnumerable<TElement?>), typeof(IEnumerable<TElement?>), typeof(ValueComparer<TElement?>)])!;
+        [typeof(IEnumerable<TElement?>), typeof(IEnumerable<TElement?>), typeof(Func<TElement?, TElement?, bool>)])!;
 
     private static readonly MethodInfo GetHashCodeMethod = typeof(ListOfNullableValueTypesComparer<TConcreteList, TElement>).GetMethod(
         nameof(GetHashCode), BindingFlags.Static | BindingFlags.NonPublic,
-        [typeof(IEnumerable<TElement?>), typeof(ValueComparer<TElement?>)])!;
+        [typeof(IEnumerable<TElement?>), typeof(Func<TElement?, int>)])!;
 
     private static readonly MethodInfo SnapshotMethod = typeof(ListOfNullableValueTypesComparer<TConcreteList, TElement>).GetMethod(
         nameof(Snapshot), BindingFlags.Static | BindingFlags.NonPublic,
-        [typeof(IEnumerable<TElement?>), typeof(ValueComparer<TElement?>)])!;
+        [typeof(IEnumerable<TElement?>), typeof(Func<TElement?, TElement?>)])!;
 
     /// <summary>
     ///     Creates a new instance of the list comparer.
@@ -67,15 +67,13 @@ ValueComparer IInfrastructure<ValueComparer>.Instance
         var prm1 = Expression.Parameter(typeof(IEnumerable<TElement?>), "a");
         var prm2 = Expression.Parameter(typeof(IEnumerable<TElement?>), "b");
 
-        //(a, b) => Compare(a, b, (ValueComparer<TElement?>)elementComparer)
+        //(a, b) => Compare(a, b, elementComparer.Equals)
         return Expression.Lambda<Func<IEnumerable<TElement?>?, IEnumerable<TElement?>?, bool>>(
             Expression.Call(
                 CompareMethod,
                 prm1,
                 prm2,
-                Expression.Convert(
-                    elementComparer.ConstructorExpression,
-                    typeof(ValueComparer<TElement?>))),
+                elementComparer.EqualsExpression),
             prm1,
             prm2);
     }
@@ -84,33 +82,29 @@ ValueComparer IInfrastructure<ValueComparer>.Instance
     {
         var prm = Expression.Parameter(typeof(IEnumerable<TElement?>), "o");
 
-        //o => GetHashCode(o, (ValueComparer<TElement?>)elementComparer)
+        //o => GetHashCode(o, elementComparer.GetHashCode)
         return Expression.Lambda<Func<IEnumerable<TElement?>, int>>(
             Expression.Call(
                 GetHashCodeMethod,
                 prm,
-                Expression.Convert(
-                    elementComparer.ConstructorExpression,
-                    typeof(ValueComparer<TElement?>))),
+                elementComparer.HashCodeExpression),
             prm);
     }
 
     private static Expression<Func<IEnumerable<TElement?>, IEnumerable<TElement?>>> SnapshotLambda(ValueComparer elementComparer)
     {
         var prm = Expression.Parameter(typeof(IEnumerable<TElement?>), "source");
 
-        //source => Snapshot(source, (ValueComparer<TElement?>)elementComparer)
+        //source => Snapshot(source, elementComparer.Snapshot)
         return Expression.Lambda<Func<IEnumerable<TElement?>, IEnumerable<TElement?>>>(
             Expression.Call(
                 SnapshotMethod,
                 prm,
-                Expression.Convert(
-                    elementComparer.ConstructorExpression,
-                    typeof(ValueComparer<TElement?>))),
+                elementComparer.SnapshotExpression),
             prm);
     }
 
-    private static bool Compare(IEnumerable<TElement?>? a, IEnumerable<TElement?>? b, ValueComparer<TElement?> elementComparer)
+    private static bool Compare(IEnumerable<TElement?>? a, IEnumerable<TElement?>? b, Func<TElement?, TElement?, bool> elementCompare)
     {
         if (ReferenceEquals(a, b))
         {
@@ -152,7 +146,7 @@ private static bool Compare(IEnumerable<TElement?>? a, IEnumerable<TElement?>? b
                     return false;
                 }
 
-                if (!elementComparer.Equals(el1, el2))
+                if (!elementCompare(el1, el2))
                 {
                     return false;
                 }
@@ -164,29 +158,29 @@ private static bool Compare(IEnumerable<TElement?>? a, IEnumerable<TElement?>? b
         throw new InvalidOperationException(
             CoreStrings.BadListType(
                 (a is IList<TElement?> ? b : a).GetType().ShortDisplayName(),
-                typeof(IList<>).MakeGenericType(elementComparer.Type.MakeNullable()).ShortDisplayName()));
+                typeof(IList<>).MakeGenericType(typeof(TElement).MakeNullable()).ShortDisplayName()));
     }
 
-    private static int GetHashCode(IEnumerable<TElement?> source, ValueComparer<TElement?> elementComparer)
+    private static int GetHashCode(IEnumerable<TElement?> source, Func<TElement?, int> elementGetHashCode)
     {
         var hash = new HashCode();
 
         foreach (var el in source)
         {
-            hash.Add(el == null ? 0 : elementComparer.GetHashCode(el));
+            hash.Add(el == null ? 0 : elementGetHashCode(el));
         }
 
         return hash.ToHashCode();
     }
 
-    private static IList<TElement?> Snapshot(IEnumerable<TElement?> source, ValueComparer<TElement?> elementComparer)
+    private static IList<TElement?> Snapshot(IEnumerable<TElement?> source, Func<TElement?, TElement?> elementSnapshot)
     {
         if (source is not IList<TElement?> sourceList)
         {
             throw new InvalidOperationException(
                 CoreStrings.BadListType(
                     source.GetType().ShortDisplayName(),
-                    typeof(IList<>).MakeGenericType(elementComparer.Type.MakeNullable()).ShortDisplayName()));
+                    typeof(IList<>).MakeGenericType(typeof(TElement).MakeNullable()).ShortDisplayName()));
         }
 
         if (IsArray)
@@ -195,7 +189,7 @@ private static int GetHashCode(IEnumerable<TElement?> source, ValueComparer<TEle
             for (var i = 0; i < sourceList.Count; i++)
             {
                 var instance = sourceList[i];
-                snapshot[i] = instance == null ? null : elementComparer.Snapshot(instance);
+                snapshot[i] = instance == null ? null : elementSnapshot(instance);
             }
 
             return snapshot;
@@ -205,7 +199,7 @@ private static int GetHashCode(IEnumerable<TElement?> source, ValueComparer<TEle
             var snapshot = IsReadOnly ? new List<TElement?>() : (IList<TElement?>)Activator.CreateInstance<TConcreteList>()!;
             foreach (var e in sourceList)
             {
-                snapshot.Add(e == null ? null : elementComparer.Snapshot(e));
+                snapshot.Add(e == null ? null : elementSnapshot(e));
             }
 
             return IsReadOnly