[wasm] Improvements to startup performance of mono_wasm_get_assembly_exports (dotnet#99924)

Change generated JSImport/JSExport initializer to not rely on Environment.Version, for faster startup

Author: kg

src/libraries/System.Runtime.InteropServices.JavaScript/gen/JSImportGenerator/Constants.cs

@@ -17,6 +17,7 @@ internal static class Constants
         public const string ModuleInitializerAttributeGlobal = "global::System.Runtime.CompilerServices.ModuleInitializerAttribute";
         public const string CompilerGeneratedAttributeGlobal = "global::System.Runtime.CompilerServices.CompilerGeneratedAttribute";
         public const string DynamicDependencyAttributeGlobal = "global::System.Diagnostics.CodeAnalysis.DynamicDependencyAttribute";
+        public const string DynamicallyAccessedMemberTypesGlobal = "global::System.Diagnostics.CodeAnalysis.DynamicallyAccessedMemberTypes";
         public const string ThreadStaticGlobal = "global::System.ThreadStaticAttribute";
         public const string TaskGlobal = "global::System.Threading.Tasks.Task";
         public const string SpanGlobal = "global::System.Span";

src/libraries/System.Runtime.InteropServices.JavaScript/gen/JSImportGenerator/JSExportGenerator.cs

@@ -215,7 +215,7 @@ private static NamespaceDeclarationSyntax GenerateRegSource(
             const string generatedNamespace = "System.Runtime.InteropServices.JavaScript";
             const string initializerClass = "__GeneratedInitializer";
             const string initializerName = "__Register_";
-            const string selfInitName = "__Net7SelfInit_";
+            const string trimmingPreserveName = "__TrimmingPreserve_";
 
             if (methods.IsEmpty) return NamespaceDeclaration(IdentifierName(generatedNamespace));
 
@@ -241,22 +241,42 @@ private static NamespaceDeclarationSyntax GenerateRegSource(
                             .WithModifiers(TokenList(new[] { Token(SyntaxKind.StaticKeyword) }))
                             .WithBody(Block(registerStatements));
 
-            // when we are running code generated by .NET8 on .NET7 runtime we need to auto initialize the assembly, because .NET7 doesn't call the registration from JS
-            // this also keeps the code protected from trimming
-            MemberDeclarationSyntax initializerMethod = MethodDeclaration(PredefinedType(Token(SyntaxKind.VoidKeyword)), Identifier(selfInitName))
-                            .WithAttributeLists(List(new[]{
-                                    AttributeList(SingletonSeparatedList(Attribute(IdentifierName(Constants.ModuleInitializerAttributeGlobal)))),
-                                }))
+            // HACK: protect the code from trimming with DynamicDependency attached to a ModuleInitializer
+            MemberDeclarationSyntax initializerMethod = MethodDeclaration(PredefinedType(Token(SyntaxKind.VoidKeyword)), Identifier(trimmingPreserveName))
+                            .WithAttributeLists(
+                                SingletonList<AttributeListSyntax>(
+                                    AttributeList(
+                                        SeparatedList<AttributeSyntax>(
+                                            new SyntaxNodeOrToken[]{
+                                                Attribute(
+                                                    IdentifierName(Constants.ModuleInitializerAttributeGlobal)),
+                                                Token(SyntaxKind.CommaToken),
+                                                Attribute(
+                                                    IdentifierName(Constants.DynamicDependencyAttributeGlobal))
+                                                .WithArgumentList(
+                                                    AttributeArgumentList(
+                                                        SeparatedList<AttributeArgumentSyntax>(
+                                                            new SyntaxNodeOrToken[]{
+                                                                AttributeArgument(
+                                                                    BinaryExpression(
+                                                                        SyntaxKind.BitwiseOrExpression,
+                                                                        MemberAccessExpression(
+                                                                            SyntaxKind.SimpleMemberAccessExpression,
+                                                                            IdentifierName(Constants.DynamicallyAccessedMemberTypesGlobal),
+                                                                            IdentifierName("PublicMethods")),
+                                                                        MemberAccessExpression(
+                                                                            SyntaxKind.SimpleMemberAccessExpression,
+                                                                            IdentifierName(Constants.DynamicallyAccessedMemberTypesGlobal),
+                                                                            IdentifierName("NonPublicMethods")))),
+                                                                Token(SyntaxKind.CommaToken),
+                                                                AttributeArgument(
+                                                                    TypeOfExpression(
+                                                                        IdentifierName(initializerClass)))})))}))))
                             .WithModifiers(TokenList(new[] {
                                 Token(SyntaxKind.StaticKeyword),
                                 Token(SyntaxKind.InternalKeyword)
                             }))
-                            .WithBody(Block(
-                                IfStatement(BinaryExpression(SyntaxKind.EqualsExpression,
-                                    IdentifierName("Environment.Version.Major"),
-                                    LiteralExpression(SyntaxKind.NumericLiteralExpression, Literal(7))),
-                                    Block(SingletonList<StatementSyntax>(
-                                        ExpressionStatement(InvocationExpression(IdentifierName(initializerName))))))));
+                            .WithBody(Block());
 
             var ns = NamespaceDeclaration(IdentifierName(generatedNamespace))
                         .WithMembers(