Add a BindingImpl attribute and use to to teach the linker look for it to search for optimizable code. (#3299)

* [ObjCRuntime] Add a BindingImplAttribute.

* [linker] Make ProviderToString an extension method on ICustomAttributeProvider to make it more discoverable.

* [generator] Use [BindingImpl] instead of [CompilerGenerated].

The entire diff is big (89MB), so it can't be gisted. However, most of it is
either removal of `using System.Runtime.CompilerServices;` or the change from
`[CompilerGenerated]` to `[BindingImpl (...)]` like this:
https://gist.github.com/rolfbjarne/8bfda3ed37b956d0342a1c1e9b079244

If I remove those parts of the diff, there's nothing significant left:
https://gist.github.com/rolfbjarne/4156164d6bdb1376366200394eb8a091

* [linker] Teach the linker about the new [BindingImpl] attribute.

In addition to the existing logic where the linker would optimize some
[CompilerGenerated] code (sometimes with additional requirements), it will now
also optimize all [BindingImpl (Optimizable)] code (without any additional
requirements).

* [tests] Add tests to make sure [BindingImpl (Optimizable)] works as expected.

* [linker] Check for [BindingImpl] before [CompilerGenerated] and stop checking for [CompilerGenerated] in XAMCORE_4_0.

Check for [BindingImpl] before checking for [CompilerGenerated], since the
former is more common.

Also stop checking for [CompilerGenerated] (at least to mean that code is
optimizable) in our next non-compatible evolutionary leap (XAMCORE_4_0):

* [introspection] Impl a better typo check.

docs/website/mtouch-errors.md

@@ -1296,6 +1296,10 @@ Mixed-mode assemblies can not be processed by the linker.
 
 See https://msdn.microsoft.com/en-us/library/x0w2664k.aspx for more information on mixed-mode assemblies.
 
+### <a name="MT2105"/>MT2105: The [BindingImpl] attribute on the member * is invalid: *
+
+The `[BindingImpl]` attribute on the mentioned member is invalid. The expected format is `[BindingImpl (BindingImplOptions.ValueA | BindingImplOptions.ValueB)]`.
+
 # MT3xxx: AOT error messages
 
 <!--

src/Makefile.generator

@@ -42,6 +42,7 @@ GENERATOR_ATTRIBUTE_SOURCES = \
 	$(TOP)/src/ObjCRuntime/AlphaAttribute.cs \
 	$(TOP)/src/ObjCRuntime/ArgumentSemantic.cs \
 	$(TOP)/src/ObjCRuntime/BindAsAttribute.cs \
+	$(TOP)/src/ObjCRuntime/BindingImplAttribute.cs \
 	$(TOP)/src/ObjCRuntime/LinkWithAttribute.cs \
 	$(TOP)/src/ObjCRuntime/NativeAttribute.cs \
 	$(TOP)/src/ObjCRuntime/PlatformAvailability2.cs \

src/ObjCRuntime/BindingImplAttribute.cs

@@ -0,0 +1,27 @@
+//
+// BindingImplAttrobute.cs: Apply this to binding methods to describe them.
+//
+
+using System;
+using System.Runtime.InteropServices;
+
+namespace XamCore.ObjCRuntime {
+
+	[AttributeUsage (AttributeTargets.Method | AttributeTargets.Property | AttributeTargets.Constructor | AttributeTargets.Field | AttributeTargets.Class, AllowMultiple = false)]
+	public class BindingImplAttribute : Attribute
+	{
+		public BindingImplAttribute (BindingImplOptions options)
+		{
+			Options = options;
+		}
+
+		public BindingImplOptions Options { get; set; }
+	}
+
+	[Flags]
+	public enum BindingImplOptions
+	{
+		GeneratedCode = 1,
+		Optimizable = 2,
+	}
+}

src/frameworks.sources

@@ -1559,6 +1559,7 @@ SHARED_SOURCES = \
 	ObjCRuntime/BackingField.cs \
 	ObjCRuntime/BaseWrapper.cs \
 	ObjCRuntime/BlockProxyAttribute.cs \
+	ObjCRuntime/BindingImplAttribute.cs \
 	ObjCRuntime/CategoryAttribute.cs \
 	ObjCRuntime/Class.mac.cs \
 	ObjCRuntime/CompileFlagsAttribute.cs \

src/generator-enums.cs

@@ -101,7 +101,7 @@ public partial class Generator {
 			print ("");
 			// the *Extensions has the same version requirement as the enum itself
 			PrintPlatformAttributes (type);
-			print ("[CompilerGenerated]");
+			print_generated_code ();
 			print ("static public partial class {0}Extensions {{", type.Name);
 			indent++;
 

src/generator-filters.cs

@@ -45,7 +45,7 @@ public partial class Generator {
 		if (!is_abstract) {
 			v = GetVisibility (filter.DefaultCtorVisibility);
 			if (v.Length > 0) {
-				print ("[CompilerGenerated]");
+				print_generated_code ();
 				print ("{0}{1} () : base (\"{2}\")", v, type.Name, native_name);
 				PrintEmptyBody ();
 			}
@@ -61,18 +61,18 @@ public partial class Generator {
 				intptrctor_visibility = MethodAttributes.Public;
 			}
 		}
-		print ("[CompilerGenerated]");
+		print_generated_code ();
 		print ("{0}{1} (IntPtr handle) : base (handle)", GetVisibility (intptrctor_visibility), type_name);
 		PrintEmptyBody ();
 
 		// NSObjectFlag constructor - always present (needed to implement NSCoder for subclasses)
-		print ("[CompilerGenerated]");
+		print_generated_code ();
 		print ("[EditorBrowsable (EditorBrowsableState.Advanced)]");
 		print ("protected {0} (NSObjectFlag t) : base (t)", type_name);
 		PrintEmptyBody ();
 
 		// NSCoder constructor - all filters conforms to NSCoding
-		print ("[CompilerGenerated]");
+		print_generated_code ();
 		print ("[EditorBrowsable (EditorBrowsableState.Advanced)]");
 		print ("[Export (\"initWithCoder:\")]");
 		print ("public {0} (NSCoder coder) : base (NSObjectFlag.Empty)", type_name);
@@ -99,7 +99,7 @@ public partial class Generator {
 		if (is_abstract && (v.Length == 0))
 			v = "protected ";
 		if (v.Length > 0) {
-			print ("[CompilerGenerated]");
+			print_generated_code ();
 			print ("{0} {1} (string name) : base (CreateFilter (name))", v, type_name);
 			PrintEmptyBody ();
 		}
@@ -110,7 +110,7 @@ public partial class Generator {
 				continue;
 			
 			print ("");
-			print ("[CompilerGenerated]");
+			print_generated_code ();
 			var ptype = p.PropertyType.Name;
 			// keep C# names as they are reserved keywords (e.g. Boolean also exists in OpenGL for Mac)
 			switch (ptype) {

src/generator.cs

@@ -671,7 +671,6 @@ public class NamespaceManager
 
 		ImplicitNamespaces = new HashSet<string> ();
 		ImplicitNamespaces.Add ("System");
-		ImplicitNamespaces.Add ("System.Runtime.CompilerServices");
 		ImplicitNamespaces.Add ("System.Runtime.InteropServices");
 		ImplicitNamespaces.Add ("System.Diagnostics");
 		ImplicitNamespaces.Add ("System.ComponentModel");
@@ -2353,6 +2352,7 @@ public partial class Generator : IMemberGatherer {
 						case "CompilerGeneratedAttribute":
 						case "ManualAttribute":
 						case "MarshalDirectiveAttribute":
+						case "BindingImplAttribute":
 							continue;
 						default:
 							throw new BindingException (1007, true, "Unknown attribute {0} on {1}.{2}", attr.GetType (), mi.DeclaringType, mi.Name);
@@ -2474,7 +2474,7 @@ public partial class Generator : IMemberGatherer {
 		Header (sw);
 		print ("namespace {0} {{", ns.CoreObjCRuntime); indent++;
 		print ("");
-		print ("[CompilerGenerated]");
+		print_generated_code ();
 		print ("static partial class Trampolines {"); indent++;
 
 		print ("");
@@ -2653,7 +2653,7 @@ public partial class Generator : IMemberGatherer {
 		
 		Header (sw);
 		print ("namespace {0} {{", ns.CoreObjCRuntime); indent++;
-		print ("[CompilerGenerated]");
+		print_generated_code ();
 		print ("static partial class Libraries {"); indent++;
 		foreach (var library_info in libraries.OrderBy (v => v.Key, StringComparer.Ordinal)) {
 			var library_name = library_info.Key;
@@ -2913,7 +2913,7 @@ public partial class Generator : IMemberGatherer {
 					print ("static IntPtr {0};", kn);
 					print ("");
 					// linker will remove the attributes (but it's useful for testing)
-					print ("[CompilerGenerated]");
+					print_generated_code ();
 					print ("{0} {1}{2} {3} {{",
 					       is_internal ? "internal" : "public",
 					       propertyType,
@@ -3039,7 +3039,7 @@ public partial class Generator : IMemberGatherer {
 	{
 		for (int i=0; i < tabs; i++)
 			sw.Write ('\t');
-		sw.WriteLine ("[CompilerGenerated]");
+		sw.WriteLine ("[BindingImpl (BindingImplOptions.GeneratedCode | BindingImplOptions.Optimizable)]");
 	}
 
 	static void WriteIsDirectBindingCondition (StreamWriter sw, ref int tabs, bool? is_direct_binding, string is_direct_binding_value, Func<string> trueCode, Func<string> falseCode)
@@ -4595,7 +4595,7 @@ public partial class Generator : IMemberGatherer {
 			if (!is_model && DoesPropertyNeedBackingField (pi) && !is_interface_impl && !minfo.is_static && !DoesPropertyNeedDirtyCheck (pi, export)) {
 				var_name = string.Format ("__mt_{0}_var{1}", pi.Name, minfo.is_static ? "_static" : "");
 
-				print ("[CompilerGenerated]");
+				print_generated_code ();
 
 				if (minfo.is_thread_static)
 					print ("[ThreadStatic]");
@@ -5961,7 +5961,7 @@ public partial class Generator : IMemberGatherer {
 					var selectorField = SelectorField (ea, true);
 					if (!InlineSelectors) {
 						selectorField = selectorField.Substring (0, selectorField.Length - 6 /* Handle */);
-						print ("[CompilerGenerated]");
+						print_generated_code ();
 						print ("const string {0} = \"{1}\";", selectorField, ea);
 						print ("static readonly IntPtr {0} = Selector.GetHandle (\"{1}\");", SelectorField (ea), ea);
 					}
@@ -5975,7 +5975,7 @@ public partial class Generator : IMemberGatherer {
 					objc_type_name = FormatCategoryClassName (bta);
 				
 				if (!is_model) {
-					print ("[CompilerGenerated]");
+					print_generated_code ();
 					print ("static readonly IntPtr class_ptr = Class.GetHandle (\"{0}\");\n", objc_type_name);
 				}
 			}
@@ -6226,7 +6226,7 @@ public partial class Generator : IMemberGatherer {
 
 					// Value types we dont cache for now, to avoid Nullable<T>
 					if (!field_pi.PropertyType.IsValueType || smartEnumTypeName != null) {
-						print ("[CompilerGenerated]");
+						print_generated_code ();
 						PrintPreserveAttribute (field_pi);
 						print ("static {0} _{1};", fieldTypeName, field_pi.Name);
 					}
@@ -6753,7 +6753,7 @@ public partial class Generator : IMemberGatherer {
 			if (!is_static_class){
 				var disposeAttr = AttributeManager.GetCustomAttributes<DisposeAttribute> (type);
 				if (disposeAttr.Length > 0 || instance_fields_to_clear_on_dispose.Count > 0){
-					print ("[CompilerGenerated]");
+					print_generated_code ();
 					print ("protected override void Dispose (bool disposing)");
 					print ("{");
 					indent++;

tests/introspection/ApiTypoTest.cs

@@ -238,6 +238,7 @@ namespace Introspection
 			"Imap",
 			"Imaps",
 			"Img",
+			"Impl", // BindingImplAttribute
 			"Indoorrun",
 			"Indoorcycle",
 			"Inklist",

tests/linker-ios/link sdk/OptimizeGeneratedCodeTest.cs

@@ -186,18 +186,30 @@ namespace Linker.Shared {
 		[Test]
 		public void IntPtrSizeTest ()
 		{
-			MethodInfo passingMethod = null;
-			MethodInfo failingMethod = null;
+			var S8methods = new MethodInfo []
+			{
+				GetType ().GetMethod (nameof (Size8Test), BindingFlags.NonPublic | BindingFlags.Instance),
+				GetType ().GetMethod (nameof (Size8Test_Optimizable), BindingFlags.NonPublic | BindingFlags.Instance)
+			};
+			var S4methods = new MethodInfo []
+			{
+				GetType ().GetMethod (nameof (Size4Test), BindingFlags.NonPublic | BindingFlags.Instance),
+				GetType ().GetMethod (nameof (Size4Test_Optimizable), BindingFlags.NonPublic | BindingFlags.Instance)
+			};
+			MethodInfo[] passingMethods = null;
+			MethodInfo[] failingMethods = null;
 			switch (IntPtr.Size) {
 			case 4:
 				Size4Test ();
-				passingMethod = GetType ().GetMethod (nameof (Size4Test), BindingFlags.NonPublic | BindingFlags.Instance);
-				failingMethod = GetType ().GetMethod (nameof (Size8Test), BindingFlags.NonPublic | BindingFlags.Instance);
+				Size4Test_Optimizable ();
+				passingMethods = S4methods;
+				failingMethods = S8methods;
 				break;
 			case 8:
 				Size8Test ();
-				passingMethod = GetType ().GetMethod (nameof (Size8Test), BindingFlags.NonPublic | BindingFlags.Instance);
-				failingMethod = GetType ().GetMethod (nameof (Size4Test), BindingFlags.NonPublic | BindingFlags.Instance);
+				Size8Test_Optimizable ();
+				passingMethods = S8methods;
+				failingMethods = S4methods;
 				break;
 			default:
 				Assert.Fail ("Invalid size: {0}", IntPtr.Size);
@@ -209,12 +221,16 @@ namespace Linker.Shared {
 			// Unfortunately in debug mode csc produces IL sequences the optimizer doesn't understand (a lot of unnecessary instructions),
 			// which means we can only check this in release mode. Also on device this will probably always pass,
 			// since we strip assemblies (and the methods will always be empty), but running the test shouldn't hurt.
-			IEnumerable<ILInstruction> passingInstructions = new ILReader (passingMethod);
-			IEnumerable<ILInstruction> failingInstructions = new ILReader (failingMethod);
-			passingInstructions = passingInstructions.Where ((v) => v.OpCode.Name != "nop");
-			failingInstructions = failingInstructions.Where ((v) => v.OpCode.Name != "nop");
-			Assert.AreEqual (1, passingInstructions.Count (), "empty body");
-			Assert.That (failingInstructions.Count (), Is.GreaterThan (1), "non-empty body");
+			foreach (var passingMethod in passingMethods) {
+				IEnumerable<ILInstruction> passingInstructions = new ILReader (passingMethod);
+				passingInstructions = passingInstructions.Where ((v) => v.OpCode.Name != "nop");
+				Assert.AreEqual (1, passingInstructions.Count (), "empty body");
+			}
+			foreach (var failingMethod in failingMethods) {
+				IEnumerable<ILInstruction> failingInstructions = new ILReader (failingMethod);
+				failingInstructions = failingInstructions.Where ((v) => v.OpCode.Name != "nop");
+				Assert.That (failingInstructions.Count (), Is.GreaterThan (1), "non-empty body");
+			}
 #endif
 		}
 
@@ -255,6 +271,42 @@ namespace Linker.Shared {
 			if (IntPtr.Size <= 3)
 				throw new NUnit.Framework.Internal.NUnitException ("6");
 		}
+
+		[BindingImplAttribute (BindingImplOptions.Optimizable)]
+		void Size8Test_Optimizable ()
+		{
+			// Everything in this method should be optimized away (when building for 64-bits)
+			if (IntPtr.Size != 8)
+				throw new NUnit.Framework.Internal.NUnitException ("1");
+			if (IntPtr.Size == 4)
+				throw new NUnit.Framework.Internal.NUnitException ("2");
+			if (IntPtr.Size > 8)
+				throw new NUnit.Framework.Internal.NUnitException ("3");
+			if (IntPtr.Size < 8)
+				throw new NUnit.Framework.Internal.NUnitException ("4");
+			if (IntPtr.Size >= 9)
+				throw new NUnit.Framework.Internal.NUnitException ("5");
+			if (IntPtr.Size <= 7)
+				throw new NUnit.Framework.Internal.NUnitException ("6");
+		}
+
+		[BindingImplAttribute (BindingImplOptions.Optimizable)]
+		void Size4Test_Optimizable ()
+		{
+			// Everything in this method should be optimized away (when building for 32-bits)
+			if (IntPtr.Size != 4)
+				throw new NUnit.Framework.Internal.NUnitException ("1");
+			if (IntPtr.Size == 8)
+				throw new NUnit.Framework.Internal.NUnitException ("2");
+			if (IntPtr.Size > 4)
+				throw new NUnit.Framework.Internal.NUnitException ("3");
+			if (IntPtr.Size < 4)
+				throw new NUnit.Framework.Internal.NUnitException ("4");
+			if (IntPtr.Size >= 5)
+				throw new NUnit.Framework.Internal.NUnitException ("5");
+			if (IntPtr.Size <= 3)
+				throw new NUnit.Framework.Internal.NUnitException ("6");
+		}
 #endif
 	}
 }

tools/common/Driver.cs

@@ -166,6 +166,8 @@ namespace Xamarin.Bundler {
 			get { return verbose; }
 		}
 
+		public const bool IsXAMCORE_4_0 = false;
+
 #if MONOMAC
 #pragma warning disable 0414
 		static string userTargetFramework = TargetFramework.Default.ToString ();

tools/linker/CoreMarkStep.cs

@@ -66,7 +66,7 @@ namespace Xamarin.Linker.Steps {
 				if (!Annotations.IsMarked (od))
 					continue;
 				// we do NOT process non-generated code - we could break user code
-				if (!od.IsGeneratedCode (LinkContext))
+				if (!od.IsOptimizableCode (LinkContext))
 					continue;
 
 				ProcessDispose (skip ? bd : cd, od);
@@ -142,7 +142,7 @@ namespace Xamarin.Linker.Steps {
 		{
 			if (!method.IsFamily || !method.IsVirtual || method.IsNewSlot || !method.HasParameters || !method.HasBody)
 				return false;
-			return ((method.Name == "Dispose") && method.IsGeneratedCode (LinkContext));
+			return ((method.Name == "Dispose") && method.IsOptimizableCode (LinkContext));
 		}
 
 		protected override TypeDefinition MarkType (TypeReference reference)
@@ -254,7 +254,7 @@ namespace Xamarin.Linker.Steps {
 					var td = i.Module.GetType (i.Namespace, i.Name.Substring (1) + "_Extensions");
 					if (td != null && td.HasMethods) {
 						foreach (var m in td.Methods) {
-							if (!m.HasParameters || (m.Name != name) || !m.IsGeneratedCode (LinkContext))
+							if (!m.HasParameters || (m.Name != name) || !m.IsOptimizableCode (LinkContext))
 							    continue;
 							bool proxy = false;
 							match = method.Parameters.Count == m.Parameters.Count - 1;

tools/linker/CoreOptimizeGeneratedCode.cs

@@ -22,7 +22,7 @@ namespace Xamarin.Linker {
 		protected override string Name { get; } = "Binding Optimizer";
 		protected override int ErrorCode { get; } = 2020;
 
-		protected bool HasGeneratedCode { get; private set; }
+		protected bool HasOptimizableCode { get; private set; }
 		protected bool IsExtensionType { get; private set; }
 
 		protected LinkerOptions Options { get; set; }
@@ -73,19 +73,24 @@ namespace Xamarin.Linker {
 			}
 			
 			// if the assembly does not refer to [CompilerGeneratedAttribute] then there's not much we can do
-			HasGeneratedCode = false;
+			HasOptimizableCode = false;
 			foreach (TypeReference tr in assembly.MainModule.GetTypeReferences ()) {
-				if (tr.Is ("System.Runtime.CompilerServices", "CompilerGeneratedAttribute")) {
+				if (tr.Is (Namespaces.ObjCRuntime, "BindingImplAttribute")) {
+					HasOptimizableCode = true;
+					break;
+				}
+
+				if (!Driver.IsXAMCORE_4_0 && tr.Is ("System.Runtime.CompilerServices", "CompilerGeneratedAttribute")) {
 #if DEBUG
 					Console.WriteLine ("Assembly {0} : processing", assembly);
 #endif
-					HasGeneratedCode = true;
+					HasOptimizableCode = true;
 					break;
 				}
 			}
 #if DEBUG
-			if (!HasGeneratedCode)
-				Console.WriteLine ("Assembly {0} : no [CompilerGeneratedAttribute] present (applying basic optimizations)", assembly);
+			if (!HasOptimizableCode)
+				Console.WriteLine ("Assembly {0} : no [CompilerGeneratedAttribute] nor [BindingImplAttribute] present (applying basic optimizations)", assembly);
 #endif
 			// we always apply the step
 			return true;
@@ -93,7 +98,7 @@ namespace Xamarin.Linker {
 
 		protected override void Process (TypeDefinition type)
 		{
-			if (!HasGeneratedCode)
+			if (!HasOptimizableCode)
 				return;
 
 			isdirectbinding_constant = type.IsNSObject (LinkContext) ? type.GetIsDirectBindingConstant (LinkContext) : null;
@@ -537,7 +542,9 @@ namespace Xamarin.Linker {
 			if (!method.HasBody)
 				return;
 
-			if (method.IsGeneratedCode (LinkContext) && (IsExtensionType || IsExport (method))) {
+			if (method.IsBindingImplOptimizableCode (LinkContext)) {
+				// We optimize all methods that have the [BindingImpl (BindingImplAttributes.Optimizable)] attribute.
+			} else if (!Driver.IsXAMCORE_4_0 && (method.IsGeneratedCode (LinkContext) && (IsExtensionType || IsExport (method)))) {
 				// We optimize methods that have the [GeneratedCodeAttribute] and is either an extension type or an exported method
 			} else {
 				// but it would be too risky to apply on user-generated code

tools/linker/CoreRemoveAttributes.cs

@@ -43,6 +43,7 @@ namespace Xamarin.Linker {
 			case "LinkWithAttribute":
 			case "DesignatedInitializerAttribute":
 			case "RequiresSuperAttribute":
+			case "BindingImplAttribute":
 				return attr_type.Namespace == Namespaces.ObjCRuntime;
 			default:
 				return base.IsRemovedAttribute (attribute);
@@ -60,6 +61,9 @@ namespace Xamarin.Linker {
 				case "IntroducedAttribute":
 					LinkContext.StoreCustomAttribute (provider, attribute, "Availability");
 					break;
+				case "BindingImplAttribute":
+					LinkContext.StoreCustomAttribute (provider, attribute);
+					break;
 				}
 			}
 

tools/linker/MobileExtensions.cs

@@ -1,15 +1,28 @@
 using System;
 using System.Collections.Generic;
+using System.Linq;
 
 using Mono.Cecil;
 using Mono.Tuner;
 
+using XamCore.ObjCRuntime;
+using Xamarin.Bundler;
 using Xamarin.Tuner;
 
 namespace Xamarin.Linker {
 
 	public static class MobileExtensions {
 
+		// Returns a string representation of the specified provider that is suitable for user-visible error/warning messages.
+		public static string AsString (this ICustomAttributeProvider provider)
+		{
+			if (provider is MemberReference member)
+				return member.DeclaringType.FullName + "." + member.Name;
+			if (provider is MethodReturnType returnType)
+				return AsString ((ICustomAttributeProvider) returnType.Method);
+			return provider.ToString ();
+		}
+
 		// This method will look in any stored attributes in the link context as well as the provider itself.
 		public static bool HasCustomAttribute (this ICustomAttributeProvider provider, DerivedLinkContext context, string @namespace, string name)
 		{
@@ -37,6 +50,48 @@ namespace Xamarin.Linker {
 			return provider.HasCustomAttribute (context, "System.Runtime.CompilerServices", "CompilerGeneratedAttribute");
 		}
 
+		// The 'provider' parameter is only used in error messages to explain where the broken attribute comes from
+		// (in particular it's not used to get the custom attributes themselves, since those may not come from this provider instance)
+		static BindingImplOptions? GetBindingImplAttribute (ICustomAttributeProvider provider, IEnumerable<ICustomAttribute> attributes)
+		{
+			if (attributes == null)
+				return null;
+			
+			foreach (var ca in attributes) {
+				TypeReference tr = ca.AttributeType;
+				if (!tr.Is (Namespaces.ObjCRuntime, "BindingImplAttribute"))
+					continue;
+
+				if (ca.HasFields)
+					throw ErrorHelper.CreateError (2105, "The [BindingImpl] attribute on the member '{0}' is invalid: did not expect fields.", provider.AsString ());
+				if (ca.HasProperties)
+					throw ErrorHelper.CreateError (2105, "The [BindingImpl] attribute on the member '{0}' is invalid: did not expect properties.", provider.AsString ());
+
+				switch (ca.ConstructorArguments.Count) {
+				case 1:
+					var arg = ca.ConstructorArguments [0];
+					if (!arg.Type.Is (Namespaces.ObjCRuntime, "BindingImplOptions"))
+						throw ErrorHelper.CreateError (2105, "The [BindingImpl] attribute on the member '{0}' is invalid: did not expect a constructor with a '{1}' parameter type (expected 'ObjCRuntime.BindingImplOptions).", provider.AsString (), arg.Type.FullName);
+					return (BindingImplOptions) (int) arg.Value;
+				default:
+					throw ErrorHelper.CreateError (2105, "The [BindingImpl] attribute on the member '{0}' is invalid: did not expect a constructor with a {1} parameters (expected 1 parameters).", provider.AsString (), ca.ConstructorArguments.Count);
+				}
+			}
+
+			return null;
+		}
+
+		static BindingImplOptions? GetBindingImplAttribute (ICustomAttributeProvider provider, DerivedLinkContext context)
+		{
+			if (provider != null && provider.HasCustomAttributes) {
+				var rv = GetBindingImplAttribute (provider, provider.CustomAttributes);
+				if (rv != null)
+					return rv;
+			}
+
+			return GetBindingImplAttribute (provider, context?.GetCustomAttributes (provider, Namespaces.ObjCRuntime, "BindingImplOptions"));
+		}
+
 		static PropertyDefinition GetPropertyByAccessor (MethodDefinition method)
 		{
 			foreach (PropertyDefinition property in method.DeclaringType.Properties) {
@@ -55,5 +110,32 @@ namespace Xamarin.Linker {
 			}
 			return HasGeneratedCodeAttribute (self, link_context);
 		}
+
+		public static bool IsBindingImplOptimizableCode (this MethodDefinition self, DerivedLinkContext link_context)
+		{
+			var attrib = GetBindingImplAttribute (self, link_context);
+			if ((attrib & BindingImplOptions.Optimizable) == BindingImplOptions.Optimizable)
+				return true;
+
+			// Check the property too
+			if (self.IsGetter || self.IsSetter) {
+				attrib = GetBindingImplAttribute (GetPropertyByAccessor (self), link_context);
+				if ((attrib & BindingImplOptions.Optimizable) == BindingImplOptions.Optimizable)
+					return true;
+			}
+
+			return false;
+		}
+
+		public static bool IsOptimizableCode (this MethodDefinition self, DerivedLinkContext link_context)
+		{
+			if (IsBindingImplOptimizableCode (self, link_context))
+				return true;
+
+			if (!Driver.IsXAMCORE_4_0 && IsGeneratedCode (self, link_context))
+				return true;
+
+			return false;
+		}
 	}
 }

tools/linker/MonoTouch.Tuner/PreserveSmartEnumConversionsSubStep.cs

@@ -51,15 +51,6 @@ namespace Xamarin.Linker.Steps
 			}
 		}
 
-		static string ProviderToString (ICustomAttributeProvider provider)
-		{
-			if (provider is MemberReference member)
-				return member.DeclaringType.FullName + "." + member.Name;
-			if (provider is MethodReturnType returnType)
-				return ProviderToString ((ICustomAttributeProvider) returnType.Method);
-			return provider.ToString ();
-		}
-
 		void ProcessAttributeProvider (ICustomAttributeProvider provider, MethodDefinition conditionA, MethodDefinition conditionB = null)
 		{
 			if (provider?.HasCustomAttributes != true)
@@ -72,14 +63,14 @@ namespace Xamarin.Linker.Steps
 					continue;
 
 				if (ca.ConstructorArguments.Count != 1) {
-					ErrorHelper.Show (ErrorHelper.CreateWarning (LinkContext.Target.App, 4124, provider, "Invalid BindAsAttribute found on '{0}': should have 1 constructor arguments, found {1}. Please file a bug report at https://bugzilla.xamarin.com", ProviderToString (provider), ca.ConstructorArguments.Count));
+					ErrorHelper.Show (ErrorHelper.CreateWarning (LinkContext.Target.App, 4124, provider, "Invalid BindAsAttribute found on '{0}': should have 1 constructor arguments, found {1}. Please file a bug report at https://bugzilla.xamarin.com", provider.AsString (), ca.ConstructorArguments.Count));
 					continue;
 				}
 
 				var managedType = ca.ConstructorArguments [0].Value as TypeReference;
 				var managedEnumType = managedType?.GetElementType ().Resolve ();
 				if (managedEnumType == null) {
-					ErrorHelper.Show (ErrorHelper.CreateWarning (LinkContext.Target.App, 4124, provider, "Invalid BindAsAttribute found on '{0}': could not find the underlying enum type of {1}. Please file a bug report at https://bugzilla.xamarin.com", ProviderToString (provider), managedType?.FullName));
+					ErrorHelper.Show (ErrorHelper.CreateWarning (LinkContext.Target.App, 4124, provider, "Invalid BindAsAttribute found on '{0}': could not find the underlying enum type of {1}. Please file a bug report at https://bugzilla.xamarin.com", provider.AsString (), managedType?.FullName));
 					continue;
 				}
 
@@ -105,7 +96,7 @@ namespace Xamarin.Linker.Steps
 					break;
 				}
 				if (extensionType == null) {
-					Driver.Log (1, $"Could not find a smart extension type for the enum {managedEnumType.FullName} (due to BindAs attribute on {ProviderToString (provider)}): most likely this is because the enum isn't a smart enum.");
+					Driver.Log (1, $"Could not find a smart extension type for the enum {managedEnumType.FullName} (due to BindAs attribute on {provider.AsString ()}): most likely this is because the enum isn't a smart enum.");
 					continue;
 				}
 
@@ -134,12 +125,12 @@ namespace Xamarin.Linker.Steps
 				}
 
 				if (getConstant == null) {
-					Driver.Log (1, $"Could not find the GetConstant method on the supposedly smart extension type {extensionType.FullName} for the enum {managedEnumType.FullName} (due to BindAs attribute on {ProviderToString (provider)}): most likely this is because the enum isn't a smart enum.");
+					Driver.Log (1, $"Could not find the GetConstant method on the supposedly smart extension type {extensionType.FullName} for the enum {managedEnumType.FullName} (due to BindAs attribute on {provider.AsString ()}): most likely this is because the enum isn't a smart enum.");
 					continue;
 				}
 
 				if (getValue == null) {
-					Driver.Log (1, $"Could not find the GetValue method on the supposedly smart extension type {extensionType.FullName} for the enum {managedEnumType.FullName} (due to BindAs attribute on {ProviderToString (provider)}): most likely this is because the enum isn't a smart enum.");
+					Driver.Log (1, $"Could not find the GetValue method on the supposedly smart extension type {extensionType.FullName} for the enum {managedEnumType.FullName} (due to BindAs attribute on {provider.AsString ()}): most likely this is because the enum isn't a smart enum.");
 					continue;
 				}
 

tools/mmp/Makefile

@@ -136,6 +136,7 @@ mmp_sources = \
 	$(TOP)/src/Foundation/ConnectAttribute.cs \
 	$(TOP)/src/Foundation/ExportAttribute.cs \
 	$(TOP)/src/ObjCRuntime/ArgumentSemantic.cs \
+	$(TOP)/src/ObjCRuntime/BindingImplAttribute.cs	\
 	$(TOP)/src/ObjCRuntime/LinkWithAttribute.cs \
 	$(TOP)/src/ObjCRuntime/PlatformAvailability2.cs \
 

tools/mmp/mmp.csproj

@@ -341,6 +341,9 @@
     <Compile Include="..\..\src\ObjCRuntime\PlatformAvailability2.cs">
       <Link>external\PlatformAvailability2.cs</Link>
     </Compile>
+    <Compile Include="..\..\src\ObjCRuntime\BindingImplAttribute.cs">
+      <Link>external\BindingImplAttribute.cs</Link>
+    </Compile>
     <Compile Include="..\common\PInvokeWrapperGenerator.cs">
       <Link>external\PInvokeWrapperGenerator.cs</Link>
     </Compile>

tools/mtouch/Makefile

@@ -105,6 +105,7 @@ MTOUCH_SOURCES = \
 	$(COMMON_SOURCES)	\
 	$(LINKER_SOURCES)	\
 	$(TOP)/src/ObjCRuntime/ArgumentSemantic.cs	\
+	$(TOP)/src/ObjCRuntime/BindingImplAttribute.cs	\
 	$(TOP)/src/ObjCRuntime/LinkWithAttribute.cs	\
 	$(TOP)/src/ObjCRuntime/NativeAttribute.cs	\
 	$(TOP)/src/ObjCRuntime/PlatformAvailability2.cs	\

tools/mtouch/mtouch.csproj

@@ -360,6 +360,9 @@
     <Compile Include="..\common\Optimizations.cs">
       <Link>external\Optimizations.cs</Link>
     </Compile>
+    <Compile Include="..\..\src\ObjCRuntime\BindingImplAttribute.cs">
+      <Link>external\maccore\BindingImplAttribute.cs</Link>
+    </Compile>
   </ItemGroup>
   <ItemGroup>
     <Reference Include="System.Core" />