[dotnet][linker] Enable the sealer optimization (#12009)

when (by default)
* the interpreter is not enabled (since new code might subclass or override the members analyzed at build time)
* building for release

Reverts c56b893
Fix #9573

Notes

* Even if possible (in metadata) there is no point in setting `final` on
a method if we remove `virtual`. This match ILLink version of the sealer
and makes the same test pass on both.

* Don't apply optimization on non-AOT builds, e.g. simulators, since some
features (like XML serialization) checks for
`RuntimeFeature.IsDynamicCodeSupported` and that requires some types
to be subclassed thru SRE

dotnet/targets/Xamarin.Shared.Sdk.targets

@@ -466,6 +466,9 @@
 			-->
 			<_ExtraTrimmerArgs>$(_ExtraTrimmerArgs) --disable-opt unusedtypechecks</_ExtraTrimmerArgs>
 
+			<!-- If a release build and the app is not extensible (no interpreter or JIT/code-loading for macOS) then the sealer optimization can be used -->
+			<_ExtraTrimmerArgs Condition="'$(_BundlerDebug)' != 'true' And '$(MtouchInterpreter)' == '' And '$(_RunAotCompiler)' == 'true' And '$(_PlatformName)' != 'macOS'">$(_ExtraTrimmerArgs) --enable-opt sealer</_ExtraTrimmerArgs>
+
 			<!-- We always want the linker to process debug symbols, even when building in Release mode, because the AOT compiler uses the managed debug symbols to output DWARF debugging symbols -->
 			<TrimmerRemoveSymbols Condition="'$(TrimmerRemoveSymbols)' == ''">false</TrimmerRemoveSymbols>
 

tests/linker/ios/link all/SealerTest.cs

@@ -36,9 +36,17 @@ public class Subclass : Base, Interface {
 	[Preserve (AllMembers = true)]
 	public class SealerTest {
 
-#if !DEBUG && NET
-		[Ignore ("https://github.com/xamarin/xamarin-macios/issues/9573")]
+#if NET
+		[SetUp]
+		public void SetUp ()
+		{
+			// XML serialization mechanism is controlled by RuntimeFeature.IsDynamicCodeSupported
+			// which will be true for simulator / JIT builds
+			// so the optimization is disabled unless AOT is used
+			TestRuntime.AssertDevice ();
+		}
 #endif
+
 		[Test]
 		public void Sealed ()
 		{
@@ -62,9 +70,6 @@ public void Sealed ()
 #endif
 		}
 
-#if !DEBUG && NET
-		[Ignore ("https://github.com/xamarin/xamarin-macios/issues/9573")]
-#endif
 		[Test]
 		public void Final ()
 		{
@@ -81,13 +86,10 @@ public void Final ()
 			// but it can be optimized / sealed as nothing else is (or can) overrides it
 			Assert.True (a.IsFinal, "A");
 			Assert.True (b.IsFinal, "B");
-			Assert.True (c.IsFinal, "C");
+			Assert.False (c.IsFinal, "C"); // devirtualized
 #endif
 		}
 
-#if !DEBUG && NET
-		[Ignore ("https://github.com/xamarin/xamarin-macios/issues/9573")]
-#endif
 		[Test]
 		public void Virtual ()
 		{

tools/linker/MonoTouch.Tuner/SealerSubStep.cs

@@ -102,6 +102,7 @@ protected override void Process (TypeDefinition type)
 				// look if this method is an override to existing _marked_ methods
 				if (!AreMarked (bases)) {
 					method.IsVirtual = false;
+					method.IsFinal = false; // since it's not virtual anymore
 #if DEBUG
 					Console.WriteLine ("Devirtualize {0} ({1})", method, ++devirtualize);
 #endif