[linker] Add the custom attributes removal step as an configurable optimization. Fix #3655

docs/website/optimizations.md

@@ -737,3 +737,21 @@ disabled  until the correct linker annotations (e.g.
 
 The default behavior can be overridden by passing
 `--optimize=[+|-]cctor-beforefieldinit` to `mtouch` or `mmp`.
+
+## Custom Attributes Removal
+
+This optimization requires the linker to be enabled and is applied globally
+on all assemblies inside the application.
+
+This optimization removes a number of, rarely used, custom attributes from
+assemblies. In turn this allows the linker to later remove the associated
+code from the base class libraries (BCL). This help reduce both the
+metadata and code size for your application.
+
+This optimization is enabled, by default, on both Xamarin.iOS and
+Xamarin.Mac. If some of the removed custom attributes are required for
+your application you can disable this optimization, without totally
+disabling the managed linker.
+
+The default behavior can be overridden by passing
+`--optimize=[+|-]custom-attributes-removal` to `mtouch` or `mmp`.

tests/mmptest/src/MMPTest.cs

@@ -646,7 +646,7 @@ public void MM0132 (string opt)
 						"<LinkMode>Full</LinkMode>",
 				};
 				var rv = TI.TestUnifiedExecutable (test, shouldFail: false);
-				rv.Messages.AssertWarning (132, $"Unknown optimization: '{opt}'. Valid optimizations are: remove-uithread-checks, dead-code-elimination, inline-isdirectbinding, inline-intptr-size, blockliteral-setupblock, register-protocols, inline-dynamic-registration-supported, static-block-to-delegate-lookup, trim-architectures, inline-is-arm64-calling-convention, cctor-beforefieldinit.");
+				rv.Messages.AssertWarning (132, $"Unknown optimization: '{opt}'. Valid optimizations are: remove-uithread-checks, dead-code-elimination, inline-isdirectbinding, inline-intptr-size, blockliteral-setupblock, register-protocols, inline-dynamic-registration-supported, static-block-to-delegate-lookup, trim-architectures, inline-is-arm64-calling-convention, cctor-beforefieldinit, custom-attributes-removal.");
 				rv.Messages.AssertErrorCount (0);
 			});
 		}

tests/mtouch/MTouch.cs

@@ -1752,7 +1752,7 @@ public void MT0132 ()
 				mtouch.Linker = MTouchLinker.LinkSdk;
 				mtouch.Optimize = new string [] { "foo" };
 				mtouch.AssertExecute (MTouchAction.BuildSim, "build");
-				mtouch.AssertWarning (132, "Unknown optimization: 'foo'. Valid optimizations are: remove-uithread-checks, dead-code-elimination, inline-isdirectbinding, inline-intptr-size, inline-runtime-arch, blockliteral-setupblock, register-protocols, inline-dynamic-registration-supported, static-block-to-delegate-lookup, remove-dynamic-registrar, remove-unsupported-il-for-bitcode, inline-is-arm64-calling-convention, seal-and-devirtualize, cctor-beforefieldinit.");
+				mtouch.AssertWarning (132, "Unknown optimization: 'foo'. Valid optimizations are: remove-uithread-checks, dead-code-elimination, inline-isdirectbinding, inline-intptr-size, inline-runtime-arch, blockliteral-setupblock, register-protocols, inline-dynamic-registration-supported, static-block-to-delegate-lookup, remove-dynamic-registrar, remove-unsupported-il-for-bitcode, inline-is-arm64-calling-convention, seal-and-devirtualize, cctor-beforefieldinit, custom-attributes-removal.");
 			}
 		}
 
@@ -3575,7 +3575,8 @@ public void MT2003 ()
 				mtouch.AssertWarning (2003, "Option '--optimize=inline-is-arm64-calling-convention' will be ignored since linking is disabled");
 				mtouch.AssertWarning (2003, "Option '--optimize=seal-and-devirtualize' will be ignored since linking is disabled");
 				mtouch.AssertWarning (2003, "Option '--optimize=cctor-beforefieldinit' will be ignored since linking is disabled");
-				mtouch.AssertWarningCount (13);
+				mtouch.AssertWarning (2003, "Option '--optimize=custom-attributes-removal' will be ignored since linking is disabled");
+				mtouch.AssertWarningCount (14);
 			}
 
 			using (var mtouch = new MTouchTool ()) {

tools/common/Optimizations.cs

@@ -41,6 +41,7 @@ public class Optimizations
 			"", // dummy value to make indices match up between XM and XI
 #endif
 			"cctor-beforefieldinit",
+			"custom-attributes-removal",
 		};
 
 		enum Opt
@@ -60,6 +61,7 @@ enum Opt
 			InlineIsARM64CallingConvention,
 			SealAndDevirtualize,
 			StaticConstructorBeforeFieldInit,
+			CustomAttributesRemoval,
 		}
 
 		bool? all;
@@ -138,6 +140,11 @@ public bool? StaticConstructorBeforeFieldInit {
 			set { values [(int) Opt.StaticConstructorBeforeFieldInit] = value; }
 		}
 
+		public bool? CustomAttributesRemoval {
+			get { return values [(int) Opt.CustomAttributesRemoval]; }
+			set { values [(int) Opt.CustomAttributesRemoval] = value; }
+		}
+
 		public Optimizations ()
 		{
 			values = new bool? [opt_names.Length];
@@ -294,6 +301,10 @@ public void Initialize (Application app)
 			if (!StaticConstructorBeforeFieldInit.HasValue)
 				StaticConstructorBeforeFieldInit = true;
 
+			// by default we remove rarely used custom attributes
+			if (!CustomAttributesRemoval.HasValue)
+				CustomAttributesRemoval = true;
+
 			if (Driver.Verbosity > 3)
 				Driver.Log (4, "Enabled optimizations: {0}", string.Join (", ", values.Select ((v, idx) => v == true ? opt_names [idx] : string.Empty).Where ((v) => !string.IsNullOrEmpty (v))));
 		}

tools/mmp/Tuning.cs

@@ -160,19 +160,7 @@ static Pipeline CreatePipeline (LinkerOptions options)
 			if (options.LinkMode != LinkMode.None) {
 				pipeline.AppendStep (new CoreTypeMapStep ());
 
-				var subdispatcher = new SubStepDispatcher {
-					new ApplyPreserveAttribute (),
-					new OptimizeGeneratedCodeSubStep (options),
-					new RemoveUserResourcesSubStep (),
-					new CoreRemoveAttributes (),
-					new CoreHttpMessageHandler (options),
-					new MarkNSObjects (),
-				};
-				// CoreRemoveSecurity can modify non-linked assemblies
-				// but the conditions for this cannot happen if only the platform assembly is linked
-				if (options.LinkMode != LinkMode.Platform)
-					subdispatcher.Add (new CoreRemoveSecurity ());
-				pipeline.AppendStep (subdispatcher);
+				pipeline.AppendStep (GetSubSteps (options));
 
 				pipeline.AppendStep (new MonoMacPreserveCode (options));
 				pipeline.AppendStep (new PreserveCrypto ());
@@ -200,6 +188,27 @@ static Pipeline CreatePipeline (LinkerOptions options)
 			return pipeline;
 		}
 
+		static SubStepDispatcher GetSubSteps (LinkerOptions options)
+		{
+			SubStepDispatcher sub = new SubStepDispatcher ();
+			sub.Add (new ApplyPreserveAttribute ());
+			sub.Add (new OptimizeGeneratedCodeSubStep (options));
+			sub.Add (new RemoveUserResourcesSubStep ());
+			// OptimizeGeneratedCodeSubStep and RemoveUserResourcesSubStep needs [GeneratedCode] so it must occurs before RemoveAttributes
+			if (options.Application.Optimizations.CustomAttributesRemoval == true)
+				sub.Add (new CoreRemoveAttributes ());
+
+			sub.Add (new CoreHttpMessageHandler (options));
+			sub.Add (new MarkNSObjects ());
+
+			// CoreRemoveSecurity can modify non-linked assemblies
+			// but the conditions for this cannot happen if only the platform assembly is linked
+			if (options.LinkMode != LinkMode.Platform)
+				sub.Add (new CoreRemoveSecurity ());
+
+			return sub;
+		}
+
 		static List<string> ListAssemblies (LinkContext context)
 		{
 			var list = new List<string> ();

tools/mtouch/Tuning.cs

@@ -150,8 +150,8 @@ static SubStepDispatcher GetSubSteps (LinkerOptions options)
 			sub.Add (new CoreRemoveSecurity ());
 			sub.Add (new OptimizeGeneratedCodeSubStep (options));
 			sub.Add (new RemoveUserResourcesSubStep (options));
-			// OptimizeGeneratedCodeSubStep and RemoveUserResourcesSubStep needs [GeneratedCode] so it must occurs before RemoveAttributes
-			sub.Add (new RemoveAttributes ());
+			if (options.Application.Optimizations.CustomAttributesRemoval == true)
+				sub.Add (new RemoveAttributes ());
 			// http://bugzilla.xamarin.com/show_bug.cgi?id=1408
 			if (options.LinkAway)
 				sub.Add (new RemoveCode (options));