[mtouch][mmp] Add control over the the linker new `BeforeFieldInit` optimization (#5820)

Add an mtouch/mmp optimization flag to control this new optimization since
3rd party code (in particular binaries) might depend on this side effect.

docs/website/optimizations.md

@@ -717,3 +717,23 @@ unless the interpreter is used.
 
 The default behavior can be overridden by passing
 `--optimize=[+|-]seal-and-devirtualize` to `mtouch`.
+
+## Static constructors for BeforeFieldInit removal
+
+This optimization requires the linker to be enabled and is applied globally
+on all code inside the application.
+
+This optimization allows the linker not to mark every `.cctor` when a type
+is preserved, e.g. whenever the class/static constructor `.cctor` is only
+used for field initialization and those fields are not marked themselves 
+then it is possible to remove the `.cctor`.
+
+This optimization is enabled, by default, on both Xamarin.iOS and 
+Xamarin.Mac. However it represent a change from older versions of the linker. 
+It is possible that some existing code depend on this side effect (i.e. 
+those `.cctor` not being removed). In such case the optimization can be 
+disabled  until the correct linker annotations (e.g. 
+`[Preserve (Conditional=true)]`) are added.
+
+The default behavior can be overridden by passing
+`--optimize=[+|-]cctor-beforefieldinit` to `mtouch` or `mmp`.

tests/mmptest/src/MMPTest.cs

@@ -646,7 +646,7 @@ namespace Xamarin.MMP.Tests
 						"<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.");
+				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.AssertErrorCount (0);
 			});
 		}

tests/mtouch/MTouch.cs

@@ -1752,7 +1752,7 @@ public class TestApp {
 				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.");
+				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.");
 			}
 		}
 
@@ -3564,7 +3564,8 @@ public partial class NotificationService : UNNotificationServiceExtension
 				mtouch.AssertWarning (2003, "Option '--optimize=static-block-to-delegate-lookup' will be ignored since the static registrar is not enabled");
 				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.AssertWarningCount (12);
+				mtouch.AssertWarning (2003, "Option '--optimize=cctor-beforefieldinit' will be ignored since linking is disabled");
+				mtouch.AssertWarningCount (13);
 			}
 
 			using (var mtouch = new MTouchTool ()) {

tools/common/Optimizations.cs

@@ -40,6 +40,7 @@ namespace Xamarin.Bundler
 #else
 			"", // dummy value to make indices match up between XM and XI
 #endif
+			"cctor-beforefieldinit",
 		};
 
 		enum Opt
@@ -58,6 +59,7 @@ namespace Xamarin.Bundler
 			RemoveUnsupportedILForBitcode,
 			InlineIsARM64CallingConvention,
 			SealAndDevirtualize,
+			StaticConstructorBeforeFieldInit,
 		}
 
 		bool? all;
@@ -131,6 +133,11 @@ namespace Xamarin.Bundler
 		}
 #endif
 
+		public bool? StaticConstructorBeforeFieldInit {
+			get { return values [(int) Opt.StaticConstructorBeforeFieldInit]; }
+			set { values [(int) Opt.StaticConstructorBeforeFieldInit] = value; }
+		}
+
 		public Optimizations ()
 		{
 			values = new bool? [opt_names.Length];
@@ -283,6 +290,10 @@ namespace Xamarin.Bundler
 			if (!InlineIsARM64CallingConvention.HasValue)
 				InlineIsARM64CallingConvention = true;
 
+			// by default we try to eliminate any .cctor we can
+			if (!StaticConstructorBeforeFieldInit.HasValue)
+				StaticConstructorBeforeFieldInit = 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/mtouch/Tuning.cs

@@ -136,6 +136,8 @@ namespace MonoTouch.Tuner {
 			context.Target = options.Target;
 			context.ExcludedFeatures = new [] { "remoting", "com", "sre" };
 			context.SymbolWriterProvider = new CustomSymbolWriterProvider ();
+			if (options.Application.Optimizations.StaticConstructorBeforeFieldInit == false)
+				context.DisabledOptimizations |= CodeOptimizations.BeforeFieldInit;
 			options.LinkContext = context;
 
 			return context;