[Xamarin.Android.Build.Tasks] Avoid br.s in generated lookup tables (#9969)

Context: 8d739f4
Context: #9962
Context: https://ecma-international.org/wp-content/uploads/ECMA-335_6th_edition_june_2012.pdf

`ManagedMarshalMethodsLookupGenerator` (8d739f4) generates IL of a
switch statement [^0] which looks like this:

	.method public hidebysig static 
	    native int '<JI>GetFunctionPointer' (
	        int32 methodIndex
	    ) cil managed 
	{
	    // Method begins at RVA 0xac270
	    // Header size: 1
	    // Code size: 59 (0x3b)
	    .maxstack 8
	
	    IL_0000: ldarg methodIndex
	    IL_0004: switch (IL_001b, IL_0022, IL_0029, IL_0030)
	
	    IL_0019: br.s IL_0037
	
	    IL_001b: ldftn void Android.App.Application::n_OnCreate_mm_wrapper(native int, native int)
	    IL_0021: ret
	
	    IL_0022: ldftn void Android.App.Application::n_OnLowMemory_mm_wrapper(native int, native int)
	    IL_0028: ret
	
	    IL_0029: ldftn void Android.App.Application::n_OnTrimMemory_I_mm_wrapper(native int, native int, int32)
	    IL_002f: ret
	
	    IL_0030: ldftn void Android.App.Application::n_OnConfigurationChanged_Landroid_content_res_Configuration__mm_wrapper(native int, native int, native int)
	    IL_0036: ret
	
	    IL_0037: nop
	    IL_0038: ldc.i4.m1
	    IL_0039: conv.i
	    IL_003a: ret
	} // end of method Application::'<JI>GetFunctionPointer'

If the `switch` instruction doesn't jump to any of the target offsets,
it continues on the next instruction.  This instruction used to be a
`br.s` [^1] which would jump over all the individual cases to the
default case.  When the switch gets too large (19 cases), the `br.s`
becomes insufficient, as the offset for `br.s` is a signed 8-bit value.

The code can be simplified by removing the branching operation
completely and simply return from the default case.

The newly generated code will look like this:

	.method public hidebysig static 
	    native int '<JI>GetFunctionPointer' (
	        int32 methodIndex
	    ) cil managed 
	{
	    // Method begins at RVA 0xac1a8
	    // Header size: 1
	    // Code size: 56 (0x38)
	    .maxstack 8
	
	    IL_0000: ldarg methodIndex
	    IL_0004: switch (IL_001c, IL_0023, IL_002a, IL_0031)
	
	    IL_0019: ldc.i4.m1
	    IL_001a: conv.i
	    IL_001b: ret
	
	    IL_001c: ldftn void Android.App.Application::n_OnCreate_mm_wrapper(native int, native int)
	    IL_0022: ret
	
	    IL_0023: ldftn void Android.App.Application::n_OnLowMemory_mm_wrapper(native int, native int)
	    IL_0029: ret
	
	    IL_002a: ldftn void Android.App.Application::n_OnTrimMemory_I_mm_wrapper(native int, native int, int32)
	    IL_0030: ret
	
	    IL_0031: ldftn void Android.App.Application::n_OnConfigurationChanged_Landroid_content_res_Configuration__mm_wrapper(native int, native int, native int)
	    IL_0037: ret
	} // end of method Application::'<JI>GetFunctionPointer'

[^0]:   IL `switch` is in ECMA-335 §III.3.66, page 392 (page numbers)
        or page 418 (Go to page…)

[^1]:   IL `br.s` is in ECMA-335 §III.3.15, page 338 (page numbers)
        or page 364 (Go to page…)

Author: simonrozsival

src/Xamarin.Android.Build.Tasks/Utilities/ManagedMarshalMethodsLookupGenerator.cs

@@ -74,25 +74,20 @@ MethodDefinition GenerateGetFunctionPointer (ManagedMarshalMethodsLookupInfo.Cla
 			targets [methodLookup.Index] = Instruction.Create (OpCodes.Ldftn, methodLookup.NativeCallbackWrapper);
 		}
 
-		var invalidUcoTarget = Instruction.Create (OpCodes.Nop);
-
 		var il = getFunctionPointerMethod.Body.GetILProcessor ();
 		il.Emit (OpCodes.Ldarg, methodIndexParameter);
 		il.Emit (OpCodes.Switch, targets);
 
-		il.Emit (OpCodes.Br_S, invalidUcoTarget);
+		// The default target
+		il.Emit (OpCodes.Ldc_I4_M1);
+		il.Emit (OpCodes.Conv_I);
+		il.Emit (OpCodes.Ret);
 
 		for (var k = 0; k < targets.Length; k++) {
 			il.Append (targets [k]);
 			il.Emit (OpCodes.Ret);
 		}
 
-		// no hit? this shouldn't happen
-		il.Append (invalidUcoTarget);
-		il.Emit (OpCodes.Ldc_I4_M1);
-		il.Emit (OpCodes.Conv_I);
-		il.Emit (OpCodes.Ret);
-
 		// in the case of private/private protected/protected nested types, we need to generate proxy method(s) in the parent type(s)
 		// so that we can call the actual GetFunctionPointer method from our assembly-level GetFunctionPointer method
 		while (declaringType.IsNested && (declaringType.IsNestedPrivate || declaringType.IsNestedFamily || declaringType.IsNestedFamilyAndAssembly)) {
@@ -153,21 +148,17 @@ MethodDefinition GenerateGetFunctionPointerPerAssembly (ManagedMarshalMethodsLoo
 		il.Emit (OpCodes.Ldarg, classIndexParameter);
 		il.Emit (OpCodes.Switch, targets);
 
-		var defaultTarget = Instruction.Create (OpCodes.Nop);
-		il.Emit (OpCodes.Br, defaultTarget);
+		// "Default target"
+		il.Emit (OpCodes.Pop); // methodIndex
+		il.Emit (OpCodes.Ldc_I4_M1);
+		il.Emit (OpCodes.Conv_I);
+		il.Emit (OpCodes.Ret);
 
 		for (int i = 0; i < targets.Length; i++) {
 			il.Append (targets [i]); // call
 			il.Emit (OpCodes.Ret);
 		}
 
-		// no hit? this shouldn't happen
-		il.Append (defaultTarget);
-		il.Emit (OpCodes.Pop); // methodIndex
-		il.Emit (OpCodes.Ldc_I4_M1);
-		il.Emit (OpCodes.Conv_I);
-		il.Emit (OpCodes.Ret);
-
 		return getFunctionPointerMethod;
 	}
 
@@ -196,21 +187,17 @@ void GenerateGlobalGetFunctionPointerMethod ()
 		il.Emit (OpCodes.Ldarg_0); // assemblyIndex
 		il.Emit (OpCodes.Switch, targets);
 
-		var defaultTarget = Instruction.Create (OpCodes.Nop);
-		il.Emit (OpCodes.Br_S, defaultTarget);
-
-		for (int i = 0; i < targets.Length; i++) {
-			il.Append (targets [i]); // call
-			il.Emit (OpCodes.Ret);
-		}
-
-		// no hit? this shouldn't happen
-		il.Append (defaultTarget);
+		// The default target
 		il.Emit (OpCodes.Pop); // methodIndex
 		il.Emit (OpCodes.Pop); // classIndex
 		il.Emit (OpCodes.Ldc_I4_M1);
 		il.Emit (OpCodes.Conv_I);
 		il.Emit (OpCodes.Ret);
+
+		for (int i = 0; i < targets.Length; i++) {
+			il.Append (targets [i]); // call
+			il.Emit (OpCodes.Ret);
+		}
 	}
 
 	TypeReference ImportReference (ModuleDefinition module, Type type)