[Java.Interop] ReadOnlyProperty<T>?

Context: #1243
Context: #1248

Java's `final` keyword is contextual, and maps to (at least?) three
separate keywords in C#:

  * `const` on fields
  * `readonly` on fields
  * `sealed` on types and methods

When binding fields, we only support "const" `final` fields: fields
for which the value is known at compile-time.

Non-`const` fields are bound as properties, requiring a lookup for
every property access.

This can be problematic, performance-wise, as `final` fields without
a compile-time value only need to be looked up once; afterward, their
value cannot change [^1].  As such, we should consider altering our
binding of "readonly" static properties to *cache* the value.

PR #1248 implemented a "nullable"-based approach to caching the field
value.  While this approach works for reference types, it is likely
not thread safe for `int?` and other value types.

[There is a comment on #1248 to make the approach thread-safe][0],
but @jonpryor isn't entirely sure if it's correct.  The
"straightfoward" approach would be to use a C# `lock` statement,
but that requires a GC-allocated lock object, which would increase
memory use.  Furthermore, if this code is wrong, the only way to fix
it is by regenerating the bindings.

@jonpryor considered moving the thread-safety logic into a separate
type, moving it outside of the generated code.  This is implemented
as `ReadOnlyProperty<T>`, in this commit.

To help figure this out, along with the performance implications,
add a `ReadOnlyPropertyTiming` test fixture to
`Java.Interop-PerformanceTests.dll` to measure performance, and
update `JavaTiming` to have the various proposed binding ideas so
that we can determine the resulting code size.

Results are as follows:

| Approach                                              | Code Size (bytes) | Total (s) | Amortized (ticks) |
| ----------------------------------------------------- | ----------------: | --------: | ----------------: |
| No caching (current)                                  |                21 | 0.0029098 |              2909 |
| "nullable" caching (not thread-safe; #1248 approach)  |                65 | 0.0000827 |                82 |
| Inlined thread-safe caching                           |                48 | 0.0000664 |                66 |
| `ReadOnlyProperty<T>` caching                         |        19+21 = 40 | 0.0001548 |               154 |

Worst performance is to not cache anything.  At least the expected
behavior is verified.

"Nullable" caching is quite performant.  Pity it isn't thread-safe.

"Inlined thread-safe caching" is ~20% faster than "nullable" caching.

`ReadOnlyProperty<T>` caching is nearly 2x slower than "nullable".

Can `ReadOnlyProperty<T>` be made faster?

[0]: #1248 (comment)

[^1]: Not strictly true; *instance* fields can change within the
      object constructor, and *static* fields change change within
      the static constructor.  As #1248 is about static fields of
      *bound* types, there should be no way for us to observe this.
      Things become trickier with instance fields.

tests/Java.Interop-PerformanceTests/Java.Interop/JavaTiming.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Collections.Concurrent;
+using System.Threading;
 
 using Java.Interop;
 using Java.Interop.GenericMarshaler;
@@ -36,6 +37,49 @@ public unsafe JavaTiming ()
 			Construct (ref peer, JniObjectReferenceOptions.CopyAndDispose);
 		}
 
+		public static int StaticReadonlyField_NoCache {
+			get {
+				const string __id = "STATIC_READONLY_FIELD.I";
+				var __v = _members.StaticFields.GetInt32Value (__id);
+				return __v;
+			}
+		}
+
+		static int? _StaticReadonlyField_Cache;
+		public static int StaticReadonlyField_ThreadUnsafeCache {
+			get {
+				if (_StaticReadonlyField_Cache.HasValue)
+					return _StaticReadonlyField_Cache.Value;
+				const string __id = "STATIC_READONLY_FIELD.I";
+				var __v = _members.StaticFields.GetInt32Value (__id);
+				return (int) (_StaticReadonlyField_Cache = __v);
+			}
+		}
+		static int _StaticReadonlyField_haveValue;
+		static int _StaticReadonlyField_value;
+
+		public static int StaticReadonlyField_ThreadSafeCache {
+			get {
+				if (1 == Interlocked.CompareExchange (ref _StaticReadonlyField_haveValue, 1, 0))
+					return _StaticReadonlyField_value;
+				const string __id = "STATIC_READONLY_FIELD.I";
+				var __v = _members.StaticFields.GetInt32Value (__id);
+				return _StaticReadonlyField_value = __v;
+			}
+		}
+
+		static ReadOnlyProperty<int> _rop_StaticReadonlyField = new ReadOnlyProperty<int> ();
+		public static unsafe int StaticReadonlyField_Rop {
+			get {
+				static int _GetInt32Value ()
+				{
+					return _members.StaticFields.GetInt32Value ("STATIC_READONLY_FIELD.I");
+				}
+				delegate *managed <int> c = &_GetInt32Value;
+				return _rop_StaticReadonlyField.GetValue (c);
+			}
+		}
+
 		static JniMethodInfo svm;
 		public static void StaticVoidMethod ()
 		{

tests/Java.Interop-PerformanceTests/Java.Interop/ReadOnlyPropertyTiming.cs

@@ -0,0 +1,71 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Globalization;
+using System.Linq;
+using System.Reflection;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+using System.Threading;
+
+using Java.Interop;
+
+using NUnit.Framework;
+
+namespace Java.Interop.PerformanceTests {
+
+
+	[TestFixture]
+	class ReadOnlyPropertyTiming : Java.InteropTests.JavaVMFixture {
+		[Test]
+		public void StaticReadOnlyPropertyTiming ()
+		{
+			const int count = 1000;
+
+			var noCache = Stopwatch.StartNew ();
+			for (int i = 0; i < count; ++i) {
+				_ = JavaTiming.StaticReadonlyField_NoCache;
+			}
+			noCache.Stop ();
+
+			var badCache = Stopwatch.StartNew ();
+			for (int i = 0; i < count; ++i) {
+				_ = JavaTiming.StaticReadonlyField_ThreadUnsafeCache;
+			}
+			badCache.Stop ();
+
+			var goodCache = Stopwatch.StartNew ();
+			for (int i = 0; i < count; ++i) {
+				_ = JavaTiming.StaticReadonlyField_ThreadSafeCache;
+			}
+			goodCache.Stop ();
+
+			var ropCache = Stopwatch.StartNew ();
+			for (int i = 0; i < count; ++i) {
+				_ = JavaTiming.StaticReadonlyField_Rop;
+			}
+			ropCache.Stop ();
+
+			Console.WriteLine ("Static ReadOnly Property Lookup + Invoke Timing:");
+			Console.WriteLine ("\t                 No caching: {0}, {1} ticks", noCache.Elapsed,      noCache.ElapsedTicks / count);
+			Console.WriteLine ("\t        Thread Unsafe Cache: {0}, {1} ticks", badCache.Elapsed,     badCache.ElapsedTicks / count);
+			Console.WriteLine ("\t          Thread-Safe Cache: {0}, {1} ticks", goodCache.Elapsed,    goodCache.ElapsedTicks / count);
+			Console.WriteLine ("\tReadOnlyProperty<int> Cache: {0}, {1} ticks", ropCache.Elapsed,     ropCache.ElapsedTicks / count);
+		}
+	}
+
+	struct ReadOnlyProperty<T> {
+		int have_value;
+		T value;
+
+		[MethodImpl (MethodImplOptions.AggressiveInlining)]
+		public unsafe T GetValue (delegate *<T> c)
+		{
+			if (1 == Interlocked.CompareExchange (ref have_value, 1, 0))
+				return value;
+			var __v = c ();
+			value = __v;
+			return __v;
+		}
+	}
+}

tests/Java.Interop-PerformanceTests/java/com/xamarin/interop/performance/JavaTiming.java

@@ -2,6 +2,13 @@
 
 public class JavaTiming {
 
+	public static final int STATIC_READONLY_FIELD = getStaticReadonlyFieldValue ();
+
+	static int getStaticReadonlyFieldValue ()
+	{
+		return 42;
+	}
+
 	public static void StaticVoidMethod ()
 	{
 	}