From 8b5fc655e90dfa581a8d1ab30dea95e34084bd5b Mon Sep 17 00:00:00 2001
From: Simon Rozsival <simon@rozsival.com>
Date: Thu, 12 Mar 2026 09:19:12 +0100
Subject: [PATCH] [TrimmableTypeMap] Add GetFunctionPointer(int) ordinal
 dispatch

Emit GetFunctionPointer(int methodIndex) on ACW proxy types that maps
ordinal indices to UnmanagedCallersOnly wrapper function pointers.

The method emits a chain of comparisons:
  if (methodIndex == 0) return (IntPtr)&wrapper_0;
  if (methodIndex == 1) return (IntPtr)&wrapper_1;
  ...
  return IntPtr.Zero;

This provides an alternative dispatch mechanism to RegisterNatives,
allowing the runtime to look up function pointers by ordinal index.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 .../Generator/TypeMapAssemblyEmitter.cs       | 49 ++++++++++++++++++-
 .../TypeMapAssemblyGeneratorTests.cs          | 18 +++++++
 2 files changed, 66 insertions(+), 1 deletion(-)

diff --git a/src/Microsoft.Android.Sdk.TrimmableTypeMap/Generator/TypeMapAssemblyEmitter.cs b/src/Microsoft.Android.Sdk.TrimmableTypeMap/Generator/TypeMapAssemblyEmitter.cs
index 775f48af607..ebf93916d4b 100644
--- a/src/Microsoft.Android.Sdk.TrimmableTypeMap/Generator/TypeMapAssemblyEmitter.cs
+++ b/src/Microsoft.Android.Sdk.TrimmableTypeMap/Generator/TypeMapAssemblyEmitter.cs
@@ -361,9 +361,10 @@ void EmitProxyType (JavaPeerProxyData proxy, Dictionary<string, MethodDefinition
 			wrapperHandles [uco.WrapperName] = handle;
 		}
 
-		// RegisterNatives
+		// RegisterNatives + GetFunctionPointer
 		if (proxy.IsAcw) {
 			EmitRegisterNatives (proxy.NativeRegistrations, wrapperHandles);
+			EmitGetFunctionPointer (proxy.NativeRegistrations, wrapperHandles);
 		}
 	}
 
@@ -729,6 +730,52 @@ void AddUnmanagedCallersOnlyAttribute (MethodDefinitionHandle handle)
 		_pe.Metadata.AddCustomAttribute (handle, _ucoAttrCtorRef, _ucoAttrBlobHandle);
 	}
 
+	/// <summary>
+	/// Emits GetFunctionPointer(int methodIndex) → IntPtr that maps ordinals to UCO wrapper function pointers.
+	/// Used as an alternative dispatch mechanism to RegisterNatives.
+	/// </summary>
+	void EmitGetFunctionPointer (List<NativeRegistrationData> registrations,
+		Dictionary<string, MethodDefinitionHandle> wrapperHandles)
+	{
+		// Collect resolved handles in registration order
+		var resolvedHandles = new List<MethodDefinitionHandle> ();
+		foreach (var reg in registrations) {
+			if (wrapperHandles.TryGetValue (reg.WrapperMethodName, out var handle)) {
+				resolvedHandles.Add (handle);
+			}
+		}
+
+		_pe.EmitBody ("GetFunctionPointer",
+			MethodAttributes.Public | MethodAttributes.Virtual | MethodAttributes.HideBySig |
+			MethodAttributes.NewSlot | MethodAttributes.Final,
+			sig => sig.MethodSignature (isInstanceMethod: true).Parameters (1,
+				rt => rt.Type ().IntPtr (),
+				p => p.AddParameter ().Type ().Int32 ()),
+			encoder => {
+				// For each registration, emit:
+				//   ldarg.1          // load methodIndex
+				//   ldc.i4 <index>   // load constant
+				//   bne.un.s <skip>  // if not equal, skip to next
+				//   ldftn <wrapper>  // load function pointer
+				//   ret
+				//   <skip>:
+				for (int i = 0; i < resolvedHandles.Count; i++) {
+					encoder.LoadArgument (1);
+					encoder.LoadConstantI4 (i);
+					// bne.un.s with offset = 7 (ldftn is 2-byte opcode + 4-byte token = 6, ret is 1 byte)
+					encoder.OpCode (ILOpCode.Bne_un_s);
+					encoder.CodeBuilder.WriteSByte (7);
+					encoder.OpCode (ILOpCode.Ldftn);
+					encoder.Token (resolvedHandles [i]);
+					encoder.OpCode (ILOpCode.Ret);
+				}
+				// Default: return IntPtr.Zero
+				encoder.OpCode (ILOpCode.Ldc_i4_0);
+				encoder.OpCode (ILOpCode.Conv_i);
+				encoder.OpCode (ILOpCode.Ret);
+			});
+	}
+
 	void EmitTypeMapAttribute (TypeMapAttributeData entry)
 	{
 		var ctorRef = entry.IsUnconditional ? _typeMapAttrCtorRef2Arg : _typeMapAttrCtorRef3Arg;
diff --git a/tests/Microsoft.Android.Sdk.TrimmableTypeMap.Tests/Generator/TypeMapAssemblyGeneratorTests.cs b/tests/Microsoft.Android.Sdk.TrimmableTypeMap.Tests/Generator/TypeMapAssemblyGeneratorTests.cs
index 4f6c595a5ba..dcd80fbb154 100644
--- a/tests/Microsoft.Android.Sdk.TrimmableTypeMap.Tests/Generator/TypeMapAssemblyGeneratorTests.cs
+++ b/tests/Microsoft.Android.Sdk.TrimmableTypeMap.Tests/Generator/TypeMapAssemblyGeneratorTests.cs
@@ -504,4 +504,22 @@ public void ParseParameterTypes_UnterminatedSignature_ReturnsEmptyList ()
 	{
 		Assert.Empty (JniSignatureHelper.ParseParameterTypes ("("));
 	}
+
+	[Fact]
+	public void Generate_AcwProxy_HasGetFunctionPointer ()
+	{
+		var peers = ScanFixtures ();
+		var acwPeer = peers.First (p => p.JavaName == "my/app/MainActivity");
+		Assert.False (acwPeer.DoNotGenerateAcw);
+
+		using var stream = GenerateAssembly (new [] { acwPeer }, "GetFnPtrTest");
+		using var pe = new PEReader (stream);
+		var reader = pe.GetMetadataReader ();
+
+		var methodDefs = reader.MethodDefinitions
+			.Select (h => reader.GetMethodDefinition (h))
+			.Select (m => reader.GetString (m.Name))
+			.ToList ();
+		Assert.Contains ("GetFunctionPointer", methodDefs);
+	}
 }
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