[Build] Add per-file up-to-date check in CompileNativeAssembly

src/Xamarin.Android.Build.Tasks/Tasks/CompileNativeAssembly.cs

@@ -22,6 +22,7 @@ sealed class Config
 			public string? AssemblerPath;
 			public string? AssemblerOptions;
 			public string? InputSource;
+			public string? OutputFile;
 		}
 
 		[Required]
@@ -43,6 +44,14 @@ public override System.Threading.Tasks.Task RunTaskAsync ()
 
 		void RunAssembler (Config config)
 		{
+			if (config.OutputFile is not null && config.InputSource is not null && File.Exists (config.OutputFile)) {
+				string sourceFile = Path.Combine (WorkingDirectory, Path.GetFileName (config.InputSource));
+				if (File.Exists (sourceFile) && File.GetLastWriteTimeUtc (config.OutputFile) >= File.GetLastWriteTimeUtc (sourceFile)) {
+					LogDebugMessage ($"[LLVM llc] Skipping '{Path.GetFileName (config.InputSource)}' because '{Path.GetFileName (config.OutputFile)}' is up to date");
+					return;
+				}
+			}
+
 			var stdout_completed = new ManualResetEvent (false);
 			var stderr_completed = new ManualResetEvent (false);
 			var psi = new ProcessStartInfo () {
@@ -118,10 +127,13 @@ IEnumerable<Config> GetAssemblerConfigs ()
 				string executableDir = Path.GetDirectoryName (llcPath);
 				string executableName = MonoAndroidHelper.GetExecutablePath (executableDir, Path.GetFileName (llcPath));
 
+				string outputFilePath = Path.Combine (WorkingDirectory, sourceFile.Replace (".ll", ".o"));
+
 				yield return new Config {
 					InputSource = item.ItemSpec,
 					AssemblerPath = Path.Combine (executableDir, executableName),
 					AssemblerOptions = $"{assemblerOptions} -o={outputFile} {QuoteFileName (sourceFile)}",
+					OutputFile = outputFilePath,
 				};
 			}
 		}

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/IncrementalBuildTest.cs

@@ -1289,6 +1289,38 @@ public void GenerateJavaStubsAndAssembly ([Values] bool isRelease, [Values] Andr
 			}
 		}
 
+		[Test]
+		public void CompileNativeAssemblySourcesSkipsUnchangedFiles ([Values (AndroidRuntime.CoreCLR)] AndroidRuntime runtime)
+		{
+			if (IgnoreUnsupportedConfiguration (runtime, release: false)) {
+				return;
+			}
+
+			var proj = new XamarinAndroidApplicationProject ();
+			proj.SetRuntime (runtime);
+
+			string abi = "arm64-v8a";
+			proj.SetRuntimeIdentifier (abi);
+
+			using (var b = CreateApkBuilder ()) {
+				b.Verbosity = LoggerVerbosity.Detailed;
+				Assert.IsTrue (b.Build (proj), "first build should have succeeded.");
+
+				// Modify MainActivity to trigger recompilation of typemap sources
+				proj.MainActivity = proj.DefaultMainActivity + Environment.NewLine + "// test comment";
+				proj.Touch ("MainActivity.cs");
+				Assert.IsTrue (b.Build (proj), "second build should have succeeded.");
+
+				Assert.IsFalse (b.Output.IsTargetSkipped ("_CompileNativeAssemblySources"), "`_CompileNativeAssemblySources` should *not* be skipped!");
+
+				// At least one .ll file should have been skipped as up to date (e.g., environment.arm64-v8a.ll)
+				Assert.IsTrue (
+					StringAssertEx.ContainsRegex (@"\[LLVM llc\] Skipping.*up to date", b.LastBuildOutput),
+					"Expected at least one .ll file to be skipped as up to date"
+				);
+			}
+		}
+
 		readonly string [] ExpectedAssemblyFiles = new [] {
 			Path.Combine ("android", "environment.@ABI@.o"),
 			Path.Combine ("android", "environment.@ABI@.ll"),