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[Xamarin.Android.Build.Tasks] aot is a valid profiler (#1253)
Mono supports an [AOT profiler][aotp]: when enabled, mono will generate a file containing the methods that were encountered while the AOT profiler is enabled, usually during the lifetime of the process. AOT profiler output can then be fed into the AOT compiler, allowing *selective AOT of assemblies*: instead of AOT'ing *everything*, only the specified methods would be AOT'd. This would hopefully allow for smaller `.apk` files containing AOT output. [aotp]: https://github.com/mono/mono/blob/4a995045915dbe45918787d158485dda26660e05/man/mono-profilers.1#L325-L330 The question: How do we make use of this functionality? We don't have an answer. We need to start *experimenting* to determine an answer. To help support that experimentation, set things up so that the AOT profiler can be used: * *Build* and package the AOT profiler, contained within the `libmono-profiler-aot.so` native library. This was done in commit 3711992. * Allow the `.apk` to contain `libmono-profiler-aot.so`. * Profile the application. * Grab the profiler output. Allow the `$(AndroidEmbedProfilers)` MSBuild property to use the value `aot` so that the `libmono-profiler-aot.so` profiling library will be embedded into the `.apk` file: msbuild /p:AndroidEmbedProfilers=aot TheApp.csproj /t:SignAndroidPackage Once the `.apk` contains `libmono-profiler-aot.so`, then the profiler can be used by setting the `debug.mono.profile` system property: adb shell setprop debug.mono.profile aot When the application is next launched, the AOT profiler will be used. If (see below) AOT output is produced, it can be extracted from the device by using the `adb pull` command. There are four known issues: 1. The *filename* of the produced file, which changes in PR #1254. 2. The *path* of the produced file, which will be in the `.__override__` directory. See also [DevelopmentTips.md][update]. 3. AOT profiler output isn't constantly written to. To ensure that *all* AOT profiler output is written to the output file, the process must currently exit. This can be done by calling `Environment.Exit()` at some point within the application. 4. `Java.Interop` doesn't support cleanly shutting down. [update]: https://github.com/xamarin/xamarin-android/blob/a10270849983aafc1d137520183b499747174e88/Documentation/DevelopmentTips.md#update-directory Specifically, if you *do* call `Environment.Exit()`, process exit is not cleanly performed: [ERROR] FATAL UNHANDLED EXCEPTION: System.ObjectDisposedException: Cannot access a disposed object. Object name: 'The ThreadLocal object has been disposed.'. at System.Threading.ThreadLocal`1[T].GetValueSlow () at System.Threading.ThreadLocal`1[T].get_Value () at Java.Interop.JniEnvironment.get_CurrentInfo () at Java.Interop.JniEnvironment.get_Runtime () at Java.Interop.JniObjectReference.Dispose (Java.Interop.JniObjectReference& reference) at Java.Interop.JniRuntime.Dispose (System.Boolean disposing) at Java.Interop.JniRuntime.Finalize () As such, AOT profiling *does not work*. However, we want to *permit enabling AOT profiling* so that we can more easily test and get things to a working state.
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