After you enable app optimization, check that your app is working as intended
and that the R8 configuration is doing what you expect. The general steps are as
follows:

1. **Test your app's critical user journeys (CUJs):** For example, make sure that users can sign in and do other important tasks.
2. **Measure performance gains using benchmarks:** [Benchmark your app](https://developer.android.com/topic/performance/benchmarking/benchmarking-overview) before and after enabling app optimization.

If you find issues, R8 provides tools to help you troubleshoot. If you're unable
to solve an issue with R8, [file a bug](https://b.corp.google.com/issues/new?component=326788&template=1025938&pli=1).

## General tips

Issues with R8 are often unique to your app, but here are some general tips to
make debugging easier:

- **Temporarily turn off obfuscation:** R8 obfuscates code as part of its optimization process. Obfuscation usually isn't the cause of errors, but obfuscation makes errors more difficult to debug. Add the `‑dontobfuscate` and `‑dontoptimize` flags to the keep rules file to help you pinpoint problematic code.
- **Check for reflection:** If you run into a *no such element* exception such
  as

  `Caused by: java.util.NoSuchElementException: Collection contains no element
  matching the predicate in the stack trace` ,

  the exception usually means the field in question is being used through
  reflection and you have to add a keep rule.
- **Inspect bytecode:** Most R8 issues require bytecode inspection, which you
  can do with tools such as the [APK Analyzer](https://developer.android.com/studio/debug/apk-analyzer).

## Check which rules are applied

To output a full report of all the rules that R8 applies when building your
project, include the following in your app's module
`proguard‑rules.pro` file:  

    -printconfiguration <output-dir>/<report-name>.txt

You can specify any path and filename. If you don't specify a path or filename,
R8 outputs the rules report at `<module-name>/build/outputs/mapping/<build-type>/configuration.txt`.

## Understand why code was kept

If you see kept code that you expected to be removed, use the configuration
option `‑whyareyoukeeping` to help understand why the code was
kept. R8 outputs a path from the kept code to one of your app's [entry
points](https://developer.android.com/topic/performance/app-optimization/add-keep-rules#how-write). For more information, see the
[`‑whyareyoukeeping`](https://www.guardsquare.com/manual/configuration/usage)
documentation in the Proguard manual.

## Recover the original stack trace

Code processed by R8 is changed in various ways so that the stack trace no
longer refers to the original code. For example, line numbers and the names of
classes and methods can change. To recover the original stack trace, R8 provides
the retrace command-line tool, which is bundled with the command-line tools
package.

To use retrace, give it a *mapping file* and a stack trace file. The mapping
file, called `mapping.txt`, is automatically bundled with your Android App
Bundle (AAB). For more details, see the [retrace](https://developer.android.com/studio/command-line/retrace) documentation and the Play
Console Help Center article about how to [de‑obfuscate crash stack
traces](https://support.google.com/googleplay/android-developer/answer/9848633).
| **Warning:** If you're shipping APKs, the `mapping.txt` file isn't automatically bundled with your APK. Android Studio saves the mapping file in the `/build/outputs/mapping/` directory. The mapping file is overwritten every time you build your project, so you must save a copy each time you publish a new release.