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# Using Profile Guided Optimizations to identify compiler optimization failures
When using Clang, the `-Rpass-missed` flag enables the verbose log of failed
compiler optimizations. However, the extensive log messages can obscure
potential optimization opportunities.
Use the following steps to generate a more transparent optimization report
using a previously created PGO profile file. The report also includes code
hotness diagnostics:
```bash
$ ../libvpx/configure --use-profile=perf.profdata \
--extra-cflags="-fsave-optimization-record -fdiagnostics-show-hotness"
```
Convert the generated YAML files into a detailed HTML report using the
[optviewer2](https://github.com/OfekShilon/optview2) tool:
```bash
$ opt-viewer.py --output-dir=out/ --source-dir=libvpx .
```
The HTML report displays each code line's relative hotness, cross-referenced
with the failed compiler optimizations.
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