make is pretty smart, and picks up what has changed from the last build, so if you run
repo sync and then build without cleaning, in most cases it should work just fine.
One exception would be after adding or updating proprietary vendor binaries. Per Google’s Android documentation, Building for devices, Google does recommend:
Cleaning up when adding proprietary binaries
In order to make sure that the newly installed binaries are properly taken into account after being extracted, the existing output of any previous build needs to be deleted with
make clean does the same thing as
make clobber, which is removing all generated files including those in (
$OUT_DIR) if defined.
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