For profile staleness report, before it only counts for the top-level function samples in the nested profile, the samples in the inlinees are ignored. This could affect the quality of the metrics when there are heavily inlined functions. This change adds a feature to flatten the nested profile and we're changing to use flatten profile as the input for stale profile detection and matching.
Example for profile flattening:
Original profile: _Z3bazi:20301:1000 1: 1000 3: 2000 5: inline1:1600 1: 600 3: inline2:500 1: 500 Flattened profile: _Z3bazi:18701:1000 1: 1000 3: 2000 5: 600 inline1:600 inline1:1100:600 1: 600 3: 500 inline2: 500 inline2:500:500 1: 500
This feature could be useful for offline analysis, like understanding the hotness of each individual function. So I'm adding the support to llvm-profdata merge under --gen-flattened-profile.