Detect Source Drift with Propeller.
Source Drift happens when the sources are updated after profiling the binary but before building the final optimized binary. If the source has changed since the profiles were obtained, optimizing basic blocks might be sub-optimal. This only applies to BasicBlockSection::List as it creates clusters of basic blocks using basic block ids. Source drift can invalidate these groupings leading to sub-optimal code generation with regards to performance.
PGO source drift for a particular function can be detected using function metadata added in D95495.