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Traces are triplets of branch source, target, and fall-through end (next branch). Traces simplify differentiation of fall-throughs into local- and external-origin, which improves performance over profile with undifferentiated fall-throughs by eliminating profile discontinuity in call to continuation fall-throughs. This makes it possible to avoid converting return profile into call to continuation profile which may introduce statistical biases. The existing format makes provisions for local- (F) and external- (f) origin fall-throughs, but the profile producer needs to know function boundaries. BOLT has that information readily available, so providing the origin branch of a fall-through is a functional replacement of the fall-through kind (f or F). This also has an effect of combining branches and fall-throughs into a single record. As traces subsume other pre-aggregated profile kinds, BOLT may drop support for them soon. Users of pre-aggregated profile format are advised to migrate to the trace format. Test Plan: Updated callcont-fallthru.s