> their proposed fix is more expensive than a HighwayTreeHash round.
For longish-strings, most of the cycles are in the preliminary rounds, so appending whatever you want onto the end should add negligible cost. This includes bitmixing (their proposed smhasher fix) or a HighwayTreeHash round.
Agreed :) We (and SipHash developers) do care about short strings, though. Scripting language hash table inputs are typically around 10 bytes, so we can't ignore finalization overhead.
For longish-strings, most of the cycles are in the preliminary rounds, so appending whatever you want onto the end should add negligible cost. This includes bitmixing (their proposed smhasher fix) or a HighwayTreeHash round.