The problem isn't simply that repeated function calls are expensive, it's that the comparator logic is very branchy in order to support the different comparator

The problem isn't simply that repeated function calls are expensive, it's that the comparator logic is very branchy in order to support the different comparator options for their queries, even though any given query will only perform the same comparator for every value. The operation being too dynamic would exist regardless. He jumps straight from the problem to codegen-based solutions, but I wonder if a simpler loop specialization would yield a big enough chunk of the performance he got. If you hoist the comparator handling higher up into a single branch per query, then have a separate loop for each, you avoid unnecessary branching in your hot loop. You'd have to duplicate logic for every comparator, which is a little ugly, but there were only a few comparators, and that seems a lot more palatable if the alternative is an elaborate codegen solution.