By using a std::vector to store the keys, the cost of key lookup goes down
in comparison with std::unordered_map. The downside is the increase in
memory use in certain situations but given the fact that the intended
purpose of worker-local data is to store global data, this is unlikely to
have observable negative side-effects. Depending on the implementations of
std::vector and std::unordered_map, the use of std::vector could even
result in a smaller memory footprint.
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