What's the benifit of flat_hash_map over unordered_map?

[zzq0119]

Hi Mike, in https://sv-lang.com/commoncomp.html , the last line

For any hash tables that do need to live outside the stack, flat_hash_map should be used over std::unordered_map, always.

Could you explain why flat_hash_map is always better in such condition?

[MikePopoloski]

It's much much faster. std::unordered_map is notoriously slow.

[zzq0119]

OK, and the reason why "outside the stack" is emphasized here is that the time difference between the two maps would much larger than it on stack?

[MikePopoloski]

If it's only on the stack, i.e. temporary, SmallMap should be used instead to potentially avoid any heap allocation altogether. Again, just for performance reasons.

[zzq0119]

OK, thanks!