Allocate empty group on heap only for std::allocator

[greg7mdp]

Otherwise leave ctrl_ as a nullptr.

[AgostonSzepessy]

Thanks for implementing this! Do you know if it would result in a perf regression if we didn't have ctrl_? The other thing we could do is user opt-in to store the ctrl_ on the raw_hash_set if there's a big perf gap.

[greg7mdp]

Thanks for implementing this! Do you know if it would result in a perf regression if we didn't have ctrl_? The other thing we could do is user opt-in to store the ctrl_ on the raw_hash_set if there's a big perf gap.

Regarding the perf regression, I very much doubt you could see any difference in real world use. This is why I prefer optimizing for correctness and overall good behavior (for example with poor hash functions) rather than for micro benchmarks.

I don't think storing the ctrl_ on the raw_hash_set is a good idea. First it probably would make no difference, and also it would do an allocation and use memory for empty hash maps, which I think is a bigger issue.

[greg7mdp]

Thanks again for bringing up this issue @AgostonSzepessy !

[AgostonSzepessy]

Thank you for fixing it so quickly!