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Noexcept affects libstdc++’s unordered_set
The other day I learned a new place where adding or removing noexcept can change the performance of your program: GNU libstdc++’s hash-based associative containers change the struct layout of their nodes depending on the noexceptness of your hash function. This is laid out fairly clearly in the docs; it’s simply bizarre enough that I’d never thought to look for such a thing in the docs!
But woe betide the hapless programmer who defines YourHash::operator() const noexcept — their hash function will be called many more times than it should, resulting in a massive slowdown (on this workload, anyway). The third column illustrates the reverse effect: libstdc++ does not recognize that hashing a vector<bool> is expensive, so they save 8 bytes and take a massive slowdown by default. (In short: They should get rid of the blacklist; pay the 8 bytes by default; introduce a whitelist for trivial hashers specifically; stop checking noexceptness for any reason.)
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