js::AtomizeString is pretty hot. Caching the two most recent lookups in its StringToAtomCache is quite effective: it hits for > 60% of the lookups on Speedometer 2 and I see numbers ranging from 30-65% on other benchmarks and websites as well.

If this cache doesn't hit, it's just two pointer comparisons which should be negligible compared to the hash table lookup(s) we do in that case.

js::AtomizeString is pretty hot and caching the most recent lookups helps avoid
slower hash table lookups in 30-65% of cases (> 60% on Speedometer 2).

https://hg.mozilla.org/mozilla-central/rev/289500d6f5c8

FYI, nsAtom uses MruCache
https://searchfox.org/mozilla-central/rev/57527d50ef5d3df412caa5d99536f0709399be6f/xpcom/ds/MruCache.h#37-39,65
, well, at least on the main thread
https://searchfox.org/mozilla-central/rev/57527d50ef5d3df412caa5d99536f0709399be6f/xpcom/ds/nsAtomTable.cpp#178