Feature #15331
Updated by alanwu (Alan Wu) over 5 years ago
## Background It's common for applications to use string literals as hash keys, especially for applications that work with YAML or JSON: ``` paylod['name'] ``` At the moment (r65895), hash lookups using a string key is about 30% slower than using a symbol key. ## Proposal We memoize the hash code for short fstrings. There is extra, currently unused space at the end of the RString struct we can use to store the hash code. The unique nature of fstrings makes it so that every user of each fstring benefit from the memoized hash. ## Evaluation The included benchmark hash_aref_fstr.rb hash_aref_str.rb is about 20% faster with this optimization. hash_aref_long_str.rb shows that for long strings which we cannot memoize, there is no visible performance penalty. vm2_freezestring.yml is also not visibly slower after this optimization. I have also attached a bechmark (string_key_vs_symbol_key.rb) that compares hash lookups with symbols against hash lookups with strings. With this optimization, the gap in performance is smaller. (10% slower post patch vs 30% slower on trunk)