Issue #15331 has been updated by ko1 (Koichi Sasada).


My opinion: For short strings, I don't think it doesn't improve.
For long strings, it can be (not sure how many long string's hash value are used). So allocating hash value (+8 bytes) in external allocated area can help (but it should be applied to enough long strings).

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Feature #15331: [PATCH] Faster hashing for short string literals
https://bugs.ruby-lang.org/issues/15331#change-76845

* Author: alanwu (Alan Wu)
* Status: Closed
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
## 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 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)

---Files--------------------------------
string_key_vs_symbol_key.rb (594 Bytes)
0001-Hash-code-memoization-for-short-fstrings.patch (3.62 KB)
0001-Hash-code-memoization-for-short-fstrings-v2.patch (4.29 KB)
0001-Hash-code-memoization-for-short-fstrings-v3.patch (4.36 KB)
0001-Hash-code-memoization-for-short-fstrings-v4.patch (4.31 KB)
0001-Hash-code-memoization-for-short-fstrings-v5.patch (12.9 KB)
0001-Add-st_update_with_hash.patch (3.09 KB)
0002-Hash-code-memoization-for-short-fstrings.patch (10 KB)


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