From: "byroot (Jean Boussier) via ruby-core" <ruby-core@...>
Date: 2024-04-09T13:10:11+00:00
Subject: [ruby-core:117473] [Ruby master Feature#20415] Precompute literal String hash code during compilation
Issue #20415 has been updated by byroot (Jean Boussier).
> if two Strings are interned it's not correct to compare them by identity, because they could still be eql? with the same bytes but different encodings.
I'm not sure I follow. Surely `x.eql?(x)` can use an identity check as a shortcut. If both refs are equal, you can immediately return true. Of course if refs aren't equal, then yes you need to do a full string comparison.
So I'm a bit surprised to see TruffleRuby doesn't have the same performance on that benchmark.
----------------------------------------
Feature #20415: Precompute literal String hash code during compilation
https://bugs.ruby-lang.org/issues/20415#change-107861
* Author: byroot (Jean Boussier)
* Status: Open
----------------------------------------
I worked on a proof of concept with @etienne which I think has some potential, but I'm looking for feedback on what would be the best implementation.
The proof of concept is here: https://github.com/Shopify/ruby/pull/553
### Idea
Most string literals are relatively short, hence embedded, and have some wasted bytes at the end of their slot. We could use that wasted space to store the string hash.
The goal being to make **looking up a literal String key in a hash, as fast as a Symbol key**. The goal isn't to memoize the hash code of all strings, but to **only selectively precompute the hash code of literal strings
in the compiler**. The compiler could even selectively do this when we literal string is used to lookup a hash (`opt_aref`).
Here's the benchmark we used:
```ruby
hash = 10.times.to_h do |i|
[i, i]
end
dyn_sym = "dynamic_symbol".to_sym
hash[:some_symbol] = 1
hash[dyn_sym] = 1
hash["small"] = 2
hash["frozen_string_literal"] = 2
Benchmark.ips do |x|
x.report("symbol") { hash[:some_symbol] }
x.report("dyn_symbol") { hash[:some_symbol] }
x.report("small_lit") { hash["small"] }
x.report("frozen_lit") { hash["frozen_string_literal"] }
x.compare!(order: :baseline)
end
```
On Ruby 3.3.0, looking up a String key is a bit slower based on the key size:
```
Calculating -------------------------------------
symbol 24.175M (� 1.7%) i/s - 122.002M in 5.048306s
dyn_symbol 24.345M (� 1.6%) i/s - 122.019M in 5.013400s
small_lit 21.252M (� 2.1%) i/s - 107.744M in 5.072042s
frozen_lit 20.095M (� 1.3%) i/s - 100.489M in 5.001681s
Comparison:
symbol: 24174848.1 i/s
dyn_symbol: 24345476.9 i/s - same-ish: difference falls within error
small_lit: 21252403.2 i/s - 1.14x slower
frozen_lit: 20094766.0 i/s - 1.20x slower
```
With the proof of concept performance is pretty much identical:
```
Calculating -------------------------------------
symbol 23.528M (� 6.9%) i/s - 117.584M in 5.033231s
dyn_symbol 23.777M (� 4.7%) i/s - 120.231M in 5.071734s
small_lit 23.066M (� 2.9%) i/s - 115.376M in 5.006947s
frozen_lit 22.729M (� 1.1%) i/s - 115.693M in 5.090700s
Comparison:
symbol: 23527823.6 i/s
dyn_symbol: 23776757.8 i/s - same-ish: difference falls within error
small_lit: 23065535.3 i/s - same-ish: difference falls within error
frozen_lit: 22729351.6 i/s - same-ish: difference falls within error
```
### Possible implementation
The reason I'm opening this issue early is to get feedback on which would be the best implementation.
#### Store hashcode after the string terminator
Right now the proof of concept simply stores the `st_index_t` after the string null terminator, and only when the string is embedded and as enough left over space.
Strings with a precomputed hash are marked with an user flag.
Pros:
- Very simple implementation, no need to change a lot of code, and very easy to strip out if we want to.
- Doesn't use any extra memory. If the string doesn't have enough left over bytes, the optimization simply isn't applied.
- The worst case overhead is a single `FL_TEST_RAW` in `rb_str_hash`.
Cons:
- The optimization won't apply to certain string sizes. e.g. strings between `17` and `23` bytes won't have a precomputed hash code.
- Extracting the hash code requires some not so nice pointer arithmetic.
#### Create another RString union
Another possibility would be to add another entry in the `RString` struct union, such as we'd have:
```c
struct RString {
struct RBasic basic;
long len;
union {
// ... existing members
struct {
st_index_t hash;
char ary[1];
} embded_literal;
} as;
};
```
Pros:
- The optimization can now be applied to all string sizes.
- The hashcode is always at the same offset and properly aligned.
Cons:
- Some strings would be bumped by one slot size, so would use marginally more memory.
- Complexify the code base more, need to modify a lot more string related code (e.g. `RSTRING_PTR` and many others)
- When compiling such string, if an equal string already exists in the `fstring` table, we'd need to replace it, we can't just mutate it in place to add the hashcode.
### Prior art
[Feature #15331] is somewhat similar in its idea, but it does it lazily for all strings. Here it's much simpler because limited to string literals, which are the ones likely to be used as Hash keys, and the overhead is on compilation, not runtime (aside from a single flag check). So I think most of the caveats of that original implementation don't apply here.
--
https://bugs.ruby-lang.org/
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