Feature #20415: Precompute literal String hash code during compilation

• Status: Open
• Assignee: -
• Target version: -

[byroot (Jean Boussier)]

I worked on a proof of concept with @etienne (Étienne Barrié) 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:

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:

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.

[Eregon (Benoit Daloze)]

FWIW TruffleRuby already does this, since frozen string literals need to be deduplicated, the hash needs to be computed, so might as well save it while doing so (the only downside being footprint).

truffleruby 24.0.0, like ruby 3.2.2, Oracle GraalVM Native [x86_64-linux]
Calculating -------------------------------------
              symbol    107.376M (± 0.7%) i/s    (9.31 ns/i) -    541.038M in   5.038971s
          dyn_symbol    106.989M (± 0.7%) i/s    (9.35 ns/i) -    543.771M in   5.082698s
           small_lit     88.014M (± 0.6%) i/s   (11.36 ns/i) -    442.996M in   5.033433s
          frozen_lit     88.174M (± 0.3%) i/s   (11.34 ns/i) -    444.293M in   5.038895s

Comparison:
              symbol: 107376115.9 i/s
          dyn_symbol: 106989494.3 i/s - same-ish: difference falls within error
          frozen_lit: 88173794.6 i/s - 1.22x  slower
           small_lit: 88013579.7 i/s - 1.22x  slower
---
ruby 3.3.0 (2023-12-25 revision 5124f9ac75) [x86_64-linux]
Calculating -------------------------------------
              symbol     17.387M (± 0.9%) i/s   (57.51 ns/i) -     88.291M in   5.078305s
          dyn_symbol     17.416M (± 1.0%) i/s   (57.42 ns/i) -     87.555M in   5.027943s
           small_lit     14.024M (± 2.6%) i/s   (71.30 ns/i) -     70.153M in   5.005519s
          frozen_lit     13.891M (± 2.1%) i/s   (71.99 ns/i) -     70.870M in   5.104221s

Comparison:
              symbol: 17387348.9 i/s
          dyn_symbol: 17415643.8 i/s - same-ish: difference falls within error
           small_lit: 14024442.1 i/s - 1.24x  slower
          frozen_lit: 13890537.2 i/s - 1.25x  slower

Strings are still slower than Symbol keys, I suspect because eql? is quite a bit more expensive for Strings.
Even 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. That case does not exist for Symbols.

[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.

[Eregon (Benoit Daloze)]

byroot (Jean Boussier) wrote in #note-2:

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.

Yes, if they are the same object of course they are eql?, but if they are not the same object they can still be eql?, even if both are interned/fstring/frozen string literals.

So I'm a bit surprised to see TruffleRuby doesn't have the same performance on that benchmark.

For this benchmark, whether eql? is called only with equal keys depends on the Hash representation, and whether they are not two keys falling in the same bucket.
For this benchmark eql? should only be called with equal keys on TruffleRuby (due to using a buckets representation), and not the case on CRuby (due to using an array instead of buckets until 8 pairs IIRC).
I'll try to take a look at some compiler graphs to have more definite insights on it.

[byroot (Jean Boussier)]

Yes, if they are the same object of course they are eql?, but if they are not the same object they can still be eql?, even if both are interned/fstring/frozen string literals.

That was my understanding.

due to using an array instead of buckets until 8 pairs IIRC

Yes, that is why my benchmark use a larger Hash.

[etienne (Étienne Barrié)]

We pushed a cleaned-up PR at https://github.com/ruby/ruby/pull/10596.

We settled on storing the hash code after the terminator as it prevents having to add yet another union in RString that would have a general performance impact and complexify the entire code base. And we decided against storing it at the end of the object slot to avoid having to access the slot size which is slower.

compare-ruby: ruby 3.4.0dev (2024-04-22T06:32:21Z main f77618c1fa) [arm64-darwin23]
built-ruby: ruby 3.4.0dev (2024-04-22T10:13:03Z interned-string-ha.. 8a1a32331b) [arm64-darwin23]
last_commit=Precompute embedded string literals hash code

|            |compare-ruby|built-ruby|
|:-----------|-----------:|---------:|
|symbol      |     39.275M|   39.753M|
|            |           -|     1.01x|
|dyn_symbol  |     37.348M|   37.704M|
|            |           -|     1.01x|
|small_lit   |     29.514M|   33.948M|
|            |           -|     1.15x|
|frozen_lit  |     27.180M|   33.056M|
|            |           -|     1.22x|
|iseq_lit    |     27.391M|   32.242M|
|            |           -|     1.18x|