Make `JSON.generate` 1.75x as fast

[mame]

This PR speeds up JSON.generate by approximately 1.75x (485k instructions per second -> 840k instructions per second) for the benchmark of Oj. This makes JSON.generate nearly as fast as Oj.dump.

Before:

$ ruby --yjit -Ilib -I ext oj-bench.rb
** Oj version 3.16.3 **
ruby 3.4.0dev (2023-12-27T05:30:20Z master 862cfcaf75) +YJIT [x86_64-linux]
Warming up --------------------------------------
             Oj.dump    83.089k i/100ms
    Oj.dump [compat]    72.410k i/100ms
     Oj.dump [rails]    57.698k i/100ms
       JSON.generate    49.706k i/100ms
Calculating -------------------------------------
             Oj.dump    836.635k (± 0.4%) i/s -     12.630M in  15.095866s
    Oj.dump [compat]    718.031k (± 0.2%) i/s -     10.789M in  15.026031s
     Oj.dump [rails]    573.882k (± 0.3%) i/s -      8.655M in  15.081085s
       JSON.generate    484.650k (± 1.0%) i/s -      7.307M in  15.078021s

After:

$ ruby --yjit -Ilib -I ext oj-bench.rb
** Oj version 3.16.3 **
ruby 3.4.0dev (2023-12-27T05:30:20Z master 862cfcaf75) +YJIT [x86_64-linux]
Warming up --------------------------------------
             Oj.dump    83.349k i/100ms
    Oj.dump [compat]    71.605k i/100ms
     Oj.dump [rails]    57.058k i/100ms
       JSON.generate    84.498k i/100ms
Calculating -------------------------------------
             Oj.dump    837.304k (± 0.3%) i/s -     12.586M in  15.031372s
    Oj.dump [compat]    718.657k (± 0.5%) i/s -     10.812M in  15.045573s
     Oj.dump [rails]    565.824k (± 0.3%) i/s -      8.502M in  15.025348s
       JSON.generate    839.614k (± 0.3%) i/s -     12.675M in  15.096044s

This PR consists of the following several improvements.

• Drop prebuild of array_delim, etc.

  • array_delim is usually a single comma character. Using memcpy to copy a single character was inefficient.
  • It is much faster to output a comma and (optional) array_nl separately without prebuild.
  • This improved the speed by about 24%, from 480k i/s to 593k i/s.

• Use faster Ruby API for encoding checks.

  • This improved the speed by 12%, from 593k i/s to 665k i/s.

• Use a fast path when string escaping is not needed.

  • This improves the performance by 16%, from 665ki/s to 770k i/s.

• Use faster Ruby API for dispatching the class of objects.

  • This improved performance by 5%, from 770k i/s to 806k i/s.

• Use generate_json_string for object keys.

  • Since object keys are already verified to be String, using generate_json in general dispatch was an unnecessary overhead.
  • This improved the performance by 3%, from 806k i/s to 830k i/s.

• Use faster Ruby API for reading array elements.

  • This improved the performance by about 4%, from 830k i/s to 854k i/s.

I made them into one PR because I thought separating this to multiple PRs would bring many conflicts between PRs. However, if you want me to do so, feel free to let me know.

[mame]

Note: I got oj-bench.rb from this article.

[hsbt]

I will re-run https://github.com/flori/json/actions/runs/7336797514/job/19976695584?pr=562 after supporting Ruby 3.3 at ruby/setup-ruby.

[Earlopain]

I'd love to see this merged, @hsbt could you take another look now that Ruby 3.3 is properly released?