Optimize JSON.dump argument parsing

[casperisfine]

JSON.dump looks terrible on micro-benchmarks because the way it handles arguments is quite allocation heavy compared to the actual JSON generation work.

Profiling the small hash benchmarked show 14% of time spent in Array#compact and 34% time spent in JSON::Ext::GeneratorState.new. Only 41% in the actual generate function.

By micro-optimizing JSON.dump, it can look much better:

Before:

== Encoding small nested array (121 bytes)
ruby 3.4.0preview2 (2024-10-07 master 32c733f57b) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
                json    91.687k i/100ms
                  oj   205.309k i/100ms
           rapidjson   161.648k i/100ms
Calculating -------------------------------------
                json    941.965k (± 1.4%) i/s    (1.06 μs/i) -      4.768M in   5.062573s
                  oj      2.138M (± 1.2%) i/s  (467.82 ns/i) -     10.881M in   5.091254s
           rapidjson      1.678M (± 1.9%) i/s  (596.04 ns/i) -      8.406M in   5.011931s

Comparison:
                json:   941964.8 i/s
                  oj:  2137586.5 i/s - 2.27x  faster
           rapidjson:  1677737.1 i/s - 1.78x  faster

== Encoding small hash (65 bytes)
ruby 3.4.0preview2 (2024-10-07 master 32c733f57b) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
                json   141.737k i/100ms
                  oj   676.871k i/100ms
           rapidjson   373.266k i/100ms
Calculating -------------------------------------
                json      1.491M (± 1.0%) i/s  (670.78 ns/i) -      7.512M in   5.039463s
                  oj      7.226M (± 1.4%) i/s  (138.39 ns/i) -     36.551M in   5.059475s
           rapidjson      3.729M (± 2.2%) i/s  (268.15 ns/i) -     18.663M in   5.007182s

Comparison:
                json:  1490798.2 i/s
                  oj:  7225766.2 i/s - 4.85x  faster
           rapidjson:  3729192.2 i/s - 2.50x  faster

After:

== Encoding small nested array (121 bytes)
ruby 3.4.0preview2 (2024-10-07 master 32c733f57b) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
                json   156.832k i/100ms
                  oj   209.769k i/100ms
           rapidjson   162.922k i/100ms
Calculating -------------------------------------
                json      1.599M (± 2.5%) i/s  (625.34 ns/i) -      7.998M in   5.005110s
                  oj      2.137M (± 1.5%) i/s  (467.99 ns/i) -     10.698M in   5.007806s
           rapidjson      1.677M (± 3.5%) i/s  (596.31 ns/i) -      8.472M in   5.059515s

Comparison:
                json:  1599141.2 i/s
                  oj:  2136785.3 i/s - 1.34x  faster
           rapidjson:  1676977.2 i/s - same-ish: difference falls within error

== Encoding small hash (65 bytes)
ruby 3.4.0preview2 (2024-10-07 master 32c733f57b) +YJIT +PRISM [arm64-darwin23]
Warming up --------------------------------------
                json   216.464k i/100ms
                  oj   661.328k i/100ms
           rapidjson   324.434k i/100ms
Calculating -------------------------------------
                json      2.301M (± 1.7%) i/s  (434.57 ns/i) -     11.689M in   5.081278s
                  oj      7.244M (± 1.2%) i/s  (138.05 ns/i) -     36.373M in   5.021985s
           rapidjson      3.323M (± 2.9%) i/s  (300.96 ns/i) -     16.871M in   5.081696s

Comparison:
                json:  2301142.2 i/s
                  oj:  7243770.3 i/s - 3.15x  faster
           rapidjson:  3322673.0 i/s - 1.44x  faster

Now profiles of the small hash benchmark show 44% in generate and 45% in GeneratorState allocation.

[eregon]

One idea (which I learned from @nobu IIRC) to optimize the most common case of no extra arguments are passed is:

def dump(obj, anIO = (no_args_set = true; nil), limit = nil, kwargs = nil)
  unless no_args_set
    ...
  end

  opts = JSON.dump_default_options
  ...

[byroot]

Nice trick.