Fix: https://github.com/ruby/json/issues/655
For very small documents, the biggest performance gap with alternatives is
that the API impose that we allocate the `State` object. In a real world app
this doesn't make much of a difference, but when running in a micro-benchmark
this doubles the allocations, causing twice the amount of GC runs, making us
look bad.
However, unless we have to call a `to_json` method, the `State` object isn't
visible, so with some refactoring, we can elude that allocation entirely.
Instead we allocate the State internal struct on the stack, and if we need
to call a `to_json` method, we allocate the `State` and spill the struct on
the heap.
As a result, `JSON.generate` is now as fast as re-using a `State` instance,
as long as only primitives are generated.
Before:
```
== Encoding small mixed (34 bytes)
ruby 3.3.4 (2024-07-09 revision be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
json (reuse) 598.654k i/100ms
json 400.542k i/100ms
oj 533.353k i/100ms
Calculating -------------------------------------
json (reuse) 6.371M (± 8.6%) i/s (156.96 ns/i) - 31.729M in 5.059195s
json 4.120M (± 6.6%) i/s (242.72 ns/i) - 20.828M in 5.090549s
oj 5.622M (± 6.4%) i/s (177.86 ns/i) - 28.268M in 5.061473s
Comparison:
json (reuse): 6371126.6 i/s
oj: 5622452.0 i/s - same-ish: difference falls within error
json: 4119991.1 i/s - 1.55x slower
== Encoding small nested array (121 bytes)
ruby 3.3.4 (2024-07-09 revision be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
json (reuse) 248.125k i/100ms
json 215.255k i/100ms
oj 217.531k i/100ms
Calculating -------------------------------------
json (reuse) 2.628M (± 6.1%) i/s (380.55 ns/i) - 13.151M in 5.030281s
json 2.185M (± 6.7%) i/s (457.74 ns/i) - 10.978M in 5.057655s
oj 2.217M (± 6.7%) i/s (451.10 ns/i) - 11.094M in 5.044844s
Comparison:
json (reuse): 2627799.4 i/s
oj: 2216824.8 i/s - 1.19x slower
json: 2184669.5 i/s - 1.20x slower
== Encoding small hash (65 bytes)
ruby 3.3.4 (2024-07-09 revision be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
json (reuse) 641.334k i/100ms
json 322.745k i/100ms
oj 642.450k i/100ms
Calculating -------------------------------------
json (reuse) 7.133M (± 6.5%) i/s (140.19 ns/i) - 35.915M in 5.068201s
json 4.615M (± 7.0%) i/s (216.70 ns/i) - 22.915M in 5.003718s
oj 6.912M (± 6.4%) i/s (144.68 ns/i) - 34.692M in 5.047690s
Comparison:
json (reuse): 7133123.3 i/s
oj: 6911977.1 i/s - same-ish: difference falls within error
json: 4614696.6 i/s - 1.55x slower
```
After:
```
== Encoding small mixed (34 bytes)
ruby 3.3.4 (2024-07-09 revision be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
json (reuse) 572.751k i/100ms
json 457.741k i/100ms
oj 512.247k i/100ms
Calculating -------------------------------------
json (reuse) 6.324M (± 6.9%) i/s (158.12 ns/i) - 31.501M in 5.023093s
json 6.263M (± 6.9%) i/s (159.66 ns/i) - 31.126M in 5.017086s
oj 5.569M (± 6.6%) i/s (179.56 ns/i) - 27.661M in 5.003739s
Comparison:
json (reuse): 6324183.5 i/s
json: 6263204.9 i/s - same-ish: difference falls within error
oj: 5569049.2 i/s - same-ish: difference falls within error
== Encoding small nested array (121 bytes)
ruby 3.3.4 (2024-07-09 revision be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
json (reuse) 258.505k i/100ms
json 242.335k i/100ms
oj 220.678k i/100ms
Calculating -------------------------------------
json (reuse) 2.589M (± 9.6%) i/s (386.17 ns/i) - 12.925M in 5.071853s
json 2.594M (± 6.6%) i/s (385.46 ns/i) - 13.086M in 5.083035s
oj 2.250M (± 2.3%) i/s (444.43 ns/i) - 11.255M in 5.004707s
Comparison:
json (reuse): 2589499.6 i/s
json: 2594321.0 i/s - same-ish: difference falls within error
oj: 2250064.0 i/s - 1.15x slower
== Encoding small hash (65 bytes)
ruby 3.3.4 (2024-07-09 revision be1089c8ec) +YJIT [arm64-darwin23]
Warming up --------------------------------------
json (reuse) 656.373k i/100ms
json 644.135k i/100ms
oj 650.283k i/100ms
Calculating -------------------------------------
json (reuse) 7.202M (± 7.1%) i/s (138.84 ns/i) - 36.101M in 5.051438s
json 7.278M (± 1.7%) i/s (137.40 ns/i) - 36.716M in 5.046300s
oj 7.036M (± 1.7%) i/s (142.12 ns/i) - 35.766M in 5.084729s
Comparison:
json (reuse): 7202447.9 i/s
json: 7277883.0 i/s - same-ish: difference falls within error
oj: 7036115.2 i/s - same-ish: difference falls within error
```