Improve Oj.dump performance

[Watson1978]

This patch uses standard C library to copy the string
because copying one byte at a time is slow.

This patch will improve Oj.dump performance as following.

–	before	after	result
Oj.dump	689.236k	1.853M	2.69x
Oj.dump (compat)	476.107k	827.446k	1.74x
Oj.dump (rails)	464.545k	644.494k	1.39x

Environment

• MacBook Air (M1, 2020)
• macOS 12.0 beta 3
• Apple M1
• Ruby 3.0.2

Before

Warming up --------------------------------------
             Oj.dump    69.210k i/100ms
    Oj.dump (compat)    47.123k i/100ms
     Oj.dump (rails)    45.911k i/100ms
Calculating -------------------------------------
             Oj.dump    689.236k (± 0.2%) i/s -      3.460M in   5.020801s
    Oj.dump (compat)    476.107k (± 0.9%) i/s -      2.403M in   5.048128s
     Oj.dump (rails)    464.545k (± 0.9%) i/s -      2.341M in   5.040711s

After

Warming up --------------------------------------
             Oj.dump   187.096k i/100ms
    Oj.dump (compat)    82.879k i/100ms
     Oj.dump (rails)    64.371k i/100ms
Calculating -------------------------------------
             Oj.dump      1.853M (± 0.3%) i/s -      9.355M in   5.049406s
    Oj.dump (compat)    827.446k (± 0.2%) i/s -      4.144M in   5.008145s
     Oj.dump (rails)    644.494k (± 0.2%) i/s -      3.283M in   5.093814s

Test code

require 'benchmark/ips'
require 'oj'

data = {
  'short_string': 'a' * 50,
  'long_string': 'b' * 255,
  'utf8_string': 'あいうえお' * 10
}

Benchmark.ips do |x|
  x.report('Oj.dump') { Oj.dump(data) }
  x.report('Oj.dump (compat)') { Oj.dump(data, mode: :compat) }
  x.report('Oj.dump (rails)') { Oj.dump(data, mode: :rails) }
end

[Watson1978]

I retrieved a benchmark result on Intel Mac.

–	before	after	result
Oj.dump	769.490k	1.338M	1.74x
Oj.dump (compat)	387.004k	482.291k	1.25x
Oj.dump (rails)	348.387k	404.250k	1.16x

Env

• MacBook Pro (16-inch, 2019)
• macOS BigSur 11.5
• CPU 2.4 GHz 8cores Intel Core i9
• Ruby 3.0.2

Before

Warming up --------------------------------------
             Oj.dump    75.561k i/100ms
    Oj.dump (compat)    37.711k i/100ms
     Oj.dump (rails)    34.858k i/100ms
Calculating -------------------------------------
             Oj.dump    769.490k (± 3.4%) i/s -      3.854M in   5.014431s
    Oj.dump (compat)    387.004k (± 2.1%) i/s -      1.961M in   5.069355s
     Oj.dump (rails)    348.387k (± 3.8%) i/s -      1.743M in   5.011002s

After

Warming up --------------------------------------
             Oj.dump   134.753k i/100ms
    Oj.dump (compat)    49.439k i/100ms
     Oj.dump (rails)    41.207k i/100ms
Calculating -------------------------------------
             Oj.dump      1.338M (± 3.3%) i/s -      6.738M in   5.040677s
    Oj.dump (compat)    482.291k (± 2.2%) i/s -      2.423M in   5.025367s
     Oj.dump (rails)    404.250k (± 3.3%) i/s -      2.019M in   5.000660s

[ohler55]

I made a few comments but please don't take that the wrong way. Your catch on the use of strncpy was a good one. I'd be curious if memcpy would be a bit better. Would you mind running the benchmarks to compare?

As for the other comments, I'd like to keep the constants on the ones that always return the same length but the two that use the sprintf return value are keepers so it is better to remove the non-used calculations of len earlier in the code.

[ohler55]

Thank you for the nice improvement. That will go nicely with the parser re-write I'm in the middle of but I'll release your improvement sooner than the parser.