Remove unnessary volatile qualifier in Oj.load

[Watson1978]

When we use volatile qualifier, seems it has some performance penalty on GCC.
The volatile qualifier is unnecessary since the VALUE type object has been kept on the local stack.
(If we manage VALUE on the heap, we need to be careful.)

−	before	after	result
GCC	4.969M	5.165M	1.039x
Clang	5.054M	5.090M	-

Environment

• Linux
  • Manjaro Linux x86_64
  • Kernel: 5.18.12-3-MANJARO
  • AMD Ryzen 7 5700G
  • gcc version 12.1.0
  • clang version 14.0.6
  • Ruby 3.1.2

GCC

Before

Warming up --------------------------------------
             Oj.load   497.783k i/100ms
Calculating -------------------------------------
             Oj.load      4.969M (± 0.1%) i/s -     49.778M in  10.017042s

After

Warming up --------------------------------------
             Oj.load   518.127k i/100ms
Calculating -------------------------------------
             Oj.load      5.165M (± 0.1%) i/s -     51.813M in  10.031075s

Clang

Before

Warming up --------------------------------------
             Oj.load   503.712k i/100ms
Calculating -------------------------------------
             Oj.load      5.054M (± 0.1%) i/s -     50.875M in  10.067027s

After

Warming up --------------------------------------
             Oj.load   510.215k i/100ms
Calculating -------------------------------------
             Oj.load      5.090M (± 0.1%) i/s -     51.022M in  10.022973s

Test code

require 'benchmark/ips'
require 'oj'

json = Oj.dump(42)

Benchmark.ips do |x|
  x.warmup = 5
  x.time = 10

  x.report('Oj.load') do |times|
    i = 0
    while i < times
      Oj.load(json)
      i += 1
    end
  end
end

[ohler55]

I'd have to see more tests with large files that cause a GC while parsing. My understanding is the volatile keyword cause the variable not to be put in a register since Ruby does not"see" register variables. Making the VALUE volatile tests the compiler it may change to put it on the stack instead of in a register.

[Watson1978]

Added the test

[ohler55]

I'll have to remember GC.stress. Looks very useful when testing.