Optimize Array#*

[asterite]

We use the same trick of multiple memcpy like we do in String#* and also consider a few edge cases like 0 and 1.

Benchmark:

require "benchmark"

Benchmark.ips do |x|
  x.report("old (1*100)") do
    [1] * 100
  end
  x.report("new (1*100)") do
    [1].mul(100)
  end
end

Benchmark.ips do |x|
  x.report("old (3*100)") do
    [1, 2, 3] * 100
  end
  x.report("new (3*100)") do
    [1, 2, 3].mul(100)
  end
end

Benchmark.ips do |x|
  x.report("old (3*1)") do
    [1, 2, 3] * 1
  end
  x.report("new (3*1)") do
    [1, 2, 3].mul(1)
  end
end

Results:

old (1*100)   1.62M (616.99ns) (± 6.18%)  528B/op   3.12× slower
new (1*100)   5.06M (197.73ns) (±13.24%)  528B/op        fastest
old (3*100)   1.25M (802.79ns) (± 3.55%)  2.08kB/op   1.58× slower
new (3*100)   1.96M (509.59ns) (± 2.59%)  2.08kB/op        fastest
old (3*1)  14.50M ( 68.96ns) (±12.02%)  96.0B/op   1.03× slower
new (3*1)  14.88M ( 67.21ns) (± 8.74%)  96.0B/op        fastest

I also got curious about this:

Benchmark.ips do |x|
  x.report("old (1*100)") do
    [1] * 100
  end
  x.report("new (1*100)") do
    [1].mul(100)
  end
  x.report("Array.new(size, value)") do
    Array.new(100, 1)
  end
end

Because they are essentially the same except that [1] * 100 is shorter to read/write and fancier.

Results:

           old (1*100)   1.74M (574.33ns) (± 0.96%)  528B/op   3.36× slower
           new (1*100)   5.48M (182.38ns) (± 1.00%)  528B/op   1.07× slower
Array.new(size, value)   5.85M (170.95ns) (± 1.37%)  480B/op        fastest

So it's not that much worse so now it becomes kind of acceptable to do.

[straight-shoota]

Nice 🥇

[sdogruyol]

More performance 😍