[ruby-core:95124] [Ruby master Feature#16119] Optimize Array#flatten and flatten! for already flattened arrays

Issue #16119 has been updated by dylants (Dylan Thacker-Smith).


It looks like I made a mistake in my benchmarking of non-flattened arrays, =
since flatten! from the first iteration would cause further iterations to a=
ctually test with a flat array.  It looks like the performance for arrays s=
tarting with a nested array are about the same with and without my patch.

The attached patch has a merge conflict.  I've opened https://github.com/ru=
by/ruby/pull/2495 with a rebase of my patch that resolves the merge conflic=
t.  That PR also has an updated benchmark that uses the benchmark driver ya=
ml format and includes the results from running that benchmark.

nobu (Nobuyoshi Nakada) wrote:
> But I could get only insignificant result, and a worse result for an unfl=
attened array.

It looks like nobu's benchmark/array_flatten.yml has the same problem with =
the non-flat arrays being mutated and affecting following iterations.  That=
 explains the improvement I am seeing locally with your benchmark, although=
 I don't know why my results differ from what you posted

```
Comparison:
                         flatten!
          built-ruby:    246214.2 i/s
        compare-ruby:    193998.4 i/s - 1.27x  slower

                          flatten
          built-ruby:    214275.0 i/s
        compare-ruby:    188818.6 i/s - 1.13x  slower

                         flatten2
          built-ruby:    159840.8 i/s
        compare-ruby:    159546.4 i/s - 1.00x  slower
```

where only the last one actually benchmarks an unflattened array properly.

methodmissing (Lourens Naud=E9) wrote:
> Attached is a patch for early return and preventing additional allocs for=
 the empty array case (however not very sure how often that happens in prac=
tice):

There is no need to special case the empty array case.  My patch would just=
 treat it as a flat array, so would still avoid extra allocations and retur=
n early.

----------------------------------------
Feature #16119: Optimize Array#flatten and flatten! for already flattened a=
rrays
https://bugs.ruby-lang.org/issues/16119#change-81758

* Author: dylants (Dylan Thacker-Smith)
* Status: Assigned
* Priority: Normal
* Assignee: nobu (Nobuyoshi Nakada)
* Target version: =

----------------------------------------
## Problem

When doing an object profile from stackprof, I noticed object allocations b=
eing made from `Array#flatten!` which was unlike other in-place methods lik=
e `Array#uniq!` and `Array#compact!`.  In this case, I wanted to optimize f=
or the array already being flattened and found that `ary.flatten! if ary.an=
y?(Array)` was significantly faster.  The code confirmed my suspicion that =
`ary.flatten!` was almost equivalent to `ary.replace(ary.flatten)` in imple=
mentation. The object allocations I noticed were from a temporary result ar=
ray, a stack array to handle nesting and a hash table to prevent infinite r=
ecursion, all of which can be avoided in the simple case of an already flat=
tened array.

## Solution

Iterate over the array to find the first nested array.  If no nested array =
is found, then `flatten!` just returns `nil` without creating additional ob=
jects and `flatten` returns a shared copy of the original array.  If a nest=
ed array is found, then it creates and initializes the temporary objects to=
 resume with the existing algorithm for flattening the array.

## Benchmark

```ruby
require 'benchmark'

N =3D 100000

def report(x, name)
  x.report(name) do
    N.times do
      yield
    end
  end
end

arrays =3D {
  small_flat_ary: 5.times.to_a,
  large_flat_ary: 100.times.to_a,
  small_pairs_ary: [[1, 2]] * 5,
  large_pairs_ary: [[1, 2]] * 100,
}

Benchmark.bmbm do |x|
  arrays.each do |name, ary|
    report(x, "#{name}.flatten!") do
      ary.flatten!
    end
    report(x, "#{name}.flatten") do
      ary.flatten
    end
  end
end
```

results on the latest master (`ruby 2.7.0dev (2019-08-22T14:10:55Z master f=
d20b32130) [x86_64-darwin18]`)

```
                               user     system      total        real
small_flat_ary.flatten!    0.082001   0.000294   0.082295 (  0.082685)
small_flat_ary.flatten     0.078655   0.000211   0.078866 (  0.079176)
large_flat_ary.flatten!    0.552551   0.001643   0.554194 (  0.556166)
large_flat_ary.flatten     0.551520   0.001327   0.552847 (  0.554091)
small_pairs_ary.flatten!   0.100073   0.000302   0.100375 (  0.100687)
small_pairs_ary.flatten    0.109440   0.000232   0.109672 (  0.109850)
large_pairs_ary.flatten!   1.021200   0.001650   1.022850 (  1.024227)
large_pairs_ary.flatten    1.049046   0.002938   1.051984 (  1.055228)
```

results with the attached patch

```
                               user     system      total        real
small_flat_ary.flatten!    0.034868   0.000150   0.035018 (  0.035180)
small_flat_ary.flatten     0.040504   0.000148   0.040652 (  0.040914)
large_flat_ary.flatten!    0.458273   0.000786   0.459059 (  0.460005)
large_flat_ary.flatten     0.453846   0.000833   0.454679 (  0.455324)
small_pairs_ary.flatten!   0.055211   0.000205   0.055416 (  0.055673)
small_pairs_ary.flatten    0.060157   0.000226   0.060383 (  0.060540)
large_pairs_ary.flatten!   0.901698   0.001650   0.903348 (  0.905246)
large_pairs_ary.flatten    0.917180   0.001370   0.918550 (  0.920008)
```

---Files--------------------------------
0001-Optimize-Array-flatten-and-flatten-for-already-fl.diff.txt (2.79 KB)
0001-Let-empty-arrays-being-flattened-not-alloc-additiona.patch (791 Bytes)


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