[ruby-core:98978] [Ruby master Bug#16996] Hash should avoid doing unnecessary rehash

Issue #16996 has been reported by marcandre (Marc-Andre Lafortune).

----------------------------------------
Bug #16996: Hash should avoid doing unnecessary rehash
https://bugs.ruby-lang.org/issues/16996

* Author: marcandre (Marc-Andre Lafortune)
* Status: Open
* Priority: Normal
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
Pop quiz: Which is the fastest way to get a copy of a Hash `h`?

If, like me, you thought `h.dup` (of course, right?), you are actually wrong.

The fastest way is to call `h.merge`. Try it:

```
require 'benchmark/ips'

lengths = 1..50

h = lengths.to_h { |i| ['x' * i, nil] }

Benchmark.ips do |x|
  x.report("dup")        { h.dup }
  x.report("merge")      { h.merge }
end
```
I get
```
Calculating -------------------------------------
                 dup    259.233k (9.2%) i/s -      1.285M in   5.013445s
               merge    944.095k (ア 8.2%) i/s -      4.693M in   5.005315s
```

Yup, it's *3.5x faster* with this example!!

Why? Because `Hash#dup` does a rehash, and `merge` does not.

Pop quiz 2: which methods of `Hash` that produce a new hash do a rehash?

Answer: it depends on the method and on the Ruby version

```

+---------------------------------+------+-----+-----+-----+-----+-----+-----+-----+-----+
| Does this rehash?               | head | 2.7 | 2.6 | 2.5 | 2.4 | 2.3 | 2.2 | 2.1 | 2.0 |
+---------------------------------+------+-----+-----+-----+-----+-----+-----+-----+-----+
| h.dup / clone                   |  Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes |
+---------------------------------+------+-----+-----+-----+-----+-----+-----+-----+-----+
| h.select{true} / reject{false}  |  Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes | Yes |
+---------------------------------+------+-----+-----+-----+-----+-----+-----+-----+-----+
| h.select!{true} / reject!{false}|   リ  |  リ  |  リ  |  リ  |  リ  |  リ  |  リ  |  リ  |  リ  |
+---------------------------------+------+-----+-----+-----+-----+-----+-----+-----+-----+
| sub_h.to_h                      |   リ  |  リ  |  リ  |  リ  |  リ  |  リ  |  リ  |  リ  |  リ  |
+---------------------------------+------+-----+-----+-----+-----+-----+-----+-----+-----+
| h.merge({})                     |   リ  |  リ  |  リ  |  リ  | Yes | Yes | Yes | Yes | Yes |
+---------------------------------+------+-----+-----+-----+-----+-----+-----+-----+-----+
| h.merge                         |   リ  |  リ  |  リ  |             n/a                   |
+---------------------------------+------+-----+-----+-----+-----+-----+-----+-----+-----+
| h.transform_values(&:itself)    |   リ  |  リ  | Yes | Yes | Yes |          n/a          |
+---------------------------------+------+-----+-----+-----+-----+-----+-----+-----+-----+
(where `sub_h = Class.new(Hash).replace(h)`, リ = no rehash)
```

So in Ruby head, doing `h.merge({})` or even `h.transform_values(&:itself)` will be much faster than `h.dup` (but slower in Ruby 2.4) (*)

Notice that `select` rehashes, but `select!` doesn't, so the fastest way to do a `select` in Ruby is... not to call select and instead to actually do a `merge.select!`! (*)

*: on hashes with non-negligible hash functions

```ruby
class Hash
  def fast_select(&block)
    merge.select!(&block) # don't call dup because it's slow
  end
end

Benchmark.ips do |x|
  x.report("select")           { h.select{true} }
  x.report("fast_select")      { h.fast_select{true} }
end
```

On my test case above, `fast_select` is *2.5x faster* than `select`. `fast_select` will always return exactly the same result (unless the receiver needed a rehash).

Pop quiz 3: Is this a bug or a feature?

It should be clear that no feature of Ruby should be re-implementable in Ruby with a 3.5x / 2.5x speed gain, so many would think "of course it's a bug".

Well, https://bugs.ruby-lang.org/issues/16121 seems to think that `Hash#dup`'s rehash is a feature...
Why?
Because there is actually a test that `dup` does a rehash
Why?
Because a test of `Set` was failing otherwise!
Commit: https://github.com/ruby/ruby/commit/a34a3c2caae4c1fbd
Short discussion: http://blade.nagaokaut.ac.jp/cgi-bin/vframe.rb/ruby/ruby-core/48040?47945-48527
Actual test: https://github.com/ruby/ruby/blob/master/test/test_set.rb#L621-L625
Why?
This test construct a `Set` that needs to be rehashed (by mutating an element of the set after it is added), and then checks that `rehash_me == rehash_me.clone`.
That test is bogus. It passes for obscure and undocumented reasons, and `rehash_me.clone == rehash_me` doesn't pass.
Today, it is official that sets with elements that are later mutated must be `Set#reset`, so it is official that this should not be relied upon.

Probably more clear is the case of `select/reject` (but I didn't check for failing test), and even more clear that `merge` changed in Ruby 2.5 and `transform_values` in 2.7, but not a single `NEWS` file mentions the word "rehash".

My conclusion is that Hash should avoid doing an unnecessary rehash: `dup`/`clone`/`select`/`reject`. We probably should add a reminder in the `NEWS` that if anyone mutates a key of a Hash, or an element of a Set and does not call `rehash`/`reset`, improper behavior should be expected.

Let's make `Hash#dup/clone/select/reject` fast please.

Any objection?



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