Issue #16121 has been reported by dylants (Dylan Thacker-Smith).

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Feature #16121: Stop making a redundant hash copy in Hash#dup
https://bugs.ruby-lang.org/issues/16121

* Author: dylants (Dylan Thacker-Smith)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
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## Problem

I noticed while profiling object allocations that Hash#dup was allocating 2 objects instead of only 1 as expected.  I looked for alternatives for comparison and found that `Hash[hash]` created a copy with only a single object allocation and seemed to be more than twice as fast.  Reading the source code revealed the difference was that Hash#dup creates a copy of the Hash, then rehashes the copy.   However, rehashing is done by making a copy of the hash, so the first copy before rehashing was unnecessary.

## Solution

I changed the code to just use rehashing to make the copy of the hash to improve performance while also preserving the existing behaviour.

## Benchmark

```ruby
require 'benchmark'

N = 100000

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

hashes = {
  small_hash: { a: 1 },
  larger_hash: 20.times.map { |i| [('a'.ord + i).chr.to_sym, i] }.to_h
}

Benchmark.bmbm do |x|
  hashes.each do |name, hash|
    report(x, "#{name}.dup") do
      hash.dup
    end
  end
end
```

results on master

```
                      user     system      total        real
small_hash.dup    0.401350   0.001638   0.402988 (  0.404608)
larger_hash.dup   7.218548   0.433616   7.652164 (  7.695990)
```

results with the attached patch

```
                      user     system      total        real
small_hash.dup    0.336733   0.002425   0.339158 (  0.341760)
larger_hash.dup   6.617343   0.398407   7.015750 (  7.070282)
```

---Files--------------------------------
0001-Stop-making-a-redundant-hash-copy-in-Hash-dup.diff.txt (1.93 KB)


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