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

Backport set to 2.5: UNKNOWN, 2.6: UNKNOWN
ruby -v set to ruby 2.7.0dev (2019-08-23T16:41:09Z master b38ab0a3a9) [x86_64-darwin18]
Tracker changed from Feature to Bug
File 0004-Stop-making-a-redundant-hash-copy-in-Hash-dup.diff.txt added
File 0003-Remove-dead-code-paths-in-rb_hash_initialize_copy.diff.txt added
File 0002-Fix-freeing-and-clearing-destination-hash-in-Hash.diff.txt added
File 0001-Remove-redundant-Check_Type-after-to_hash.diff.txt added

I split up the patch into the 4 attached patches, because I realized there were a few changes that could be emphasized independently, each with their own description of the change.  Hopefully that will make the optimization itself in the last patch easier to review.

Technically that makes this a bug fix, since "Patch 2/4" does fix a memory leak in the following contrived code

```ruby
h = 9.times.map { |i| [i, i] }.to_h
h.send(:initialize_copy, {})
```

where the st_table for the hash `h` won't get freed with `st_free_table` because it doesn't try to call that on the `if (RHASH_AR_TABLE_P(hash2)) {` conditional block in `rb_hash_initialize_copy`.  Mostly I didn't want to ignore it and turn into undefined behaviour from the optimization.

----------------------------------------
Bug #16121: Stop making a redundant hash copy in Hash#dup
https://bugs.ruby-lang.org/issues/16121#change-80942

* Author: dylants (Dylan Thacker-Smith)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.7.0dev (2019-08-23T16:41:09Z master b38ab0a3a9) [x86_64-darwin18]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN
----------------------------------------
## 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)
0001-Remove-redundant-Check_Type-after-to_hash.diff.txt (624 Bytes)
0002-Fix-freeing-and-clearing-destination-hash-in-Hash.diff.txt (1.57 KB)
0003-Remove-dead-code-paths-in-rb_hash_initialize_copy.diff.txt (1.12 KB)
0004-Stop-making-a-redundant-hash-copy-in-Hash-dup.diff.txt (1.35 KB)


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