[ruby-core:94507] [Ruby master Feature#16121] Stop making a redundant hash copy in Hash#dup

[ruby-core:94507] [Ruby master Feature#16121] Stop making a redundant hash copy in Hash#dup

[dylan.smith]

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

----------------------------------------
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: 
----------------------------------------
## 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)


-- 
https://bugs.ruby-lang.org/

[ruby-core:94510] [Ruby master Bug#16121] Stop making a redundant hash copy in Hash#dup

[dylan.smith]

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)


-- 
https://bugs.ruby-lang.org/

[ruby-core:94983] [Ruby master Bug#16121] Stop making a redundant hash copy in Hash#dup

[ko1]

Issue #16121 has been updated by ko1 (Koichi Sasada).

Assignee set to ko1 (Koichi Sasada)

I'll review it.

----------------------------------------
Bug #16121: Stop making a redundant hash copy in Hash#dup
https://bugs.ruby-lang.org/issues/16121#change-81602

* Author: dylants (Dylan Thacker-Smith)
* Status: Open
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
* 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-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)


-- 
https://bugs.ruby-lang.org/

[ruby-core:95000] [Ruby master Bug#16121] Stop making a redundant hash copy in Hash#dup

[ko1]

Issue #16121 has been updated by ko1 (Koichi Sasada).


Thank you. Completely fine.
Do you want to merge via github PR or attached patch by my commit?

----------------------------------------
Bug #16121: Stop making a redundant hash copy in Hash#dup
https://bugs.ruby-lang.org/issues/16121#change-81628

* Author: dylants (Dylan Thacker-Smith)
* Status: Open
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
* 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-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)


-- 
https://bugs.ruby-lang.org/

[ruby-core:95002] [Ruby master Bug#16121] Stop making a redundant hash copy in Hash#dup

[ko1]

Issue #16121 has been updated by ko1 (Koichi Sasada).


I found that

> `    rb_define_method(rb_cArray, "initialize_copy", rb_ary_replace, 1);`

`Array#initialize_copy` == `Array#replace`. Can we use same technique?

----------------------------------------
Bug #16121: Stop making a redundant hash copy in Hash#dup
https://bugs.ruby-lang.org/issues/16121#change-81630

* Author: dylants (Dylan Thacker-Smith)
* Status: Open
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
* 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-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)


-- 
https://bugs.ruby-lang.org/

[ruby-core:95137] [Ruby master Bug#16121] Stop making a redundant hash copy in Hash#dup

[dylan.smith]

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


ko1 (Koichi Sasada) wrote:
> I found that
> 
> > `    rb_define_method(rb_cArray, "initialize_copy", rb_ary_replace, 1);`
> 
> `Array#initialize_copy` == `Array#replace`. Can we use same technique?

I tried doing that and it caused test failures because Hash#replace stop rehashing keys in ruby 2.6.  I've opened a bug for that: https://bugs.ruby-lang.org/issues/16187

I've opened https://github.com/ruby/ruby/pull/2489 which replaces the implementation of both of these methods with a single optimized implementation that always rehashes.

I benchmarked it with the following benchmark/hash_dup.yml file

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

benchmark:
  dup_small: small_hash.dup
  dup_larger: larger_hash.dup
  replace_small: '{}.replace(small_hash)'
  replace_larger: '{}.replace(larger_hash)'
loop_count: 100000
```

and here are the results against the latest ruby master commit

```
Comparison:
                        dup_small
          built-ruby:   4105090.4 i/s
        compare-ruby:   3089471.1 i/s - 1.33x  slower

                       dup_larger
          built-ruby:    682873.5 i/s
        compare-ruby:    518623.8 i/s - 1.32x  slower

                    replace_small
        compare-ruby:  10697475.5 i/s
          built-ruby:   9399379.9 i/s - 1.14x  slower

                   replace_larger
          built-ruby:    807767.5 i/s
        compare-ruby:    358383.1 i/s - 2.25x  slower
```

The `replace_small` case is slower due to rehashing, but is still much faster than on ruby version 2.5.6 that did rehash keys of small hashes:

```
                    replace_small
          built-ruby:   8847991.6 i/s
        compare-ruby:   2178174.7 i/s - 4.06x  slower
```

----------------------------------------
Bug #16121: Stop making a redundant hash copy in Hash#dup
https://bugs.ruby-lang.org/issues/16121#change-81776

* Author: dylants (Dylan Thacker-Smith)
* Status: Open
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
* 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-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)


-- 
https://bugs.ruby-lang.org/

[ruby-core:95452] [Ruby master Bug#16121] Stop making a redundant hash copy in Hash#dup

[ko1]

Issue #16121 has been updated by ko1 (Koichi Sasada).


Thank you, I merged it!

----------------------------------------
Bug #16121: Stop making a redundant hash copy in Hash#dup
https://bugs.ruby-lang.org/issues/16121#change-82202

* Author: dylants (Dylan Thacker-Smith)
* Status: Open
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
* 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-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)


-- 
https://bugs.ruby-lang.org/