[ruby-core:116941] [Ruby master Bug#20301] `Set#add?` does two hash look-ups

["AMomchilov (Alexander Momchilov) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #20301 has been reported by AMomchilov (Alexander Momchilov).

----------------------------------------
Bug #20301: `Set#add?` does two hash look-ups
https://bugs.ruby-lang.org/issues/20301

* Author: AMomchilov (Alexander Momchilov)
* Status: Open
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
A common usage of `Set`s is to keep track of seen objects, and do something different whenever an object is seen for the first time, e.g.:

```ruby
SEEN_VALUES = Set.new
	
def receive_value(value)
	if SEEN_VALUES.add?(value)
		puts "Saw #{value} for the first time."
	else
		puts "Already seen #{value}, ignoring."
	end
end

receive_value(1) # Saw 1 for the first time.
receive_value(2) # Saw 2 for the first time.
receive_value(3) # Saw 3 for the first time.
receive_value(1) # Already seen 1, ignoring.
```

Readers might reasonably assume that `add?` is only looking up into the set a single time, but it's actually doing two separate look-ups! ([source](https://github.com/ruby/ruby/blob/c976cb5/lib/set.rb#L517-L525))

```rb
class Set
  def add?(o
    # 1. `include?(o)` looks up into `@hash`
    # 2. if the value isn't there, `add(o)` does a second look-up into `@hash`
    add(o) unless include?(o)
  end
end
```

This gets especially expensive if the values are large hash/arrays/objects, whose `#hash` is expensive to compute.

We can optimize this if it was possible to set a value in hash, *and* retrieve the value that was already there, in a single go. I propose adding `Hash#update_value` to do exactly that. If that existed, we can re-implement `#add?` as:

```rb
class Set
  def add?(o)
    # Only requires a single look-up into `@hash`!
    self unless @hash.update_value(o, true)
  end
```

Here's a PR: https://github.com/ruby/ruby/pull/10093

How much of a benefit this has depends on two things:

1. How much `#hash` is called, which depends on how many new objects are added to the set.
    * If every object is new, then `#hash` is called twice on every `#add?`. This is where this improvement makes the biggest (2x!) change.
    * If every object has already been seen, then `#hash` was never being called twice before anyway, so there would be no improvement
    * Every other case lies somewhere in between those two.
2. How slow `#hash` is to compute for the key
    * If the hash is slow to compute, this change will make a bigger improvement
    * If the hash value is fast to compute, then it won't matter as much. Even if we called it half as much, it's a minority of the total time, so it won't have much net impact.

Here is a summary of the benchmark results:

|                           | All objects are new | All objects are preexisting |
|---------------------------|-------:|------:|
| objects with slow `#hash` | 100.0% | ~0.0% |
| objects with fast `#hash` |  24.5% |  4.6% |





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