[ruby-core:117177] [Ruby master Bug#20301] `Set#add?` does two hash look-ups

["Eregon (Benoit Daloze) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #20301 has been updated by Eregon (Benoit Daloze).


That implementation using `size` is not thread-safe, even on CRuby AFAIK.
For example, if T2 calls `add?` with a new element while T1 calls `add?` with an existing element.
If T1 is just before `m = size` when T2 executes `add(o)`, then both threads return "element added" but T1 did not add an element (incorrect result).

The original code with two lookups does not have that race condition.
However it can have the race condition that two threads adding the same new element both return "element added".
`Hash#exchange_value` would fix that.

----------------------------------------
Bug #20301: `Set#add?` does two hash look-ups
https://bugs.ruby-lang.org/issues/20301#change-107267

* 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#exchange_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.exchange_value(o, true)
  end
```

Here's a proof-of-concept implementation: https://github.com/ruby/ruby/pull/10093

# Theory

How much of a benefit this has depends on 2 factors:

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` used to be 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.
        * It's important to not regress in this case, because many use cases of sets don't deal with many distinct objects, but just need to do quick checks against an existing set.
    * Every other case lies somewhere in between those two, depending on the % of objects which are new.
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.

# Benchmark summary

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

As we see, this change makes a huge improvement the cases where it helps, and crucially, doesn't slow down the cases where it can't.

For the complete benchmark source code and results, see the PR: https://github.com/ruby/ruby/pull/10093



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