Misc #16188


What are the performance implications of the new keyword arguments in 2.7 and 3.0?

Added by Eregon (Benoit Daloze) over 2 years ago. Updated over 2 years ago.



In #14183, keyword arguments became further separated from positional arguments.

Contrary to the original design though, keyword and positional arguments are not fully separated for methods not accepting keyword arguments.
Example: foo(key: :value) will def foo(hash) will pass a positional argument.
This is of course better for compatibility, but I wonder what are the performance implications.

The block argument is completely separate in all versions, so no need to concern ourselves about that.

In Ruby <= 2.6:

  • The caller never needs to know about the callee's arguments, it can just take all arguments and pass them as an array.
    The last argument might be used to extract keyword, but this is all done at the callee side.
  • Splitting kwargs composed of Symbol and non-Symbol keys can be fairly expensive, but it is a rare occurrence.
    If inlining the callee and kwargs are all passed as a literal Hash at the call site, there shouldn't be any overhead compared to positional arguments once JIT'ed.

In Ruby 2.7:

  • The caller needs to pass positional and keyword arguments separately, at least when calling a method accepting kwargs.
    But, if it calls a methods not accepting kwargs, then the "kwargs" (e.g. foo(key: :value)) should be treated just like a final Hash positional argument.
  • (If we had complete separation, then we could always pass positional and keyword arguments separately, so the caller could once again ignore the callee)

How is the logic implemented in MRI for 2.7?

Specializing the caller for a given callee is a well-known technique.
However, it becomes more difficult if different methods are called from the same callsite (polymorphic call), especially if one accepts kwargs and another does not.
In that case, I think we will see a performance cost to this approach, by having to pass arguments differently based on the method to be called.

What about delegation using ruby2_keywords?
Which checks does that add (compared to 2.6) in the merged approach with the Hash flag?

Related issues 4 (2 open2 closed)

Related to Ruby master - Feature #14183: "Real" keyword argumentClosedActions
Related to Ruby master - Misc #16157: What is the correct and *portable* way to do generic delegation?OpenActions
Related to Ruby master - Feature #16897: General purpose memoizer in Ruby 3 with Ruby 2 performanceOpenActions
Related to Ruby master - Feature #16463: Fixing *args-delegation in Ruby 2.7: ruby2_keywords semantics by default in 2.7.1Closedmatz (Yukihiro Matsumoto)Actions

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