Issue #16188 has been updated by Eregon (Benoit Daloze).


Here is another benchmark, where no keyword arguments are used, yet we see a slowdown of up to 11.5% in MRI 2.7:
https://gist.github.com/eregon/31e155901c995925bd1c661dfa1a71d8

`length([1], [2], [3])` and `length({a: 1}, {b: 2}, {c: 3})` are essentially the baselines.
On TruffleRuby, those two incur no overhead because they cause no allocation (escape analysis) and
the JIT can see arguments don't have the `ruby2_keywords` flag.

The other cases cannot be optimized that way, and need to check for the `ruby2_keywords` flag.

TruffleRuby:
* `length(*ARRAYS)`: 5.9% slower
* `length(*HASHES)`: 4.4% slower

MRI 2.7.0preview3 vs MRI 2.6.5:
* `length(*ARRAYS)`: 2.7 is 11.5% slower
* `length(*HASHES)`: 2.7 is 10.3% slower

Do we want all `foo(*args)` calls to get that overhead in Ruby 3+?

I think we should either:
* Remove `ruby2_keywords` in Ruby 3.0, just have it in Ruby 2.7 where it's needed. Ruby 3.0 (with the keyword arg separation) doesn't need `ruby2_keywords`.
* Combine `ruby2_keywords` with `send_keyword_hash`, which solves the performance issue and is explicit, therefore improving readability and debug-ability.
* Use another way for delegation in Ruby 2.7 (e.g. the lexical `pass_keywords` or `...`, see https://eregon.me/blog/2019/11/10/the-delegation-challenge-of-ruby27.html )

cc @matz @mame @jeremyevans0

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Misc #16188: What are the performance implications of the new keyword arguments in 2.7 and 3.0?
https://bugs.ruby-lang.org/issues/16188#change-82767

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
* Assignee: jeremyevans0 (Jeremy Evans)
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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?



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