[ruby-core:79826] [Ruby trunk Feature#12901] Anonymous functions without scope lookup overhead

Issue #12901 has been updated by Benoit Daloze.


Shyouhei Urabe wrote:
> I think it's impossible to optimize lambdas without breaking backwards compatibility.

It is possible with deoptimization though.
An optimizer might assume that a block does not call binding and in case it does, deoptimize.
Removing the scope lookup overhead for accessing captured variables
is possible if one considers a whole method containing the lambda for analysis.
However, it seems much harder to optimize the scope lookup for a lambda capturing a non-optimized scope.

I do not think scope lookup is the only overhead of lambdas though,
there is a lot of implicitly captured state which needs to be restored for every call
(but the same optimizations apply when optimizing a method containing the lambda).

----------------------------------------
Feature #12901: Anonymous functions without scope lookup overhead
https://bugs.ruby-lang.org/issues/12901#change-63261

* Author: Richard Schneeman
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
Right now if you are writing performance sensitive code you can get a performance bump by not using anonymous functions (procs, lambdas, blocks) due to the scope lookup overhead.

https://twitter.com/apotonick/status/791167782532493312?lang=en

I would like to be able to create an anonymous function and specify I don't need it to perform the lookup when called.

I am thinking that this code:

~~~
Proc.new(scope: false) {|var| puts var }
~~~

Would be the equivalent of 

~~~
def anon(var)
  puts var
end
~~~

If you call it while trying to access something from outside of the scope it would error

~~~
var = "hello"
Proc.new(scope: false) { puts var }.call
# => NameError: undefined local variable or method `var' for main:Object
~~~

An example of a case where this could be used is found in https://github.com/rails/sprockets/pull/417. In this case we are getting rid of anonymous functions in favor of a method created with refinements. This solution works but it was not an obvious optimization. It would be convenient to have a syntax for defining anonymous functions that do not need access to caller scope.






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