[ruby-core:58381] [ruby-trunk - Feature #9118] In Enumerable#to_a, use size to set array capa when possible

Issue #9118 has been updated by HonoreDB (Aaron Weiner).


Ah, right! This seems like an opportunity to improve on existing behavior: right now that just silently hangs forever. Do you think we should warn, then hang, or just raise? I'd lean towards the warn because it's possible size is returning the wrong thing.
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Feature #9118: In Enumerable#to_a, use size to set array capa when possible
https://bugs.ruby-lang.org/issues/9118#change-42978

Author: HonoreDB (Aaron Weiner)
Status: Open
Priority: Normal
Assignee: 
Category: 
Target version: 


Cross-post from https://github.com/ruby/ruby/pull/444.

Enumerable#to_a works by creating an empty array with small capacity, then populating it and expanding the capacity as it goes. For large enumerables, this causes several resizes, which can hurt performance. When an enumerable exposes a size method, we can guess that the resulting array's size will usually be equal to the enumerable's size. If we're right, we only have to set capacity once, and if we're wrong, we don't lose anything.

The attached file (or linked PR) adjusts enum.c's to_a method to take advantage of the size method when it's there. In my tests this makes Range#to_a about 10% faster, and doesn't have any significant effect on a vanilla enum with no size method. I couldn't find any existing benchmark that this consistently made better or worse.

If you like this idea, this could also be done in other classes with custom to_a, like Hash.


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