[ruby-core:113269] [Ruby master Bug#4040] SystemStackError with Hash[*a] for Large _a_

Issue #4040 has been updated by Eregon (Benoit Daloze).


jeremyevans0 (Jeremy Evans) wrote in #note-24:
> So if anything, the patch encourages users not to pass large arrays as splats, as doing so is even worse for performance than before.

Thank you for the precision. Then at least it's not going to encourage large splats due to changed performance, good.
Of course it remains a concern that users/code start to depend on this i.e., on no SystemStackError in calls with a large splat, and then changing that back would be a breaking change.
But maybe it's rare and stays rare enough that it might be possible to change it again (e.g. if valuable enough for performance), unsure.

> We may want to consider adding a performance warning for passing large arrays via splats.

That's a good idea.

----------------------------------------
Bug #4040: SystemStackError with Hash[*a] for Large _a_
https://bugs.ruby-lang.org/issues/4040#change-102831

* Author: runpaint (Run Paint Run Run)
* Status: Open
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
* ruby -v: ruby 1.9.3dev (2010-11-09 trunk 29737) [x86_64-linux] 
* Backport: 2.2: UNKNOWN, 2.3: UNKNOWN, 2.4: UNKNOWN
----------------------------------------
=begin
 I've been hesitating over whether to file a ticket about this, so please feel free to close if I've made the wrong choice.
 
 I often use Hash[*array.flatten] in IRB to convert arrays of arrays into hashes. Today I noticed that if the array is big enough, this would raise a SystemStackError. Puzzled, I looked deeper. I assumed I was hitting the maximum number of arguments a method's argc can hold, but realised that the minimum size of the array needed to trigger this exception differed depending on whether I used IRB or not. So, presumably this is indeed exhausting the stack...
 
 In IRB, the following is the minimal reproduction of this problem:
 
   Hash[*130648.times.map{ 1 }]; true
 
 I haven't looked for the minimum value needed with `ruby -e`, but the following reproduces:
 
   ruby -e 'Hash[*1380888.times.map{ 1 }]'
 
 I suppose this isn't technically a bug, but maybe it offers another argument for either #666 or an extension of #3131.
=end




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