[ruby-core:66683] [ruby-trunk - Bug #10511] fix for #9940 causes dramatic performance regression

Issue #10511 has been updated by Koichi Sasada.


I understood.

First hash (Hash.new) contains empty st table.
Second hash ({}) does not have st table (NULL).

So the performance is different.

Making an entry for second hash like that:

```ruby
theHash = {}
theHash[Thread.current] = nil
```

also shows slow down:

```
ruby 2.2.0dev (2014-12-02 trunk 48684) [i386-mswin32_110]
  6.084000   0.000000   6.084000 (  6.107212)
```


----------------------------------------
Bug #10511: fix for #9940 causes dramatic performance regression
https://bugs.ruby-lang.org/issues/10511#change-50288

* Author: Oleg Sukhodolsky
* Status: Feedback
* Priority: Normal
* Assignee: Koichi Sasada
* Category: core
* Target version: current: 2.2.0
* ruby -v: ruby 2.1.3p242 (2014-09-19 revision 47630) [x86_64-darwin14.0]
* Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN
----------------------------------------
Originally reported against debase gem (https://github.com/denofevil/debase/issues/16)
After fix for #9940 running anything under debugger became about 4 times slower.
I've verified that we I revert the fix performance is restored.
To reproduce you could get https://github.com/railstutorial/sample_app_rails_4 and run rspec rake task under debugger (I've used RubyMine for this because debase is used as 
debugger's backend there)
Performance with 2.1.2 it takes about 20 seconds on my laptop, with 2.1.3 - almost 80 seconds :(
The same problem is observable with 2.1.4 and 2.1.5 too.




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