[ruby-core:63497] [ruby-trunk - Bug #10003] Hash#fetch performance issue

Issue #10003 has been updated by Eric Wong.


 Jeremy is correct, Hash#[] (and Array#[]) have optimized dispatch.
 I hardly see Hash#fetch used anywhere, and optimizing dispatch for
 uncommon cases hurts performance of common cases.
 
 P.S. even if Hash#fetch were optimized, it would still be slower
 because it has optional args to parse.

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Bug #10003: Hash#fetch performance issue
https://bugs.ruby-lang.org/issues/10003#change-47540

* Author: Richard Schneeman
* Status: Open
* Priority: Normal
* Assignee: 
* Category: core
* Target version: 
* ruby -v: 2.1.2
* Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN
----------------------------------------
It looks like there is a performance issue with Hash#fetch compared to Hash#[]. I found this issue via this gist: https://gist.github.com/jonleighton/3552829. I could not find it reported anywhere on the tracker, forgive me if this is a known issue:

```
require 'benchmark/ips'
 
h = { foo: :bar }
 
Benchmark.ips do |r|
  r.report('#[]')    { h[:foo] }
  r.report('#fetch') { h.fetch(:foo) }
end

Calculating -------------------------------------
                 #[]    124893 i/100ms
              #fetch    121729 i/100ms
-------------------------------------------------
                 #[]  6917857.4 (賊20.4%) i/s -   32472180 in   5.008832s
              #fetch  4570251.5 (賊18.8%) i/s -   22032949 in   5.015712s
```

This is in Ruby 2.1.2



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