[ruby-core:78485] [Ruby trunk Bug#13002] Hash calculations no longer using universal hashing

Issue #13002 has been updated by Vladimir Makarov.


Nobuyoshi Nakada wrote:
> `test_wrapper_of_special_const` failed, that is, it's impossible to emulate switching weak and strong versions under the hood.
> I think this behavior is quirky.

You are right, the behavior with strong/weak hashes is wrong for Ruby.  This test is not guaranteed to work.  After reading the documentation thoroughly, I got a conclusion that hash value used for the hash tables should be *always* the same.  So the hash tables can not use two different hash functions at all.

Although strong/weak hash approach gave about 5-6% improvement out of 45% on Ruby hash table benchmarks, I think we should not use it for Ruby.  I will provide a patch to get rid of it in 2 days.

Sorry for inconvenience.


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Bug #13002: Hash calculations no longer using universal hashing
https://bugs.ruby-lang.org/issues/13002#change-61863

* Author: Martin D端rst
* Status: Open
* Priority: Normal
* Assignee: 
* ruby -v: 
* Backport: 2.1: DONTNEED, 2.2: DONTNEED, 2.3: DONTNEED
----------------------------------------
When preparing for my lecture on hash tables last week, I found that Ruby trunk doesn't do universal hashing anymore. See http://events.ccc.de/congress/2011/Fahrplan/attachments/2007_28C3_Effective_DoS_on_web_application_platforms.pdf for background.

I contacted [e-mail], but was told by Shugo that because trunk is not a published version, we can talk about it publicly.

Shugo also said that the change was introduced in r56650.

Following is some output from two different versions of Ruby that show the problem:

On Ruby 2.2.3, different hash value for the same number every time Ruby is restarted:

C:\Users\duerst>ruby -v
ruby 2.2.3p173 (2015-08-18 revision 51636) [i386-mingw32]

C:\Users\duerst>ruby -e 'puts 12345678.hash'
611647260

C:\Users\duerst>ruby -e 'puts 12345678.hash'
-844752827

C:\Users\duerst>ruby -e 'puts 12345678.hash'
387106497

On Ruby trunk, always the same value:

duerst@Arnisee /cygdrive/c/Data/ruby
$ ruby -v
ruby 2.4.0dev (2016-12-02 trunk 56965) [x86_64-cygwin]

duerst@Arnisee /cygdrive/c/Data/ruby
$ ruby -e 'puts 12345678.hash'
1846311797112760547

duerst@Arnisee /cygdrive/c/Data/ruby
$ ruby -e 'puts 12345678.hash'
1846311797112760547

duerst@Arnisee /cygdrive/c/Data/ruby
$ ruby -e 'puts 12345678.hash'
1846311797112760547




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