[ruby-core:91660] [Ruby trunk Bug#15625] Module#name performance has exponential-time worst case by aliased constants

Issue #15625 has been updated by nobu (Nobuyoshi Nakada).

Subject changed from Module#name performance has exponential-time worst case to Module#name performance has exponential-time worst case by aliased constants
File bug-15625.patch added

Of course it is possible to check duplicated paths to refine worst cases, but it is a trade-off for usual case; no aliased constants.

### trunk
      |       user|     system|      total|        real
------+----------:|----------:|----------:|-----------:
before|   2.117049|   0.003000|   2.120049|(  2.123071)
a9    |   5.685656|   0.006082|   5.691738|(  5.697850)
a10   |   9.309634|   0.013036|   9.322670|(  9.336729)
a11   |  16.784519|   0.049488|  16.834007|( 16.883028)

### patched
      |       user|     system|      total|        real
------+----------:|----------:|----------:|-----------:
before|   3.161939|   0.055769|   3.217708|(  3.223522)
a9    |   3.317957|   0.046668|   3.364625|(  3.369195)
a10   |   3.316404|   0.045762|   3.362166|(  3.366455)
a11   |   3.321730|   0.048193|   3.369923|(  3.374927)


----------------------------------------
Bug #15625: Module#name performance has exponential-time worst case by aliased constants
https://bugs.ruby-lang.org/issues/15625#change-76922

* Author: nelhage (Nelson Elhage)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.6.1p33 (2019-01-30 revision 66950) [x86_64-darwin18]
* Backport: 2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN
----------------------------------------
It's well-known (c.f #11119) that `Module#name` has poor performance on anonymous classes, due to searching the entire constant namespace of the VM.

However, we recently discovered that it's even worse than that. In the presence of aliased constants (e.g. `module M; end; Alias = M`), the `find_class_path` method actually searches every *path* to a given module (e.g. it will visit that module under both the `M` and `Alias` names). This can lead to exponential-time behavior in cases of repeated aliases. See the attached test case, where performance gets twice as slow with every additional layer of aliasing:

On my laptop:

```
       user     system      total        real
before  2.183899   0.012151   2.196050 (  2.355187)
       user     system      total        real
a19  5.614596   0.024394   5.638990 (  5.702878)
       user     system      total        real
a10  9.204399   0.052817   9.257216 (  9.391010)
       user     system      total        real
a11 16.184035   0.060854  16.244889 ( 16.561101)
```

Our house style makes extensive use of aliases as an import-like mechanism to provide short names within a given scope. We discovered this bug while exploring an actual performance problem -- this is not just a theoretical report.

---Files--------------------------------
classname.rb (854 Bytes)
bug-15625.patch (1.38 KB)


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