[ruby-core:117263] [Ruby master Feature#20351] Optionally extract common GC routines into a DSO

["eightbitraptor (Matthew Valentine-House) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #20351 has been reported by eightbitraptor (Matthew Valentine-House).

----------------------------------------
Feature #20351: Optionally extract common GC routines into a DSO
https://bugs.ruby-lang.org/issues/20351

* Author: eightbitraptor (Matthew Valentine-House)
* Status: Open
----------------------------------------
[Github PR#10302](https://github.com/ruby/ruby/pull/10302)

**NOTE: This proposal does not change the default build of Ruby, and therefore
should NOT cause performance degradation for Ruby built in the usual way**

Our long term goal is to standardise Ruby's GC interface, allowing alternative
GC implementations to be used. This will be acheived by optionally building
Ruby's GC as a shared object; enabling it to be replaced at runtime using using
`LD_LIBRARY_PATH`. eg:

```
LD_LIBRARY_PATH=/custom_gc_location ruby script.rb
```

This ticket proposes the first step towards this goal. A new experimental build
option, `--enable-shared-gc`, that will compile and link a module into the built
`ruby` binary as a shared object - `miniruby` will remain statically linked to
the existing GC in all cases.


Similar methods of replacing functionality relied on by Ruby have
precedent. `jemalloc` uses `LD_PRELOAD` to replace `glibc` provided `malloc` and
`free` at runtime. Although this project will be the first time a technique such
as this has been used to replace core Ruby functionality.

This flag will be marked as experimental & **disabled by default**.

[The PR linked from this ticket](https://github.com/ruby/ruby/pull/10302) implements the new build flag, along with the
absolute minimum code required to test it's implementation (a single debug
function).

The implementation of the new build flag is based on the existing implementation
of `--enable-shared` and behaves as follows:

- `--enable-shared --enable-shared-gc`
  
  This will build both `libruby` and `librubygc` as shared objects. `ruby` will
  link dynamically to both `libruby` and `librubygc`.
  
- `--disable-shared --enable-shared-gc`

  This will build `librubygc` as a shared object, and build `libruby` as a
  static object. `libruby` will link dynamically to `librubygc` and `ruby` will
  be statically linked to `libruby`.
  
- `--disable-shared-gc`

  **This will be the default**, and when this case is true the build behaviour
  will be exactly the same as it is currently. ie. the existing Ruby GC will be
  built and linked statically into either `ruby` or `libruby.so` depending on
  the state of `--enable-shared`.
  
We are aware that there will be a small performance penalty from moving the GC
logic into a shared object, but this is an opt-in configuration turned on at
build time intended to be used by experienced users. 

Still, we anticipate that, even with this configuration turned on, this penalty
will be negligible compared the the benefit that being able to use high
performance GC algorithms will provide.

This performance penalty is also the reason that **this feature will be disabled
by default**. There will be no performance impact for anyone compiling Ruby in
the usual manner, without explicitly enabling this feature.

We have discussed this proposal with @matz who has approved our work on this
project - having a clear abstraction between the VM and the GC will enable us to
iterate faster on improvements to Ruby's existing GC.

## Motivation

In the long term we want to provide the ability to override the current Ruby GC
implementation in order to:

* Experiment with modern high-performance GC implementations, such as Immix, G1,
  LXR etc.
* Easily split-test changes to the GC, or the GC tuning, in production without
  having to rebuild Ruby
* Easily use debug builds of the GC to help identify production problems and
  bottlenecks without having to rebuild Ruby
* Encourage the academic memory management research community to consider Ruby
  for their research (the current work on [MMTk & Ruby]() is a good example of
  this).

## Future work

The initial implementation of the shared GC module in this PR is deliberately
small, and exists only for testing the build system integration.

The next steps are to identify boundaries between the GC and the VM and begin to
extract common functionality into this GC wrapper module to serve as the
foundation of our GC interface.

## Who's working on this
  
- Matt Valentine-House (@eightbitraptor) 
- Aaron Patterson (@tenderlove)
- Peter Zhu(@peterzhu2118)
- Eileen Uchitelle(@eileencodes)



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