[ruby-core:112242] [Ruby master Feature#19420] Simplify MJIT implementation

Issue #19420 has been updated by ioquatix (Samuel Williams).


I'm very happy about this, it sounds like a good path forward.

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Feature #19420: Simplify MJIT implementation
https://bugs.ruby-lang.org/issues/19420#change-101667

* Author: k0kubun (Takashi Kokubun)
* Status: Open
* Priority: Normal
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# Background
### Implementation complexity
Because MJIT needs to run a C compiler and be able to call almost everything in vm.c, it has brought MJIT-specific complexity to the Ruby VM implementation:

* GVL and waitpid hacks for `Process.waitall`
  * @ko1 and @ioquatix have had problems dealing with it.
* `vm_base_ptr` dependency from MJIT
  * @ko1 wanted to remove `cfp->__bp__` commit:8dd0fb9039bbe6152ea5542e6bc70de152871e23
  * This is technically not the fault of the MJIT design, but the current implementation relies on it.
* MJIT header and `MJIT_HEADER` macro
  * It seems like some build dependency is missing for it and my local build with `-j` randomly fails on MJIT header build.
* MJIT-specific frame handling
  * `catch_except_p` flag in ISeq
	* `jit_enable_p` flag in `vm_exec`
* `MJIT_FUNC_EXPORTED` functions

### Third-party JIT experiments
Despite YJIT being the only non-experimental JIT compiler in CRuby from Ruby 3.2, people still experiment with other JIT implementations, e.g. [vnmakarov/ruby (sir-mirjit)](https://github.com/vnmakarov/ruby), [tenderlove/tenderjit](https://github.com/tenderlove/tenderjit), [jhawthorn/hawthjit](https://github.com/jhawthorn/hawthjit), which I think is a good thing for the future of Ruby, and for YJIT as well.

In Ruby 3.2, MJIT was rewritten in Ruby. It also meant that you could replace CRuby's JIT implementation by monkey-patching MJIT, which became the first ever "third-party JIT backdoor". I know two people started using it for that purpose. I want people (including myself through MJIT) to keep experimenting with Ruby performance, so I don't want to just get rid of this. `RubyVM::MJIT::C` and `RubyVM::MJIT::Hooks` are something that third-party gems can't easily provide. They should be considered as something similar to JVMCI (Java's JIT interface) whether we make them private or not.

# Proposal
* Change MJIT to a simple JIT implementation that doesn't rely on a C compiler at runtime, which works like YJIT (to remove all the complexity mentioned above) but is written in Ruby (to keep offerring modules for third-party JITs).
  * I have [a PoC branch](https://github.com/k0kubun/ruby/compare/07c19cf551e58e1b82af5efeb0047f10588fff9f...5af15fb59bdae3f5e70c7742abc3b4d1e423e0e8), which needs rebase and cleanup but passes `make check`. It currently supports very limited operations and is not really useful, but I intend to keep improving this outside my work hours (that I dedicate to YJIT), sometimes experimenting with things that are not done in YJIT. Note that we could still solve the above problems even with the current patch.
  * It shouldn't increase the maintenance cost of CRuby too much. You're free to just mark something "can't compile" in MJIT when VM is changed. It should also not support platforms that YJIT doesn't support.
  * It's also kind of useful in the sense that it requires neither a C compiler at runtime nor a Rust compiler at build time.
  * Note that part of the implementation cannot be implemented as a gem. `RubyVM::MJIT::C` (that uses `Primitive`) and `RubyVM::MJIT::Hooks` (that are hooked from VM internals) must be part of the interpreter. I'm open to making other parts a gem if other people want me to, but it might come with a maintainability trade-off because of that interpreter dependency.



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