I agree - I think MIR is trying to produce better machine code more quickly, but that isn't the problem Ruby faces! The problem Ruby faces is having to inline through ten levels of metaprogramming in order to get any meaningfully-sized compilation unit that you can optimise, and only then it makes sense to worry about code generation.
However, the reason that this is not so simple is that Ruby is a vastly more complicated language than JavaScript (I've worked on implementing both.) Ruby has an enormous standard library and most Ruby programs are just endless calls to the that library, so your compiler must be able to understand the library semantically, either by rewriting it in Ruby (not likely at scale in MRI) or by adding tens of thousands of individually optimised intrinsics (again not likely).
Ruby is digging itself further into a local optima with these approaches optimisations, rather than looking further around for a better global optima.
Your description of optimizing Ruby sounds like it is exactly like optimizing JS. JSC’s main optimizing IR (the DFG) is all about understanding the standard library semantically. We write the standard library in JS+hacks (called “builtins”) or C++ (pick on a per function basis). Some of those functions have opcodes in DFG, or are built out of primitives that have their own specialized opcodes in DFG.
Maybe the reason why Ruby optimization has problems is nobody has done it the JSC way.
> Your description of optimizing Ruby sounds like it is exactly like optimizing JS.
Yes it's the same problem and it's unique to neither Ruby nor JS - but the problem is just scale.
Ruby has a larger library, so needs more rewritten from the current C into Ruby+builtins. And then that rewrite doesn't maintain Ruby semantics (the C API currently used does not exactly match Ruby semantics) so a rewrite matching semantics is often very hard.
> Maybe the reason why Ruby optimization has problems is nobody has done it the JSC way.
People have been trying the approach you use in Core (for over a decade, starting with Rubinius, now TruffleRuby and others), but I think (based on practical experience working on optimising both languages) that it's just a larger problem in Ruby which is why it hasn't been conquered yet.
I think that the way you’re describing the solution for Ruby tells me that you don’t see the problem the way that I see it. The problem isn’t rewriting things in builtins. The problem is making an IR in which you can reason about the standard library at scale: reason about its speculation opportunities, all of the implied dependencies and effects, it’s GC impacts, etc.
None of the IRs I’ve seen people try for Ruby does that. JSC’s DFG IR does a lot of this. Hence I don’t think folks have really tried the JSC approach for Ruby.
Forgot to mention: baseline JIT with PICs. I keep hearing about sophisticated top tier JITs but that won’t do shit if you don’t have a solid baseline JIT.
> The C++ function is understood by the JIT so well that it just emits the code for it itself.
But how would that work for Ruby, where C functions on the critical performance path are often third-party code that the compiler author has never seen before?
We'd need to let the JIT understand third-party unseen C functions. There are experiments to do that (Sulong, MIR, Rubinius sort of tried it) but I think it's more of an open problem than you're implying.
If you treat calls to unknown third-party C functions as an opaque native call then you're really going to struggle to build a meaningful compilation unit, in my experience.
It looks like the author is intending to use a C to MIR compiler so that existing CRuby code can be inlined into generated MIR code. Third party code could be compiled the same way, right?
"The blue parts show the new data-flow for MJIT. When building CRuby, we could generate MIR code for the standard Ruby methods written in C. We can load this MIR code as a MIR binary. This part could be done very quickly."
That's what MIR is doing, not what JavaScriptCore is doing.
Flip thinks this isn't needed for Ruby - '[t]his project is trying to do too many things' - and that JavaScriptCore could do it already.
I think that MIR and TruffleRuby think they have to do something else (lifting C code into their IR) shows us that JavaScriptCore's approach isn't quite as immediately applicable as Filip thinks it is.
I buy that third party native code is critical path.
I buy that native code often calls back to Ruby.
I get why you would assume that therefore you need to make it fast for third party native code to call into Ruby. But that’s not how you want to think to succeed at VM optimizations.
You can make native code fast. It’s probably already about as fast as it’s going to be. Design a VM that makes it continue to be fast. Truffle won’t give you that since it will have to DBT the native code to make it fast (and that’s the good case).
I would bet you that first party native code that calls into Ruby is by far the most common kind of yield invocation. Like Array#each/map and equivalents for Hash. You want to treat those specially for two reasons:
- their fastest path for baseline code if done the way I describe is faster than any alternative. Baseline isn’t going to have a chance to inline arbitrary functions.
- they are likely to make up a large fraction of cases where native calls back to a Ruby.
For third parties, there’s a future where someone just exposes the JITing API that JSC gives to the DOM.
Well I'm not sure what you think you're referring to then.
Do you mean an IR where all core library routines are first-class citizens, with their own nodes and information about their semantics encoded so that the compiler can reason about them?
With the caveat that core routines that are very cool don’t get included no matter how simple they are and warm/hot ones get opcodes even if it’s very annoying to do it.
Right, so I get that, and I think so do other people in the small Ruby compiler community. It's just a much larger problem in Ruby due to the size and complexity of the standard library and the presence of more non-local effects (like accessing dynamic caller frames is often required). It's possibly so large that it's not reasonable to do it like that and people are looking for other options, like Truffle's PE. That's why it hasn't been achieved yet - it's not because anyone is unaware of common VM techniques.
Another big problem JavaScript doesn't have is that user C extensions are very often on the critical path in Ruby, so you must be able to optimise through them somehow. I don't think JavaScriptCore has any solutions for that we should be trying?
Chris, JSC’s IRs have extensive support for accessing non local callframes and doing a great job on effects generally. JS has _lots_ of this kind of nonsense in real code and the language supports it at least as much as Ruby does (like in super evils ways, like function.arguments).
JSC has extensive support for fast calls into native code because of the DOM. For example we have the Snippet JIT that allows the DOM to turn hot functions into almost first class compiler ops. Like, callbacks in the DOM can dictate codegen and effect analysis.
Josh, the canonical JSC approach to the iterator callback problem wouldn’t be to use builtins. That wouldn’t necessarily achieve great perf. It’s certainly not great for the baseline tier. I think the only good option is to make those iterators (like Array#each) be intrinsic as fuck: all call ICs can ask for inline machine code generation of the loop along with the call back to the passed in block. You could imagine this enabling inlining of the loop and its body even in the baseline JIT.
I get that people probably see the builtins in JSC and start having wild fantasies about what this can achieve. In reality for things where perf matters, you deploy ICs and custom template codegen.
I don't really see a good solution here other than lifting the control flow to Ruby with specific bytecode ops to minimize overhead and a good, typed FFI like LuaJIT has? Am I missing something?
Could establish an interface for those extensions to participate in the JIT and ICs. JSC does that for the DOM.
Other than that just try to make calls into JITed code as fast as possible. But that needs to be the backup plan, with the main plan being that perf-sensitive extensions play with the JIT.
But we have no control over these C extensions. And there's a huge corpus of existing C extensions that need to keep working. 500 million lines of C and C++, in fact, even for just the publicly available code.
And by the way - there isn't really any proper interface at all! There's just basically the whole internals of the C Ruby implementation exposed to C extension authors. There's no abstraction with handles and things like that.
There was an effort at one point to get C extension authors to use the FFI instead, and a JNI-style API to permit a moving garbage collector was proposed, but these approaches weren't successful in gaining any momentum.
All in all... that's why we don't just do it the JavaScriptCore way. We have different constraints to yours.
You don’t need to make all native extensions fast. You need to make some of the most common things fast and keep the rest of them working. It’s fine if not all extensions adopt a JIT interface.
The thing about moving GC is a red herring. Ruby wouldn’t benefit from it. JSC doesn’t use moving GC.
I think for CRuby to use the JSC approach we would need to translate methods written entirely in C which opaquely iterate over collections and invoke blocks of Ruby into Ruby which uses bytecodes to get an iterator and then iterate?
It looks like Mir's approach to this is to build a jit compiler that has access to the C source of the standard library and can inline it into the JITted code of Ruby functions, and vice versa. Building a compiler that can handle mixed-language input might be easier than rewriting the standard Library to all be in one language.
"inlining everything" is actually maybe not the best approach. It's been talked about a lot, but it's not always helpful. For example, C compilers (or LLVM) are unable to optimize boxing and then unboxing a double into a Ruby Flonum and back.
However, the reason that this is not so simple is that Ruby is a vastly more complicated language than JavaScript (I've worked on implementing both.) Ruby has an enormous standard library and most Ruby programs are just endless calls to the that library, so your compiler must be able to understand the library semantically, either by rewriting it in Ruby (not likely at scale in MRI) or by adding tens of thousands of individually optimised intrinsics (again not likely).
Ruby is digging itself further into a local optima with these approaches optimisations, rather than looking further around for a better global optima.