On the other hand, have a look at Dylan, as it might inspire you:
Something possible in AOT as well to certain extent, but it requires a mix of profile guided optimizations coupled with whole programm analysis.
Which have issues with dll/so anyway, as those calls cannot be optimized away as in JITs.
That's what I referred to with "inline caches". The problem is that for Ruby you need fully polymorphic inline caches, with guards all over the place, because unless you do tons of analysis upfront, you will have problems knowing whether or not the world has totally changed on you after any method call, and almost anything is a method call. (call into code you have not verified can't possibly call "eval", and you might find that adding two integers afterwards does in fact not add them, but returns a string and changes global variables, and what-not)
The upshot, is that compared to vtables, you're not actually saving all that much. E.g., take "1 + 2 - 3". You could inline Fixnum#+ (and could reasonably do so with an AOT compiler too). But you need to add a type guard before the inlined fragment to verify that Fixnum#+ still is the Fixnum#+ you inlined, which at the minimum costs you a comparison and a branch or you need to record every call-site with inlined code and be prepared to overwrite it with fixups if the implementation changes.
And if Fixnum#+ has been overridden, or the Fixnum#+ implementation has method calls, chances are you will need another guard before "-" too, because you might not even know for sure whether or not the object returned from "1 + 2" will be a Fixnum, so you might find that the inlined method suddenly is for the wrong class.
I'm planning on benchmarking inline caching for my compiler against vtables, but absent evidence to the contrary I'm expecting that there will be a very substantial number of cases where the complexity isn't worth it, or where they might even turn out to be slower.
> Something possible in AOT as well to certain extent, but it requires a mix of profile guided optimizations coupled with whole programm analysis.
It does if you want to do everything upfront, but you can pull things into inline caches with a mostly-AOT compiler relatively easily with just a little bit of extra information, and a few guards thrown in to do some basic tracing.
For example, mapping a few pages for vtables/method dictionaries read only. When something like `def` or `define_method` comes along, catch the segfault (which in this case would actually mean "segmentation fault" instead of "I fucked up") and rewrite all JIT blocks or method caches that depend on that method table. Once everything has settled, generally after startup and the vtables tend to stay more stable, the overhead seems like it'd be negligible.
The tricky bit is if you have gone as far as inlining the method.
Another way to handle this is to assume that Fixnum#+ hasn't changed when compiling a method that is using it (maybe add a check at method entry); but when it does get redefined you "deoptimize" the methods that you compiled while holding that assumption.
> or you need to record every call-site with inlined code and be prepared to overwrite it with fixups if the implementation changes.
Compilers was one of the main focus on my CS degree, so I am really into this type of discussions.
Good luck for the project.
It's really fascinating, and what fascinates me in particular with Ruby is exactly that once you start looking into it, there are new problems around every corner, and trying to make it as "ahead of time as possible" makes it even trickier - I absolutely agree with you that there are parts that are much easier to do if you JIT, though, and I'll have to go there anyway to handle "eval"..
Currently my feeling is that most languages could benefit from both.
A JIT like environment for live coding and portable deployment.
And a AOT one for certain types of deployment where thin runtimes are desired.
For starters, for most method calls there's no reason to do the expensive method lookups that MRI still uses - cache or no cache - you can use C++ style vtables, as long as you propagate updates to them downwards when a method is re-defined. You do need to be able to fall back to handle dynamically created methods with names not present when you generate the vtables, and optionally reduce waste (as the vtables needs to be the same size for all classes, with unimplemented methods replaced with pointers to method_missing thunks), but in terms of performance you can do fairly well and compared to this GC blowup, the memory waste would be small.
But there's also not much alternative but going for proper JIT'ing of at least some things.