Re: [Caml-list] LLVM OCaml bindings

[Peter Zotov <whitequark@whitequark.org>]

Jeff Meister писал 06.11.2013 01:00:
> I can call const_int to get an llvalue representing an integer, then
> pass this llvalue as the first argument to build_call, which expects a
> function. Since both have type llvalue, the OCaml type checker will
> not complain, but a segfault or other undefined behavior will likely
> result at runtime.

As mentioned elsewhere, it's defined in Asserts build. Personally,
I'm going to ship my compiler with LLVM built in Release+Asserts mode.

> What I did to fix this was reintroduce the inheritance hierarchy
> present in the C++ code. Using some module system tricks to make
> OCaml's structural subtyping act like nominal subtyping, I refactored
> everything into classes. In my version, const_int would return a
> ConstInt.t, which is a subtype of Value.t, but not compatible with the
> Function.t required by build_call.

One thing to consider is that the C++ API does not use subtyping for
this kind of safety, either. Consider the prototype of
IRBuilder::CreateCall:
CallInst *CreateCall (Value *Callee, ArrayRef< Value * > Args, const 
Twine &Name="")

This means that there is no guarantee you can convert the API to use
the subtyping safety measure you devised. Indeed, there are several
functions which do not have an equivalent type in hierarchy.
Llvm.set_volatile expects a [load] or [store], where Llvm.set_alignment
expects a [load], [store] or a global value, and 
Llvm.set_atomic_ordering
expects a [load], [store], [atomicrmw], [fence], or [cmpxchg].
LLVM itself represents this by trying to explicitly cast the 
instruction
to each supported one, and then calling the corresponding method.

Interestingly, this kind of relationship can be expressed better by 
OCaml's
structural OO system than C++'s nominative one. I think that a project
to wrap either LLVM's C or C++ API into OCaml's classes and objects may
be a good solution to at least part of the problem. I don't have time
to develop this myself, though.

> I will try to find my code and put it up on GitHub if it would be 
> useful to you.

I would be interested to look into it, but I probably will not 
integrate
it into the bindings.

-- 
   WBR, Peter Zotov.