sizeoff(), ABI, and code duplication

I'm looking at implementing more ABI stuff. Right now, I have the vtable layout stuff fixed, I hope- the vtable layout can now contain things other than virtual functions, like virtual destructors, deleting destructors, offsets (glory) and RTTI pointers.

What I'm really thinking about right now though is layout.

In order for Wide and C++ to communicate using a type, they have to agree on it's layout. You could not have a type where Wide thinks one subobject is in a different place. The problem with this is that Clang cannot lay out arbitrary types in the way that Wide can- it can only lay out Clang types, and it can only do so in the context of a particular translation unit.

The real problem is sizeof(). Since sizeof() is a constant, I have to know when you request the size how to lay out the class. If I lay it out in one way, and then Clang lays it out in another, I can't simply drop my own layout. I have to know beforehand. This means either strictly laying out all classes in the same way as Clang (sucks), and duplicating their layout code, or, change sizeof().

I've been thinking about introducing a new class of value- a semi-constant, you could call it. The value would not be a constant (since it's only semi) but not vary at run-time. There would be some language features that could accept semi-constants instead of constants- say, array size.

Another advantage of this would be that strictly speaking, the code would be more platform-independent. One of the reasons that C++ is not platform independent right now is that when you use sizeof(), it has to tell you the size. You can't port the IR output of Clang from x86 to x64 because the sizeof()s will be incorrect.

But in principle, a hypothetical Wide VM could use the same LLVM IR across multiple platforms. There's already work in this direction with PNaCL.

Quick edit: First, LLVM's array types take only integer values, not constantexprs, so that would be fun. Second, turns out that Itanium ABI specifies a bunch of secondary virtual tables which are complete duplicates of the primary ones for ... some reason. I didn't have these secondary tables. This just shows that I really, really need more tests. But onwards and upwards, as they say. I will stop implementing new features when I have Itanium-compatible EH. And my code count is now 18,500. Feel the growth.

Thirdly, my laparoscopy is scheduled for Monday. If the surgeon gives me the all clear then I'm done, done, done.