Can you point out to me where in the VMKit code I should be looking?
The compilation of an exception table in Java is in lib/JnJVM/VMCore/JavaJIT.cpp, function "readExceptionTable".
The interaction with the libgcc runtime is in lib/JnJVM/VMCore/JavaThread.cpp.
I spent some additional time reading the docs, and I have some specific questions:
1) I'm trying to figure out the relationship between the __cxa_throw function, and the _Unwind_RaiseException function mentioned in the ABI doc. My guess is that _Unwind_RaiseException is the language-neutral implementation of stack unwinding, and __cxa_throw is the C++ exception semantics that are implemented on top of it. If that is the case, should I be calling _Unwind_RaiseException since my exceptions are more like Java than C++ exceptions?
I don't know... Last time I use __cxa_throw, because it's allocating an exception object and setting the correct values in the header of this exception. _Unwind_RaiseException just takes an exception object as parameter and if it's not filled correctly, some other functions may not work.
2) How portable is _Unwind_RaiseException and similar functions? In particular, if I call those functions, will I be limiting myself to a subset of the platforms that LLVM supports? (The same question can be asked of __cxa_throw).
I think __cxa_throw is fairly portable. That's what g++ uses to assembly generate exception throwing. So it depends on libgcc being implemented on multiple platforms. I have no idea though for _Unwind_RaiseException.
Although _Unwind_RaiseException is mentioned as part of the Itanium ABI, it is apparently used on many more platform types than just that one processor. A code search shows that it is in fact part of the standard gcc-provided runtime libs (see http://www.google.com/codesearch/p?hl=en#47R7EH5FbNk/trunk/gcc/gcc/unwind-generic.h&q=_Unwind_RaiseException%20lang:c). There is also a mention of "libunwind". Is libunwind a portable implementation of the Itanium unwinding ABI?
You'd have to ask the libunwind developers: http://www.nongnu.org/libunwind/people.html. Apparently, i runs on ia64, x86, x86-64, ppc64. I have no idea of how well libunwind fits with libgcc.
3) I really like the "invoke/unwind" abstraction that LLVM provides, as it divorces the mechanism of unwinding from the creation of an exception object. However, looking at the Itanium unwinding API, I don't see how you can implement LLVM invoke/unwind on top of it, since the API functions take an exception object as a parameter. (I suppose you could use a dummy exception to satisfy the ABI, and then implement your own, separate exception object, but this destroys some of the efficiency of the unwinding implementation.)
Yeah, I don't think both match very well. VMKit uses a dummy exception.
Also, the Itanium ABI appears to offer some interesting features that aren't exposed by the invoke/unwind mechanism which would be useful. For example, the fact that the stack frames are traversed in two passes seems like it might make it possible to implement human-readable stack dumps, which would be difficult with invoke/unwind.
What's a human-readable stack dump? I don't see how invoke/unwind would limit this.