[LLVMdev] More questions on exception handling

[Nicolas Geoffray]

Talin wrote:
> I've read and re-read the LLVM exception handling doc, and there are a 
> number of things that aren't clear to me.
>
> In the section describing llvm.eh.exception, it says "The backend 
> replaces this intrinsic with the code that accesses the first argument 
> of a call." I don't understand this sentence. A call to what? What is 
> meant by 'accesses' - do you mean 'returns'?
>
> In the paragraph that describes llvm.eh.selector, it says "The first 
> argument is the reference to the exception structure.", but later down 
> it says "To express this in LLVM code the landing pad will call 
> llvm.eh.selector. The arguments are the length of the filter 
> expression (the number of type infos plus one) ...". This seems 
> contradictory.
>
> If I am calling the _Unwind_ functions directly, do I need to use any 
> of the llvm.eh.* functions?

That's the only question I can answer :) Yes you need the llvm.eh.* 
functions even with the _Unwind_* functions and __cxa_throw. The 
llvm.eh.* functions allow the llvm code generator to produce dwarf 
exception tables. I don't know why the initial developers of exception 
handling in LLVM didn't use the unwind destination of an invoke instruction.

> Can I just use invoke and write my own landing pad?
>
> How do you handle throwing an exception with a catch handler in the 
> same function?

You'd need to do that by hand. Making an exceplicit __cxa_throw call in 
the llvm code.

> Invoke won't work since it assumes that the exception is being thrown 
> in a subroutine.
>
> -- 
> -- Talin
>
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