[LLVMdev] RFC: New Exception Handling Proposal

[Bill Wendling]

On Nov 24, 2009, at 5:42 AM, Jay Foad wrote:

>>> Are you saying that, in the LLVM IR, it would be legal to have an
>>> llvm.eh.exception that *isn't* dominated by convokes (because  
>>> there's
>>> a direct branch to that catch block), and in that case the call
>>> returns an undefined value?
>>
>> this is already the case (with invoke substituted for convoke).
>
> Good to know. It would be nice if this was clearly documented!
>
>>> And it would be up to codegen to "peel it out into its own basic  
>>> block"?
>>
>> Not sure what that means?
>
> I was trying to work out what Bill meant by this:
>
>> The only other problem I can see is if code branches into a catch  
>> block. It's gross, but could happen. In that case, the  
>> llvm.eh.exception should *not* be executed, because it's metadata  
>> constructed by the compiler, not user code. It would have to be  
>> peeled out into its own basic block in such instances.
>
> What exactly would be "peeled out into its own basic block", by whom,
> at what stage of the compilation process?
>
That wasn't clear. :-) This should be totally doable by the front-end,  
since it's the one generating the llvm.eh.exception() call.

-bw