[LLVMdev] unwinds to in the CFG

Dale Johannesen dalej at apple.com
Fri Apr 4 10:30:23 PDT 2008

I'm chasing a wrong-codegen bug that looks very much like this, except
that I do not have a second call to foo; I have %x being referenced in  
unwinding code, where it hasn't been set.  Was there a resolution for  

On Apr 3, 2008, at 10:58 AM, Duncan Sands wrote:
> Hi Devang,
>>> Just as a quick recap the problem I encountered is how to deal
>>> instructions in a block being used as operands in the unwind dest.
>>> Such
>>> as this:
>>> bb1: unwinds to %cleanup
>>>  call void @foo() ; might throw, might not
>>>  %x = add i32 %y, %z
>>>  call void @foo() ; might throw, might not
>>>  ret void
>>> cleanup:
>>>  call void @use(i32 %x)
>>> The problem is that %x might not have been executed before we enter
>>> %cleanup.
>> This means bb1 has multiple exit points, which defeats the "single
>> entry, single exit" idea. Did I miss anything here ?
> you are correct, this fundamental property of basic blocks is being
> discarded.  Very nasty!  This is why I argued against this approach
> in favour of the mini-basic-blocks approach, in which you have lots
> of basic blocks which under the hood share common info to reduce  
> memory
> usage.  However Chris convinced me that in fact not that many places
> really use that there is a single exit, and that only a wimpy quiche
> eater would shrink at the idea of auditing all of LLVM! :)
> Ciao,
> Duncan.
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