[LLVMdev] Is LLVM expressive enough to represent asynchronous exceptions?

[Cameron Zwarich]

On 2011-06-12, at 4:40 PM, John McCall wrote:

> On Jun 12, 2011, at 2:14 PM, Cameron Zwarich wrote:
> 
>> On Jun 12, 2011, at 1:25 AM, Duncan Sands wrote:
>> 
>>> Hi Sohail,
>>> 
>>>> Is LLVM expressive enough to represent asynchronous exceptions?
>>> 
>>> not currently.  The first step in this direction is to get rid of the invoke
>>> instruction and attach exception handling information to basic blocks.  See
>>> http://nondot.org/sabre/LLVMNotes/ExceptionHandlingChanges.txt
>>> for a discussion.
>> 
>> Is this really a good idea? Why have a control flow graph if it doesn't actually capture control flow? There are lots of compilers for languages with more pervasive exceptions that represent them explicitly, e.g. the Hotspot server compiler for Java or several ML compilers (where integer overflow throws an exception).
> 
> You and Bill seem to be responding to a different question, namely "Is LLVM expressive enough to represent synchronous exceptions from non-call instructions?"  This really has nothing to do with Sohail's question.  Duncan is quite correct:  the only reasonable representation for asynchronous exceptions is to attach EH information to basic blocks.

I was just commenting on the reference to Chris' original proposal to allow for non-terminators that throw (synchronous) exceptions. I don't think it is a good tradeoff.

Adding proper support for SEH would complicate things. If you only allow extra edges for asynchronous exceptions to be down edges in the (ordinary) dominator tree, then at least the standard CFG traversals will continue to visit instructions in dominance order. However, you still have to modify dominance queries to take these extra edges into account. 

Cameron