[LLVMdev] exact semantics of 'nounwind'

[Nick Lewycky]

Hi everyone,

Since I'm busy muddying the waters by changing how exception handling 
works, I thought I should ask for clarification on the exact behaviour 
of the current 'nounwind' attribute found on functions, calls and invokes.

I was thinking these would be similar to the AA analysis notes like 
"doesNotAccessMemory" which is a provable property of the function or 
call site being analyzed. Duncan mentioned that doesNotThrow is actually 
an important language semantic meaning that an unwind calls a 
language-defined behaviour such as calling terminate(). What happens in 
other languages?

Chris and I also couldn't agree on what the semantics ought to be going 
forward. He suggested having two bits, one to memoize an analysis 
proving that it can't unwind, and one to mean that an unwind triggers 
terminate. I happen to think that this ought to be explicitly modelled 
in the IR by arcing to another BB that calls terminate.

We do agree that we need crystal-clear semantics in the language, so I'm 
taking it to the mailing list to see what if we can form a consensus.

Nick