[LLVMdev] Proposal for Poison Semantics

[John Regehr]

> I think such a thing would be great.  However, there is a problem that the RFC wasn't aware of when it was
> written:
> 
> consider:
> %S = select %A, %B, undef
> 
> without us knowing anything about %A or %B, we will replace all uses of %S with %B.  This transform would be
> considered wrong with the RFC in mind.
> 
> If this transform was valid, there could not be any value or value-like property in LLVM with semantics more
> powerful than undef.  This makes me think that what LLVM *actually* implements is not poison or something like
> it.

Is it possible that the new weaker poison subsumes undef?  The interaction 
between these different but related UB-like concepts is confusing at best.

John