[LLVMdev] Jump Theading/GVN bug - moving discussion to llvm-dev

[Philip Reames]

On 02/23/2015 08:45 PM, Hal Finkel wrote:
> I don't disagree with this, but I think there are two (separable) issues here:
>
>   1. Should transformations produce unreachable code? On the face of it, they must be able to, because the problem of determining dynamic reachability is generally undecidable. Should passes produce trivially dead code? Well, probably not if they avoid it. Even this is not locally decidable, and so you're right, removing it will add extra expense (but also provide savings, so we'd need to experiment).
>
>   2. Should unreachable code be allowed to contain nonsense (like instructions that depend on themselves)? To this, I believe the answer is no. We currently permit this, and I think that a lot of the bugs regarding unreachable code some from this. I've yet to hear a good argument why, for example, JumpThreading must produce self-referential instructions in trivially-dead regions.
>
>   -Hal
I agree with Hal here.  Fixing (2) does not necessarily involve changing 
(1).  Can someone sketch out how we'd ever get an instance of (2)?  I'd 
naively expect to at least see a PHI in the cycle.

Philip