[llvm-dev] Killing undef and spreading poison

[Mehdi Amini via llvm-dev]

> On Oct 18, 2016, at 12:44 PM, Sanjoy Das via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hi,
> 
> Nuno Lopes wrote:
> 
>>> Okay; so the problem is that an instruction that is value-equivalent
>>> to a poison value is not itself necessarily poison?
>> 
>> Right.
>> I think there were other examples, but I don't have them here handy. But
>> at least this one is very problematic for GVN.
> 
> Another example is this:
> 
> void f(int k) {
>  if (k != 0) {
>    for (< finite loop >) {
>      if (always_false_at_runtime) {
>        print(1 / k);
>      }
>    }
>  }
> }
> 
> We'd like to hoist the `1 / k` computation to the preheader.  However, we can't do that today if `k` is undef, and we've defined branching on undef to be a non-deterministic choice.

This one isn’t clear to me: you can fold 1/k -> 1 when k is undef. And then it is a constant which makes no point to hoist?

— 
Mehdi


> 
> I have some more details at: http://www.playingwithpointers.com/problem-with-undef.html
> 
> -- Sanjoy
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