llvm dev - Feb 2019 - funnel shift, select, and poison

On 2/25/2019 11:15 AM, John Regehr via llvm-dev wrote:
> I'd just like to add that the general question here is "where does
> poison stop propagating" and this question needs to be definitively 
> answered by this community.
Does a call stop poison?

Whatever the decision is may be contradicted after inlining, so what 
should such a call return?  A superposition of poison and non-poison, 
until the function is inlined? It's a serious question.

-Krzysztof

Poison has to propagate through calls and loads/stores, or else 
basically nothing works.

John



On 2/25/19 10:29 AM, Krzysztof Parzyszek via llvm-dev
wrote:
> On 2/25/2019 11:15 AM, John Regehr via llvm-dev wrote:
>> I'd just like to add that the general question here is "where
does
>> poison stop propagating" and this question needs to be
definitively
>> answered by this community.
> 
> Does a call stop poison?
> 
> Whatever the decision is may be contradicted after inlining, so what 
> should such a call return?  A superposition of poison and non-poison, 
> until the function is inlined? It's a serious question.
> 
> -Krzysztof
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

On 2/25/2019 12:28 PM, John Regehr via llvm-dev wrote:
> Poison has to propagate through calls and loads/stores, or else 
> basically nothing works.
Consider this:

   %v0 = call i32 @foo(poison) nounwind/readnone
   store i32 %v0, i32* %valid_address

If we assume that poison propagates through calls, we could then 
optimize this to

   %v0 = poison
   store poison, i32* %valid_address

If we somehow realized that foo is

   define i32 @foo(i32) { ret i32 0 }

then the resulting code would be different.

-Krzysztof