[llvm-dev] RFC: Killing undef and spreading poison

[Nuno Lopes via llvm-dev]

> On 10/18/2016 3:12 PM, Sanjoy Das wrote:
>> But in the new proposal, in:
>>
>>   %x = freeze(poison)
>>   %y = xor %x, %x
>>
>> that is no longer allowed (%y _has_ to be 0) -- all uses of %x will see
>> some garbage, but fixed bit pattern.
>
> What about this:
>    %x = phi poison, poison  (I'm simplifying the syntax here)
> Can this be simplified to "%x = poison", i.e. can we rauw(%x, poison)?

Yes, that's ok.


>    %x = load %uninitialized_var
>    %y = load %uninitialized_var
>    // are %x and %y equal (i.e. is "cmp eq %x, %y" == true)?
>    // is freeze(%x) equal to freeze(%y)?

"icmp %x, %y" would be poison (so you can chose true or false if you wish). 
There's no change here with respect to current poison semantics.
freeze(%x) is not necessarily the same as freeze(%y).  Even %a and %b might 
not be the same in "%a = freeze(%x), %b = freeze(%x)"  (each freeze returns 
an arbitrary, but fixed, value).

Nuno