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

[Sanjoy Das via llvm-dev]

Hi Krzysztof,

Krzysztof Parzyszek wrote:
> On 10/18/2016 4:29 PM, Nuno Lopes wrote:
>> Even %a and %b might not be the same in "%a = freeze(%x), %b =
>> freeze(%x)" (each freeze returns an arbitrary, but fixed, value).
>
> Assume that %x is known to be a poison value and have:
> %a = freeze(%x)
> %b = freeze(%x)
>
> Is %a == %a true?

Yes, %a is always == %a.  It is a normal SSA value with some unspecific 
content.

> Is %a == %b true?

Not necessarily; but the compiler can make it true by (consistently) 
choosing equal values for %a and %b.

By consistently I mean it can't fold one instance of %a == %b to true 
and fold another instance of %a == %b to false.

-- Sanjoy