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

[Sanjoy Das via llvm-dev]

Hi Krzysztof,

freeze(poison) is different from undef today, in the sense that it is an 
instruction that produces some random, but fixed bit pattern.

E.g. today in

   %x = undef
   %y = xor %x, %x

we can fold %y to undef since each use of %x can independently see some 
arbitrary (up to the compiler / environment) bit pattern.

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.

-- Sanjoy


Krzysztof Parzyszek via llvm-dev wrote:
> On 10/18/2016 2:25 PM, Nuno Lopes via llvm-dev wrote:
>> Right, a load touching a single uninitialized bit results in poison.
>> The trick is that on the first bitfield write, all the remaining
>> untouched fields become initialized (with an arbitrary value, though).
>> Essentially we are making the adjacent bitfields undef.
>
> So "undef" still exists, except now it's obtainable via "freeze(poison)"?
>
> -Krzysztof
>