[llvm-dev] RFC: Killing undef and spreading poison (original) (raw)

MARCELLO MAGGIONI via llvm-dev llvm-dev at lists.llvm.org
Mon Oct 24 16:31:02 PDT 2016

On Oct 18, 2016, at 1:12 PM, Sanjoy Das via llvm-dev <llvm-dev at lists.llvm.org> wrote:

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

What is

%x = freeze(poison) %y = freeze(poison) %z = xor %x, %y

going to result into?

Marcello

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

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