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

[Alexandre Isoard via llvm-dev]

Hi Sanjoy,

Really interesting read. I am perplexed now, and am not even sure what is
the meaning of undef anymore.

Example (unrelated to your blog post, but still weird):
%x = sext i1 undef to i2

I understand that I can replace it by either of:
%x = i2 0
%x = i2 -1

But can I replace it by:
%x = i2 undef

I would have said no, at first sight, because -2 and 1 should not be
possible values.
But if I look at each bit, independently, each one can be either 0 or 1.
Then, if we
forget their "entanglement" (like we do shamelessly with xor %x, %x), and
then
concatenate them back together, we get the i2 undef...

So I have no clue what is the actual semantic of undef.

On Wed, Oct 19, 2016 at 8:38 PM, Sanjoy Das <sanjoy at playingwithpointers.com>
wrote:

> Hi Alexandre,
>
> On Wed, Oct 19, 2016 at 11:19 AM, Alexandre Isoard
> <alexandre.isoard at gmail.com> wrote:
> > I am probably missing something important, but what I mean is that you
> can
> > always convert:
> >
> > %y = xor %x, %x
> > to
> > %y = 0
> >
> > Regardless of if %x is/might be an undef. That is, consider that reading
> %x
> > any number of times always give the value of its definition (as SSA
> > suggest). Therefore, disregarding the special semantic of undef (which is
> > always safe in this direction).
> >
> > Is there an example of transformation which is forbidden by the fact
> that a
> > variable might be undef?
>
> There are some examples earlier in this thread, but this is one:
> http://www.playingwithpointers.com/problem-with-undef.html
>
> (Writing the blog post is already paying off :) )
>
> -- Sanjoy
>



-- 
*Alexandre Isoard*
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