[llvm-dev] killing undef and spreading poison

[Sanjoy Das via llvm-dev]

Hi  Peter,

On Fri, Jun 16, 2017 at 5:32 PM, Peter Lawrence via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Nuno,
>           we still need some examples showing that the definition
> “Branching on poison is immediate-UB” is the best choice,
> so far we only have arguments against it (the one for loop-switching).

The alternative that allows us to propagate equalities in GVN (which
you seem to be alluding at) is to use an undef "instruction" that
produces a fixed but arbitrary value (just like "freeze(poison)" would
today).  However, that's bad for codegen since in some cases we will
have to keep around a live register containing consistent garbage.
Having both freeze and poison lets us use poison when we can (with
better codegen), and freeze when we must (with worse codegen) -- this
is the having-our-cake-and-eating-it-too scenario I was referring to
earlier.

-- Sanjoy

>
>
> Excerpts from the Paper [1]
>
> Here’s the example you give for GVN
>
>         t = x + 1;
>         if (t == y) {
>           w = x + 1;
>           foo(w);
>         }
>         Therefore, GVN can pick y as the representative value and transform
> the code into:
>         t = x + 1;
>         if (t == y) {
>           foo(y);
>         }
>         However, if y is a poison value and w is not
>
> This really hinges on an incorrect choice for representative value of  w.
>
> Any value with the “poison” attribute is not a valid representative for any
> seemingly equivalent value.  If GVN is actually doing this it is a bug in
> GVN.
>
> So this example doesn’t really show anything about branching.
>
>
> Peter Lawrence.
>
>
> [1.  http://www.cs.utah.edu/~regehr/papers/undef-pldi17.pdf ]
>
>
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