[llvm-dev] killing undef and spreading poison

[Peter Lawrence via llvm-dev]

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).


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 <http://www.cs.utah.edu/~regehr/papers/undef-pldi17.pdf> ]

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