[PATCH] D71799: [Attributor] AAUndefinedBehavior: Check for branches on undef value.

[Johannes Doerfert via Phabricator via llvm-commits]

jdoerfert added a comment.

In D71799#1795961 <https://reviews.llvm.org/D71799#1795961>, @baziotis wrote:

> In D71799#1795803 <https://reviews.llvm.org/D71799#1795803>, @jdoerfert wrote:
>
> > I'd wait wrt. AAValueSimplify.
>
>
> I guess you meant "wait to be committed" (and not wait as to not use it in the memory accessing functions yet).


Yes. Also wrt. modifying AAVAlueSimply further. Using it is not a problem.

>> I'd do the following two first:
>> 
>> 1. UnreachableInst is UB (though no need to replace it in the manifest.
>> 2. Given an instruction, determine if that instruction or one later that is known to be executed is known to cause UB. We then hook that up to the AAIsDead.
> 
> Regarding 2), would it suffice to: Go through the next instructions and follow (alive) branches either by taking unconditional branches, or branches that have known value (e.g. using `AAValueSimplify`) `true`.
>  Note: Walk them in a DFS kind of way until we find a UB instruction (or none at all).

You want to use `MustBeExecutedContextExplorer`. You get it like this:

  MustBeExecutedContextExplorer &Explorer =
      A.getInfoCache().getMustBeExecutedContextExplorer();

You iterate over it like you would with a container like this:

  for (MustBeExecutedIterator &It : Explorer.range(Instruction))



---

There was a patch by @uenoku to deal with conditional and the merging of states when we are exploring but I don't see it in-tree and I forgot which one it was.
Nevertheless, it should immediately work for straight line code and code that is in "some merge block" after a conditional.


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