[PATCH] D70749: [InstCombine] do not insert nonnull assumption for undef

[Nuno Lopes via Phabricator via llvm-commits]

nlopes added a comment.

In D70749#1767639 <https://reviews.llvm.org/D70749#1767639>, @jdoerfert wrote:

> You said "Changing the semantics on non-respecting the tags from UB to poison doesn't help either.", could you elaborate why?
>  If there are no uses of a violating instantiation (undef/null passed to a non-null) we would not get UB but an unused poison value.


The issue I had in mind was a function call where the return value is actually used. e.g.:

  x = foo(nonnull ptr)
  ->
  x = foo(nonnull undef)

With the current semantics, we introduce UB. If changed to "the function returns poison", then `x` would change from a regular value to poison, also incorrect.

---

Just to add to the attribute stripping issue, see this example:

  define @foo(nonnull)
  
  x = @foo(nonnull %ptr)
  ->
  x = @foo(%ptr)

Dropping `nonnull` from the call site doesn't help because we inherit it from the callee definition. We can only go this way if we guarantee we have a strong way of stripping attributes from definitions (as @efriedma mentioned). I don't think that's guaranteed in linking ATM, but if so, problem solved :)


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