[llvm] [IR] LangRef: document behavior of double-lifetime.start (PR #184296)

[Ralf Jung via llvm-commits]

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@@ -27726,15 +27726,18 @@ value.
 Semantics:
 """"""""""
 
-If ``ptr`` is a ``poison`` value, the intrinsic has no effect.
+If ``ptr`` is a ``poison`` value or not a pointer to a stack-allocated object,
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RalfJung wrote:

So, if I write a program where conditional on some program input, the pointer points to a stack object or a heap object, is the program valid? What if this is conditional on whether we find a counterexample to the Collatz conjecture? It seems like this clause makes IR validity an undecidable property, which doesn't sound desirable.

https://github.com/llvm/llvm-project/pull/184296