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

[Nikita Popov 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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nikic wrote:

Yes, phi nodes are not allowed.

Poison is only allowed so that dead allocas can be replaced with poison. That's also why your "no effect" -> "Undefined Behavior" change is incorrect. Lifetime intrinsics with poison argument must have no effect.

Poison monotonicity is not relevant here, specifically because these intrinsics do not take arbitrary values.

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