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

[Ralf Jung via llvm-commits]

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@@ -27734,7 +27721,8 @@ Arguments:
 """"""""""
 
 The argument is either a pointer to an ``alloca`` instruction or a ``poison``
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RalfJung wrote:

I think I'd suggest something that remains on the syntactic level: "the argument must be an SSA variable that is initialized with an alloca instruction or a poison value".

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