[llvm] [ExtendLifetimes] Implement llvm.fake.use to extend variable lifetimes (PR #86149)

[David Blaikie via llvm-commits]

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+# Parsing dwarfdump's output to determine whether the location list for the
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dwblaikie wrote:

if it works right now, and reasonably can continue to do so for the specific example being tested - I'd keep it simple/test it exactly. If there's a specific case where we can't get the whole range - maybe add a test for that and, agian, test the exact behavior with a note "this doesn't extend to the very end of the function, and that's OK or that's a known limitation, or that's a known bug (with link to the bug)"

https://github.com/llvm/llvm-project/pull/86149