[llvm] [LangRef] Specify that load of alloca outside lifetime is poison (PR #157852)

[Nikita Popov via llvm-commits]

nikic wrote:

@dtcxzyw No, this store does unfortunately get executed, resulting in observable miscompiles when stack coloring is enabled. We still need a solution for that case. (Likely by actually checking whether we are within the alloca lifetime or not.)

https://github.com/llvm/llvm-project/pull/157852