[PATCH] D133036: [InstCombine] Treat passing undef to noundef params as UB

[Tim Neumann via Phabricator via cfe-commits]

TimNN added a comment.

I'm sorry for the noise. Further investigation has shown that this happens when Rust is doing (thin) LTO, and I don't think this patch can be considered in any way "at fault" here, so this is the last you'll hear from me on the topic here.

I don't know whether the fault is with how Rust implements (thin) LTO or something on the LLVM side. Basically, after running the `FunctionImporter` on a module, we end up with a `define available_externally void outer` and a `declare void inner`, presumably coming from different modules. `outer` contains the call to `inner` with `poison`, but the parameter is `noundef` on the declaration on `inner`.


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https://reviews.llvm.org/D133036