[PATCH] D130224: [Clang][Attribute] Introduce maybe_undef attribute for function arguments which accepts undef values

[Matt Arsenault via Phabricator via cfe-commits]

arsenm added a comment.

In D130224#3668243 <https://reviews.llvm.org/D130224#3668243>, @aaron.ballman wrote:

> I'm still not seeing the issue fully. My understanding of the situation (which may be wrong) is that Clang lowers to LLVM IR and adds `noundef` markings at call sites that this patch attempts to let the user then undo. However, why can Clang's CodeGen not notice the special builtin and walk up the call chain to mark all the wrapper functions to ensure everything is marked appropriately? There might be a small perf loss (if there are wrappers around wrappers around wrappers kind of situation), but this means we don't have to expose this problem to users.

This requires maintaining logic in clang to specially recognize these N wrapper functions, and there would be no indication in the source that this magic is occurring. It's less maintainable because it requires tight, magic coupling between these functions in the source and special handling in clang to be maintained if anything were to change


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https://reviews.llvm.org/D130224