[PATCH] D70749: [InstCombine] do not insert nonnull assumption for undef

[Reid Kleckner via Phabricator via llvm-commits]

rnk added a comment.

In D70749#1761120 <https://reviews.llvm.org/D70749#1761120>, @jdoerfert wrote:

> The only alternative I see is to make passes that replace dead values aware of attributes, e.g., let them clear them out. Though, I fear that also hides the problems only a few indirections deeper.


I guess this, then, would be the "simple" fix that will allow us to move forward without making LangRef changes. DAE is already pretty attribute aware. I think it only surveys functions with local linkage, so stripping nonnull from known-dead arguments would be pretty safe.

Another fix would be to treat arguments marked nonnull as live, but that would effectively make all C++ `this` pointers immune to DAE, since they are all nonnull.


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