[PATCH] D93183: [BasicAA] Make sure context instruction is symmetric

Mon Dec 14 13:31:46 PST 2020

nikic added a comment.

In D93183#2452415 <https://reviews.llvm.org/D93183#2452415>, @jdoerfert wrote:

> Can't we use any dominator for the context? If so, why not use `findNearestCommonDominator` instead of giving up?

Yes, that would be legal as well (but also sub-optimal). Annoyingly we don't have an instruction-level NCD API (D91767 <https://reviews.llvm.org/D91767> was recently not accepted), though in this case just taking the terminator of the BB NCD should work fine.

Taking a step back though, I think my original direction here probably wasn't the best, and it would make more sense to use whichever GEP the index actually belonged to as context. That may also be non-optimal (if the index is on a dominating GEP), but I think it's more predictable and doesn't have any odd "cliffs". I have updated the patch to that effect.

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