[llvm] [InstCombine] Fold `select Cond, not X, X` into `Cond ^ X` (PR #90089)

[Nikita Popov via llvm-commits]

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@@ -3985,5 +3985,10 @@ Instruction *InstCombinerImpl::visitSelectInst(SelectInst &SI) {
     }
   }
 
+  if (CondVal->getType() == SI.getType() && impliesPoison(FalseVal, TrueVal) &&
+      isImpliedCondition(FalseVal, TrueVal, DL, /*LHSIsTrue=*/true) == false &&
+      isImpliedCondition(FalseVal, TrueVal, DL, /*LHSIsTrue=*/false) == true)
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nikic wrote:

I'd still like an answer to this question. This is a very roundabout way to check whether the conditions are inverses of each other, and it does have some overhead (http://llvm-compile-time-tracker.com/compare.php?from=76a3be7c766bd55221c3d0d0a74c42f82c5d76ed&to=1731798e2ef457fb5cabd29ae054c700b8b13c14&stat=instructions%3Au). Not enough to make me overly concerned, about enough to contribute to the death by a thousand cuts :)

https://github.com/llvm/llvm-project/pull/90089