[llvm] [InstCombine] Simplify select using KnownBits of condition (PR #95923)

[Nikita Popov via llvm-commits]

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@@ -4018,5 +4018,30 @@ Instruction *InstCombinerImpl::visitSelectInst(SelectInst &SI) {
   if (CondVal->getType() == SI.getType() && isKnownInversion(FalseVal, TrueVal))
     return BinaryOperator::CreateXor(CondVal, FalseVal);
 
+  if (SelType->isIntOrIntVectorTy()) {
+    // Try to simplify select arms based on KnownBits implied by the condition.
+    CondContext CC(CondVal);
+    findValuesAffectedByCondition(CondVal, /*IsAssume=*/false, [&](Value *V) {
+      CC.AffectedValues.insert(V);
+    });
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nikic wrote:

> I mean do the full transform in a single shot as opposed to going through the work list twice.

Do you mean the case where it's possible to simplify *both* select operands? If so, this should be very rare, and as such not worth optimizing.

https://github.com/llvm/llvm-project/pull/95923