[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 didn't get what you refer to with "saving an iteration of InstCombine" -- how can we save an InstCombine iteration?

For the record, this is the impact of the KnownBits calculation (as opposed to the extra member and the affected value calculation): http://llvm-compile-time-tracker.com/compare.php?from=c9edd08d49137e47fe3acda3ea5b2bae563cb3ac&to=db730283063dff0dc72338413f346bd0174dafdd&stat=instructions:u

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