[llvm] [SimplifyCFG] Transform switch to select when common bits uniquely identify one case (PR #145233)

[Antonio Frighetto via llvm-commits]

================
@@ -6290,10 +6290,37 @@ static Value *foldSwitchToSelect(const SwitchCaseResultVectorTy &ResultVector,
     // case 0,2,8,10 -> Cond & 0b1..0101 == 0 ? result : default
     if (isPowerOf2_32(CaseCount)) {
       ConstantInt *MinCaseVal = CaseValues[0];
-      // Find mininal value.
-      for (auto *Case : CaseValues)
+      // In case, there are bits, that can only be present in the CaseValues we
+      // can transform the switch into a select if the conjunction of
+      // all the values uniquely identify the CaseValues.
+      APInt AndMask = APInt::getAllOnes(MinCaseVal->getBitWidth());
+
+      for (auto *Case : CaseValues) {
         if (Case->getValue().slt(MinCaseVal->getValue()))
           MinCaseVal = Case;
+        AndMask &= Case->getValue();
+      }
+
+      ConstantRange CR = computeConstantRange(
+          Condition, /* ForSigned */ Condition->getType()->isSingleValueType());
+      unsigned int ConditionWidth = Condition->getType()->getIntegerBitWidth();
+      APInt ActiveBits = APInt(ConditionWidth, CR.getActiveBits(),
+                               Condition->getType()->isSingleValueType());
+
+      APInt One(ConditionWidth, 1, false);
+      APInt ActiveBitsMask = (One << ActiveBits) - 1;
----------------
antoniofrighetto wrote:

Meant to use ActiveBitsMask?

https://github.com/llvm/llvm-project/pull/145233