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

[Antonio Frighetto via llvm-commits]

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@@ -6234,10 +6234,35 @@ 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());
+
+      // Find mininal value and compute the conjuction of the values.
+      for (auto *Case : CaseValues) {
         if (Case->getValue().slt(MinCaseVal->getValue()))
           MinCaseVal = Case;
+        AndMask &= Case->getValue();
+      }
+
+      if (!AndMask.isZero()) {
+        KnownBits Known = computeKnownBits(Condition, DL);
+        // Compute the number of bits that are free to vary.
+        unsigned FreeBits = Known.countMaxActiveBits() - AndMask.popcount();
+        // Compute 2^FreeBits in order to check whether all the possible
+        // combination of the free bits matches the number of cases.
----------------
antoniofrighetto wrote:

```suggestion
        // combinations of the free bits matches the number of cases.
```

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