[llvm] [SimplifyCFG] When only one case value is missing, replace default with that case (PR #76669)

[Yingwei Zheng via llvm-commits]

================
@@ -5609,10 +5612,26 @@ static bool eliminateDeadSwitchCases(SwitchInst *SI, DomTreeUpdater *DTU,
       Known.getBitWidth() - (Known.Zero | Known.One).popcount();
   assert(NumUnknownBits <= Known.getBitWidth());
   if (HasDefault && DeadCases.empty() &&
-      NumUnknownBits < 64 /* avoid overflow */ &&
-      SI->getNumCases() == (1ULL << NumUnknownBits)) {
-    createUnreachableSwitchDefault(SI, DTU);
-    return true;
+      NumUnknownBits < 64 /* avoid overflow */) {
+    uint64_t AllNumCases = 1ULL << NumUnknownBits;
+    if (SI->getNumCases() == AllNumCases) {
+      createUnreachableSwitchDefault(SI, DTU);
+      return true;
+    }
+    // When only one case value is missing, replace default with that case.
+    if (SI->getNumCases() == AllNumCases - 1) {
+      assert(NumUnknownBits > 1 && "Should be canonicalized to a branch");
+      uint64_t MissingCaseVal = 0;
+      for (const auto &Case : SI->cases())
+        MissingCaseVal ^= Case.getCaseValue()->getValue().getLimitedValue();
+      auto *MissingCase =
+          cast<ConstantInt>(ConstantInt::get(Cond->getType(), MissingCaseVal));
+      SwitchInstProfUpdateWrapper SIW(*SI);
+      SIW.addCase(MissingCase, SI->getDefaultDest(), SIW.getSuccessorWeight(0));
+      createUnreachableSwitchDefault(SI, DTU, false);
----------------
dtcxzyw wrote:

```suggestion
      createUnreachableSwitchDefault(SI, DTU, /*RemoveOrigDefaultBlock*/ false);
```

https://github.com/llvm/llvm-project/pull/76669