[llvm] [SimplifyCFG] Don't use a mask for lookup tables generated from switches with an unreachable default case (PR #94468)

[Nikita Popov via llvm-commits]

================
@@ -6743,8 +6743,25 @@ static bool SwitchToLookupTable(SwitchInst *SI, IRBuilder<> &Builder,
     TableSize =
         (MaxCaseVal->getValue() - MinCaseVal->getValue()).getLimitedValue() + 1;
 
+  // If the default destination is unreachable, or if the lookup table covers
+  // all values of the conditional variable, branch directly to the lookup table
+  // BB. Otherwise, check that the condition is within the case range.
+  bool DefaultIsReachable = !SI->defaultDestUndefined();
+
   bool TableHasHoles = (NumResults < TableSize);
-  bool NeedMask = (TableHasHoles && !HasDefaultResults);
+
+  // If the table has holes but the default destination doesn't produce any
+  // constant results, the lookup table entries corresponding to the holes will
+  // contain undefined values.
+  bool HolesHaveUndefinedResults = (TableHasHoles && !HasDefaultResults);
+
+  // If the default destination doesn't produce a constant result but is still
+  // reachable, and the lookup table has holes, we need to use a mask to
+  // determine if the current index should load from the lookup table or jump
+  // to the default case.
+  // The mask is unnecessary if the table has holes but the default destination
+  // is unreachable, as in that case the holes must also be unreachable.
+  bool NeedMask = (HolesHaveUndefinedResults && DefaultIsReachable);
----------------
nikic wrote:

```suggestion
  bool NeedMask = HolesHaveUndefinedResults && DefaultIsReachable;
```

https://github.com/llvm/llvm-project/pull/94468