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

[via llvm-commits]

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@@ -6743,8 +6743,14 @@ 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);
+  bool HolesHaveUndefinedResults = (TableHasHoles && !HasDefaultResults);
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DaMatrix wrote:

I've pushed some new comments explaining this in more detail.

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