[llvm] [SimplifyCFG] Find the minimal table considering overflow in `switchToLookupTable` (PR #67885)

[via llvm-commits]

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@@ -6519,17 +6518,60 @@ static bool SwitchToLookupTable(SwitchInst *SI, IRBuilder<> &Builder,
   SmallDenseMap<PHINode *, Type *> ResultTypes;
   SmallVector<PHINode *, 4> PHIs;
 
-  for (SwitchInst::CaseIt E = SI->case_end(); CI != E; ++CI) {
-    ConstantInt *CaseVal = CI->getCaseValue();
-    if (CaseVal->getValue().slt(MinCaseVal->getValue()))
-      MinCaseVal = CaseVal;
-    if (CaseVal->getValue().sgt(MaxCaseVal->getValue()))
-      MaxCaseVal = CaseVal;
+  SmallVector<ConstantInt *, 8> CaseVals;
+  for (auto CI : SI->cases()) {
+    ConstantInt *CaseVal = CI.getCaseValue();
+    CaseVals.push_back(CaseVal);
+  }
+
+  // We want to find a range of indexes that will create the minimal table.
+  // We can treat all possible index values as a circle. For example, the i8 is
+  // [-128, -1] and [0, 127]. After that find the minimal range from this circle
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DianQK wrote:

Updated.
> When considering overflow, the next indexed value of the maximum signed integer is the minimum signed integer.

https://github.com/llvm/llvm-project/pull/67885