[llvm] [SimplifyCFG] Fold the contiguous wrapping cases into ICmp. (PR #161000)

[Yingwei Zheng via llvm-commits]

================
@@ -5797,37 +5847,65 @@ bool SimplifyCFGOpt::turnSwitchRangeIntoICmp(SwitchInst *SI,
   assert(!CasesA.empty() || HasDefault);
 
   // Figure out if one of the sets of cases form a contiguous range.
-  SmallVectorImpl<ConstantInt *> *ContiguousCases = nullptr;
-  BasicBlock *ContiguousDest = nullptr;
-  BasicBlock *OtherDest = nullptr;
-  if (!CasesA.empty() && casesAreContiguous(CasesA)) {
-    ContiguousCases = &CasesA;
-    ContiguousDest = DestA;
-    OtherDest = DestB;
-  } else if (casesAreContiguous(CasesB)) {
-    ContiguousCases = &CasesB;
-    ContiguousDest = DestB;
-    OtherDest = DestA;
-  } else
-    return false;
+  std::optional<ContiguousCasesResult> ContiguousCases;
+
+  // Only one icmp is needed when there is only one case.
+  if (!HasDefault && CasesA.size() == 1)
+    ContiguousCases = ContiguousCasesResult{
+        /*Min=*/CasesA[0],
+        /*Max=*/CasesA[0],
+        /*Dest=*/DestA,
+        /*OtherDest=*/DestB,
+        /*Cases=*/&CasesA,
+        /*OtherCases=*/&CasesB,
+    };
+  else if (CasesB.size() == 1)
+    ContiguousCases = ContiguousCasesResult{
+        /*Min=*/CasesB[0],
+        /*Max=*/CasesB[0],
+        /*Dest=*/DestB,
+        /*OtherDest=*/DestA,
+        /*Cases=*/&CasesB,
+        /*OtherCases=*/&CasesA,
+    };
 
-  // Start building the compare and branch.
+  // Correctness: Cases to the default destination cannot be contiguous cases.
+  if (!ContiguousCases && !HasDefault && !CasesA.empty())
----------------
dtcxzyw wrote:

```suggestion
  else if (!HasDefault)
```
`!HasDefault` implies `!CasesA.empty()`.


https://github.com/llvm/llvm-project/pull/161000