Index: llvm/lib/Transforms/Utils/SimplifyCFG.cpp
===================================================================
--- llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -6471,6 +6471,21 @@
 
   std::vector<DominatorTree::UpdateType> Updates;
 
+  // Compute the maximum table size representable by the integer type we are
+  // switching upon.
+  unsigned CaseSize = MinCaseVal->getType()->getPrimitiveSizeInBits();
+  uint64_t MaxTableSize = CaseSize > 63 ? UINT64_MAX : 1ULL << CaseSize;
+  assert(MaxTableSize >= TableSize &&
+         "It is impossible for a switch to have more entries than the max "
+         "representable value of its input integer type's size.");
+
+  // 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.
+  const bool DefaultIsReachable =
+      !isa<UnreachableInst>(SI->getDefaultDest()->getFirstNonPHIOrDbg());
+  const bool GeneratingCoveredLookupTable = (MaxTableSize == TableSize);
+
   // Create the BB that does the lookups.
   Module &Mod = *CommonDest->getParent()->getParent();
   BasicBlock *LookupBB = BasicBlock::Create(
@@ -6485,24 +6500,13 @@
     TableIndex = SI->getCondition();
   } else {
     TableIndexOffset = MinCaseVal;
-    TableIndex =
-        Builder.CreateSub(SI->getCondition(), TableIndexOffset, "switch.tableidx");
+    // If the default is unreachable, all case values are s>= MinCaseVal, so the
+    // subtraction cannot wrap in a signed sense.
+    TableIndex = Builder.CreateSub(SI->getCondition(), TableIndexOffset,
+                                   "switch.tableidx", /*HasNUW =*/false,
+                                   /*HasNSW =*/!DefaultIsReachable);
   }
 
-  // Compute the maximum table size representable by the integer type we are
-  // switching upon.
-  unsigned CaseSize = MinCaseVal->getType()->getPrimitiveSizeInBits();
-  uint64_t MaxTableSize = CaseSize > 63 ? UINT64_MAX : 1ULL << CaseSize;
-  assert(MaxTableSize >= TableSize &&
-         "It is impossible for a switch to have more entries than the max "
-         "representable value of its input integer type's size.");
-
-  // 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.
-  const bool DefaultIsReachable =
-      !isa<UnreachableInst>(SI->getDefaultDest()->getFirstNonPHIOrDbg());
-  const bool GeneratingCoveredLookupTable = (MaxTableSize == TableSize);
   BranchInst *RangeCheckBranch = nullptr;
 
   if (!DefaultIsReachable || GeneratingCoveredLookupTable) {