Index: llvm/lib/Transforms/Utils/SimplifyCFG.cpp
===================================================================
--- llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -5942,7 +5942,7 @@
 
   /// Build instructions with Builder to retrieve the value at
   /// the position given by Index in the lookup table.
-  Value *BuildLookup(Value *Index, IRBuilder<> &Builder);
+  Value *BuildLookup(Value *Index, IRBuilder<> &Builder, APInt &MaxIndex);
 
   /// Return true if a table with TableSize elements of
   /// type ElementType would fit in a target-legal register.
@@ -6106,7 +6106,7 @@
   Kind = ArrayKind;
 }
 
-Value *SwitchLookupTable::BuildLookup(Value *Index, IRBuilder<> &Builder) {
+Value *SwitchLookupTable::BuildLookup(Value *Index, IRBuilder<> &Builder, APInt &MaxIndex) {
   switch (Kind) {
   case SingleValueKind:
     return SingleValue;
@@ -6114,10 +6114,18 @@
     // Derive the result value from the input value.
     Value *Result = Builder.CreateIntCast(Index, LinearMultiplier->getType(),
                                           false, "switch.idx.cast");
+    // Check whether signed wrap needed.
+    bool MulSW, AddSW;
+    // TODO: cast BitWidth by cleaner way
+    MaxIndex = APInt(LinearMultiplier->getBitWidth(), MaxIndex.getZExtValue());
+    LinearOffset->getValue().sadd_ov(
+        LinearMultiplier->getValue().smul_ov(MaxIndex, MulSW), AddSW);
     if (!LinearMultiplier->isOne())
-      Result = Builder.CreateMul(Result, LinearMultiplier, "switch.idx.mult");
+      Result = Builder.CreateMul(Result, LinearMultiplier, "switch.idx.mult",
+                                 /*HasNUW = */ false, /*HasNSW = */ !MulSW);
     if (!LinearOffset->isZero())
-      Result = Builder.CreateAdd(Result, LinearOffset, "switch.offset");
+      Result = Builder.CreateAdd(Result, LinearOffset, "switch.offset",
+                                 /*HasNUW = */ false, /*HasNSW = */ !AddSW);
     return Result;
   }
   case BitMapKind: {
@@ -6497,20 +6505,22 @@
   Builder.SetInsertPoint(SI);
   Value *TableIndex;
   ConstantInt *TableIndexOffset;
+  APInt MaxIndex(MaxCaseVal->getBitWidth(), 0);
   if (UseSwitchConditionAsTableIndex) {
     TableIndexOffset = ConstantInt::get(MaxCaseVal->getType(), 0);
     TableIndex = SI->getCondition();
+    MaxIndex = MaxCaseVal->getValue();
   } else {
     TableIndexOffset = MinCaseVal;
     // If the default is unreachable, all case values are s>= MinCaseVal. Then
     // we can try to attach nsw.
     bool CanBeWrapped = true;
     if (!DefaultIsReachable)
-      MaxCaseVal->getValue().ssub_ov(MinCaseVal->getValue(), CanBeWrapped);
-
+      MaxIndex =
+          MaxCaseVal->getValue().ssub_ov(MinCaseVal->getValue(), CanBeWrapped);
     TableIndex = Builder.CreateSub(SI->getCondition(), TableIndexOffset,
                                    "switch.tableidx", /*HasNUW =*/false,
-                                   /*HasNSW =*/!CanBeWrapped);
+                                   /*HasNSW =*/!(CanBeWrapped || DefaultIsReachable));
   }
 
   BranchInst *RangeCheckBranch = nullptr;
@@ -6591,7 +6601,7 @@
     SwitchLookupTable Table(Mod, TableSize, TableIndexOffset, ResultList, DV,
                             DL, FuncName);
 
-    Value *Result = Table.BuildLookup(TableIndex, Builder);
+    Value *Result = Table.BuildLookup(TableIndex, Builder, MaxIndex);
 
     // Do a small peephole optimization: re-use the switch table compare if
     // possible.
Index: llvm/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll
===================================================================
--- llvm/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll
+++ llvm/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll
@@ -1223,8 +1223,8 @@
 ; CHECK-NEXT:    [[SWITCH_TABLEIDX:%.*]] = sub i32 [[C:%.*]], 10
 ; CHECK-NEXT:    [[TMP0:%.*]] = icmp ult i32 [[SWITCH_TABLEIDX]], 4
 ; CHECK-NEXT:    [[SWITCH_IDX_CAST:%.*]] = trunc i32 [[SWITCH_TABLEIDX]] to i8
-; CHECK-NEXT:    [[SWITCH_IDX_MULT:%.*]] = mul i8 [[SWITCH_IDX_CAST]], -5
-; CHECK-NEXT:    [[SWITCH_OFFSET:%.*]] = add i8 [[SWITCH_IDX_MULT]], 18
+; CHECK-NEXT:    [[SWITCH_IDX_MULT:%.*]] = mul nsw i8 [[SWITCH_IDX_CAST]], -5
+; CHECK-NEXT:    [[SWITCH_OFFSET:%.*]] = add nsw i8 [[SWITCH_IDX_MULT]], 18
 ; CHECK-NEXT:    [[X:%.*]] = select i1 [[TMP0]], i8 [[SWITCH_OFFSET]], i8 3
 ; CHECK-NEXT:    ret i8 [[X]]
 ;
@@ -1251,7 +1251,7 @@
 ; CHECK-NEXT:    [[SWITCH_TABLEIDX:%.*]] = sub i8 [[C:%.*]], -13
 ; CHECK-NEXT:    [[TMP0:%.*]] = icmp ult i8 [[SWITCH_TABLEIDX]], 4
 ; CHECK-NEXT:    [[SWITCH_IDX_CAST:%.*]] = zext i8 [[SWITCH_TABLEIDX]] to i32
-; CHECK-NEXT:    [[SWITCH_OFFSET:%.*]] = add i32 [[SWITCH_IDX_CAST]], 18
+; CHECK-NEXT:    [[SWITCH_OFFSET:%.*]] = add nsw i32 [[SWITCH_IDX_CAST]], 18
 ; CHECK-NEXT:    [[X:%.*]] = select i1 [[TMP0]], i32 [[SWITCH_OFFSET]], i32 3
 ; CHECK-NEXT:    ret i32 [[X]]
 ;
@@ -1278,7 +1278,7 @@
 ; CHECK-NEXT:    [[SWITCH_TABLEIDX:%.*]] = sub i32 [[C:%.*]], 10
 ; CHECK-NEXT:    [[TMP0:%.*]] = icmp ult i32 [[SWITCH_TABLEIDX]], 4
 ; CHECK-NEXT:    [[SWITCH_IDX_CAST:%.*]] = trunc i32 [[SWITCH_TABLEIDX]] to i8
-; CHECK-NEXT:    [[SWITCH_IDX_MULT:%.*]] = mul i8 [[SWITCH_IDX_CAST]], 100
+; CHECK-NEXT:    [[SWITCH_IDX_MULT:%.*]] = mul nsw i8 [[SWITCH_IDX_CAST]], 100
 ; CHECK-NEXT:    [[X:%.*]] = select i1 [[TMP0]], i8 [[SWITCH_IDX_MULT]], i8 3
 ; CHECK-NEXT:    ret i8 [[X]]
 ;
@@ -1330,7 +1330,7 @@
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[TMP0:%.*]] = icmp ult i32 [[X:%.*]], 4
 ; CHECK-NEXT:    [[INVERTED_CMP:%.*]] = xor i1 [[TMP0]], true
-; CHECK-NEXT:    [[SWITCH_OFFSET:%.*]] = add i32 [[X]], 10
+; CHECK-NEXT:    [[SWITCH_OFFSET:%.*]] = add nsw i32 [[X]], 10
 ; CHECK-NEXT:    [[R_0:%.*]] = select i1 [[TMP0]], i32 [[SWITCH_OFFSET]], i32 0
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[R_0]], 0
 ; CHECK-NEXT:    [[RETVAL_0:%.*]] = select i1 [[INVERTED_CMP]], i32 100, i32 [[R_0]]
@@ -1398,7 +1398,7 @@
 ; CHECK-LABEL: @no_reuse_cmp(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[TMP0:%.*]] = icmp ult i32 [[X:%.*]], 4
-; CHECK-NEXT:    [[SWITCH_OFFSET:%.*]] = add i32 [[X]], 10
+; CHECK-NEXT:    [[SWITCH_OFFSET:%.*]] = add nsw i32 [[X]], 10
 ; CHECK-NEXT:    [[R_0:%.*]] = select i1 [[TMP0]], i32 [[SWITCH_OFFSET]], i32 12
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp ne i32 [[R_0]], 0
 ; CHECK-NEXT:    [[RETVAL_0:%.*]] = select i1 [[CMP]], i32 [[R_0]], i32 100
@@ -1434,7 +1434,7 @@
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[EC:%.*]] = icmp ne i32 [[Y:%.*]], 0
 ; CHECK-NEXT:    [[TMP0:%.*]] = icmp ult i32 [[X:%.*]], 4
-; CHECK-NEXT:    [[SWITCH_OFFSET:%.*]] = add i32 [[X]], 10
+; CHECK-NEXT:    [[SWITCH_OFFSET:%.*]] = add nsw i32 [[X]], 10
 ; CHECK-NEXT:    [[SPEC_SELECT:%.*]] = select i1 [[TMP0]], i32 [[SWITCH_OFFSET]], i32 0
 ; CHECK-NEXT:    [[R_0:%.*]] = select i1 [[EC]], i32 [[SPEC_SELECT]], i32 100
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[R_0]], 0
@@ -1778,8 +1778,8 @@
 ; CHECK-NEXT:    [[TRUNC:%.*]] = trunc i8 [[N:%.*]] to i2
 ; CHECK-NEXT:    [[SWITCH_TABLEIDX:%.*]] = sub i2 [[TRUNC]], -2
 ; CHECK-NEXT:    [[SWITCH_IDX_CAST:%.*]] = zext i2 [[SWITCH_TABLEIDX]] to i32
-; CHECK-NEXT:    [[SWITCH_IDX_MULT:%.*]] = mul i32 [[SWITCH_IDX_CAST]], 1111
-; CHECK-NEXT:    [[SWITCH_OFFSET:%.*]] = add i32 [[SWITCH_IDX_MULT]], 1111
+; CHECK-NEXT:    [[SWITCH_IDX_MULT:%.*]] = mul nsw i32 [[SWITCH_IDX_CAST]], 1111
+; CHECK-NEXT:    [[SWITCH_OFFSET:%.*]] = add nsw i32 [[SWITCH_IDX_MULT]], 1111
 ; CHECK-NEXT:    ret i32 [[SWITCH_OFFSET]]
 ;
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