diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 55e375670cc61..61d891d65346b 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -5414,11 +5414,13 @@ static bool CasesAreContiguous(SmallVectorImpl<ConstantInt *> &Cases) {
 }
 
 static void createUnreachableSwitchDefault(SwitchInst *Switch,
-                                           DomTreeUpdater *DTU) {
+                                           DomTreeUpdater *DTU,
+                                           bool RemoveOrigDefaultBlock = true) {
   LLVM_DEBUG(dbgs() << "SimplifyCFG: switch default is dead.\n");
   auto *BB = Switch->getParent();
   auto *OrigDefaultBlock = Switch->getDefaultDest();
-  OrigDefaultBlock->removePredecessor(BB);
+  if (RemoveOrigDefaultBlock)
+    OrigDefaultBlock->removePredecessor(BB);
   BasicBlock *NewDefaultBlock = BasicBlock::Create(
       BB->getContext(), BB->getName() + ".unreachabledefault", BB->getParent(),
       OrigDefaultBlock);
@@ -5427,7 +5429,8 @@ static void createUnreachableSwitchDefault(SwitchInst *Switch,
   if (DTU) {
     SmallVector<DominatorTree::UpdateType, 2> Updates;
     Updates.push_back({DominatorTree::Insert, BB, &*NewDefaultBlock});
-    if (!is_contained(successors(BB), OrigDefaultBlock))
+    if (RemoveOrigDefaultBlock &&
+        !is_contained(successors(BB), OrigDefaultBlock))
       Updates.push_back({DominatorTree::Delete, BB, &*OrigDefaultBlock});
     DTU->applyUpdates(Updates);
   }
@@ -5609,10 +5612,28 @@ static bool eliminateDeadSwitchCases(SwitchInst *SI, DomTreeUpdater *DTU,
       Known.getBitWidth() - (Known.Zero | Known.One).popcount();
   assert(NumUnknownBits <= Known.getBitWidth());
   if (HasDefault && DeadCases.empty() &&
-      NumUnknownBits < 64 /* avoid overflow */ &&
-      SI->getNumCases() == (1ULL << NumUnknownBits)) {
-    createUnreachableSwitchDefault(SI, DTU);
-    return true;
+      NumUnknownBits < 64 /* avoid overflow */) {
+    uint64_t AllNumCases = 1ULL << NumUnknownBits;
+    if (SI->getNumCases() == AllNumCases) {
+      createUnreachableSwitchDefault(SI, DTU);
+      return true;
+    }
+    // When only one case value is missing, replace default with that case.
+    // Eliminating the default branch will provide more opportunities for
+    // optimization, such as lookup tables.
+    if (SI->getNumCases() == AllNumCases - 1) {
+      assert(NumUnknownBits > 1 && "Should be canonicalized to a branch");
+      uint64_t MissingCaseVal = 0;
+      for (const auto &Case : SI->cases())
+        MissingCaseVal ^= Case.getCaseValue()->getValue().getLimitedValue();
+      auto *MissingCase =
+          cast<ConstantInt>(ConstantInt::get(Cond->getType(), MissingCaseVal));
+      SwitchInstProfUpdateWrapper SIW(*SI);
+      SIW.addCase(MissingCase, SI->getDefaultDest(), SIW.getSuccessorWeight(0));
+      createUnreachableSwitchDefault(SI, DTU, /*RemoveOrigDefaultBlock*/ false);
+      SIW.setSuccessorWeight(0, 0);
+      return true;
+    }
   }
 
   if (DeadCases.empty())
diff --git a/llvm/test/Transforms/SimplifyCFG/switch-dead-default-lookup-table.ll b/llvm/test/Transforms/SimplifyCFG/switch-dead-default-lookup-table.ll
new file mode 100644
index 0000000000000..bead0dc4c567a
--- /dev/null
+++ b/llvm/test/Transforms/SimplifyCFG/switch-dead-default-lookup-table.ll
@@ -0,0 +1,61 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
+; RUN: opt %s -S -passes='simplifycfg<switch-to-lookup>' -simplifycfg-require-and-preserve-domtree=1 -switch-range-to-icmp | FileCheck %s
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+
+define i64 @test_1(i64 %0) {
+; CHECK-LABEL: define i64 @test_1(
+; CHECK-SAME: i64 [[TMP0:%.*]]) {
+; CHECK-NEXT:  switch.lookup:
+; CHECK-NEXT:    [[TMP1:%.*]] = urem i64 [[TMP0]], 4
+; CHECK-NEXT:    [[SWITCH_GEP:%.*]] = getelementptr inbounds [4 x i64], ptr @switch.table.test_1, i32 0, i64 [[TMP1]]
+; CHECK-NEXT:    [[SWITCH_LOAD:%.*]] = load i64, ptr [[SWITCH_GEP]], align 8
+; CHECK-NEXT:    ret i64 [[SWITCH_LOAD]]
+;
+  %2 = urem i64 %0, 4
+  switch i64 %2, label %5 [
+  i64 1, label %3
+  i64 2, label %3
+  i64 3, label %4
+  ]
+
+3:
+  br label %5
+
+4:
+  br label %5
+
+5:
+  %.0 = phi i64 [ 2, %4 ], [ 1, %3 ], [ 0, %1 ]
+  ret i64 %.0
+}
+
+
+define i64 @test_2(i64 %0) {
+; CHECK-LABEL: define i64 @test_2(
+; CHECK-SAME: i64 [[TMP0:%.*]]) {
+; CHECK-NEXT:  switch.lookup:
+; CHECK-NEXT:    [[TMP1:%.*]] = urem i64 [[TMP0]], 4
+; CHECK-NEXT:    ret i64 [[TMP1]]
+;
+  %2 = urem i64 %0, 4
+  switch i64 %2, label %6 [
+  i64 1, label %3
+  i64 2, label %4
+  i64 3, label %5
+  ]
+
+3:
+  br label %6
+
+4:
+  br label %6
+
+5:
+  br label %6
+
+6:
+  %.0 = phi i64 [ 0, %1 ], [ 1, %3 ], [ 2, %4 ], [ 3, %5 ]
+  ret i64 %.0
+}
+
diff --git a/llvm/test/Transforms/SimplifyCFG/switch-dead-default.ll b/llvm/test/Transforms/SimplifyCFG/switch-dead-default.ll
index 7c0d5e4f2b653..e30a535c52323 100644
--- a/llvm/test/Transforms/SimplifyCFG/switch-dead-default.ll
+++ b/llvm/test/Transforms/SimplifyCFG/switch-dead-default.ll
@@ -79,15 +79,15 @@ default:
   ret void
 }
 
-; This one is a negative test - we know the value of the default,
-; but that's about it
+; We can replace the default branch with case 3 since it is the only case that is missing.
 define void @test3(i2 %a) {
 ; CHECK-LABEL: define void @test3(
 ; CHECK-SAME: i2 [[A:%.*]]) {
-; CHECK-NEXT:    switch i2 [[A]], label [[DEFAULT:%.*]] [
+; CHECK-NEXT:    switch i2 [[A]], label [[DOTUNREACHABLEDEFAULT:%.*]] [
 ; CHECK-NEXT:      i2 0, label [[CASE0:%.*]]
 ; CHECK-NEXT:      i2 1, label [[CASE1:%.*]]
 ; CHECK-NEXT:      i2 -2, label [[CASE2:%.*]]
+; CHECK-NEXT:      i2 -1, label [[DEFAULT:%.*]]
 ; CHECK-NEXT:    ]
 ; CHECK:       common.ret:
 ; CHECK-NEXT:    ret void
@@ -100,6 +100,8 @@ define void @test3(i2 %a) {
 ; CHECK:       case2:
 ; CHECK-NEXT:    call void @foo(i32 2)
 ; CHECK-NEXT:    br label [[COMMON_RET]]
+; CHECK:       .unreachabledefault:
+; CHECK-NEXT:    unreachable
 ; CHECK:       default:
 ; CHECK-NEXT:    call void @foo(i32 3)
 ; CHECK-NEXT:    br label [[COMMON_RET]]
@@ -122,6 +124,50 @@ default:
   ret void
 }
 
+define void @test3_prof(i2 %a) {
+; CHECK-LABEL: define void @test3_prof(
+; CHECK-SAME: i2 [[A:%.*]]) {
+; CHECK-NEXT:    switch i2 [[A]], label [[DOTUNREACHABLEDEFAULT:%.*]] [
+; CHECK-NEXT:      i2 0, label [[CASE0:%.*]]
+; CHECK-NEXT:      i2 1, label [[CASE1:%.*]]
+; CHECK-NEXT:      i2 -2, label [[CASE2:%.*]]
+; CHECK-NEXT:      i2 -1, label [[DEFAULT:%.*]]
+; CHECK-NEXT:    ], !prof [[PROF0:![0-9]+]]
+; CHECK:       common.ret:
+; CHECK-NEXT:    ret void
+; CHECK:       case0:
+; CHECK-NEXT:    call void @foo(i32 0)
+; CHECK-NEXT:    br label [[COMMON_RET:%.*]]
+; CHECK:       case1:
+; CHECK-NEXT:    call void @foo(i32 1)
+; CHECK-NEXT:    br label [[COMMON_RET]]
+; CHECK:       case2:
+; CHECK-NEXT:    call void @foo(i32 2)
+; CHECK-NEXT:    br label [[COMMON_RET]]
+; CHECK:       .unreachabledefault:
+; CHECK-NEXT:    unreachable
+; CHECK:       default:
+; CHECK-NEXT:    call void @foo(i32 3)
+; CHECK-NEXT:    br label [[COMMON_RET]]
+;
+  switch i2 %a, label %default [i2 0, label %case0
+  i2 1, label %case1
+  i2 2, label %case2], !prof !0
+
+case0:
+  call void @foo(i32 0)
+  ret void
+case1:
+  call void @foo(i32 1)
+  ret void
+case2:
+  call void @foo(i32 2)
+  ret void
+default:
+  call void @foo(i32 3)
+  ret void
+}
+
 ; Negative test - check for possible overflow when computing
 ; number of possible cases.
 define void @test4(i128 %a) {
@@ -267,3 +313,7 @@ default:
 
 declare void @llvm.assume(i1)
 
+!0 = !{!"branch_weights", i32 8, i32 4, i32 2, i32 1}
+;.
+; CHECK: [[PROF0]] = !{!"branch_weights", i32 0, i32 4, i32 2, i32 1, i32 8}
+;.