diff --git a/llvm/include/llvm/Analysis/ValueTracking.h b/llvm/include/llvm/Analysis/ValueTracking.h
index af218ba564081..093309cb8bbee 100644
--- a/llvm/include/llvm/Analysis/ValueTracking.h
+++ b/llvm/include/llvm/Analysis/ValueTracking.h
@@ -1024,6 +1024,16 @@ findValuesAffectedByCondition(Value *Cond, bool IsAssume,
 LLVM_ABI Value *stripNullTest(Value *V);
 LLVM_ABI const Value *stripNullTest(const Value *V);
 
+/// Enumerates all possible values of V and inserts them into the set \p
+/// Constants. If \p AllowUndefOrPoison is false, it fails when V may contain
+/// undef/poison elements. Returns true if the result is complete. Otherwise,
+/// the result is incomplete (more than MaxCount values).
+/// NOTE: The constant values are not distinct.
+LLVM_ABI bool
+collectPossibleValues(const Value *V,
+                      SmallPtrSetImpl<const Constant *> &Constants,
+                      unsigned MaxCount, bool AllowUndefOrPoison = true);
+
 } // end namespace llvm
 
 #endif // LLVM_ANALYSIS_VALUETRACKING_H
diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index 523374bdc472f..789a98366cead 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -10341,3 +10341,55 @@ const Value *llvm::stripNullTest(const Value *V) {
 Value *llvm::stripNullTest(Value *V) {
   return const_cast<Value *>(stripNullTest(const_cast<const Value *>(V)));
 }
+
+bool llvm::collectPossibleValues(const Value *V,
+                                 SmallPtrSetImpl<const Constant *> &Constants,
+                                 unsigned MaxCount, bool AllowUndefOrPoison) {
+  SmallPtrSet<const Instruction *, 8> Visited;
+  SmallVector<const Instruction *, 8> Worklist;
+  auto Push = [&](const Value *V) -> bool {
+    if (auto *C = dyn_cast<Constant>(V)) {
+      if (!AllowUndefOrPoison && !isGuaranteedNotToBeUndefOrPoison(C))
+        return false;
+      // Check existence first to avoid unnecessary allocations.
+      if (Constants.contains(C))
+        return true;
+      if (Constants.size() == MaxCount)
+        return false;
+      Constants.insert(C);
+      return true;
+    }
+
+    if (auto *Inst = dyn_cast<Instruction>(V)) {
+      if (Visited.insert(Inst).second)
+        Worklist.push_back(Inst);
+      return true;
+    }
+    return false;
+  };
+  if (!Push(V))
+    return false;
+  while (!Worklist.empty()) {
+    const Instruction *CurInst = Worklist.pop_back_val();
+    switch (CurInst->getOpcode()) {
+    case Instruction::Select:
+      if (!Push(CurInst->getOperand(1)))
+        return false;
+      if (!Push(CurInst->getOperand(2)))
+        return false;
+      break;
+    case Instruction::PHI:
+      for (Value *IncomingValue : cast<PHINode>(CurInst)->incoming_values()) {
+        // Fast path for recurrence PHI.
+        if (IncomingValue == CurInst)
+          continue;
+        if (!Push(IncomingValue))
+          return false;
+      }
+      break;
+    default:
+      return false;
+    }
+  }
+  return true;
+}
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index bb733277e0fad..532511dcf91b0 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -6020,6 +6020,8 @@ static bool eliminateDeadSwitchCases(SwitchInst *SI, DomTreeUpdater *DTU,
                                      const DataLayout &DL) {
   Value *Cond = SI->getCondition();
   KnownBits Known = computeKnownBits(Cond, DL, AC, SI);
+  SmallPtrSet<const Constant *, 4> KnownValues;
+  bool IsKnownValuesValid = collectPossibleValues(Cond, KnownValues, 4);
 
   // We can also eliminate cases by determining that their values are outside of
   // the limited range of the condition based on how many significant (non-sign)
@@ -6039,15 +6041,18 @@ static bool eliminateDeadSwitchCases(SwitchInst *SI, DomTreeUpdater *DTU,
         UniqueSuccessors.push_back(Successor);
       ++It->second;
     }
-    const APInt &CaseVal = Case.getCaseValue()->getValue();
+    ConstantInt *CaseC = Case.getCaseValue();
+    const APInt &CaseVal = CaseC->getValue();
     if (Known.Zero.intersects(CaseVal) || !Known.One.isSubsetOf(CaseVal) ||
-        (CaseVal.getSignificantBits() > MaxSignificantBitsInCond)) {
-      DeadCases.push_back(Case.getCaseValue());
+        (CaseVal.getSignificantBits() > MaxSignificantBitsInCond) ||
+        (IsKnownValuesValid && !KnownValues.contains(CaseC))) {
+      DeadCases.push_back(CaseC);
       if (DTU)
         --NumPerSuccessorCases[Successor];
       LLVM_DEBUG(dbgs() << "SimplifyCFG: switch case " << CaseVal
                         << " is dead.\n");
-    }
+    } else if (IsKnownValuesValid)
+      KnownValues.erase(CaseC);
   }
 
   // If we can prove that the cases must cover all possible values, the
@@ -6058,33 +6063,41 @@ static bool eliminateDeadSwitchCases(SwitchInst *SI, DomTreeUpdater *DTU,
   const unsigned NumUnknownBits =
       Known.getBitWidth() - (Known.Zero | Known.One).popcount();
   assert(NumUnknownBits <= Known.getBitWidth());
-  if (HasDefault && DeadCases.empty() &&
-      NumUnknownBits < 64 /* avoid overflow */) {
-    uint64_t AllNumCases = 1ULL << NumUnknownBits;
-    if (SI->getNumCases() == AllNumCases) {
+  if (HasDefault && DeadCases.empty()) {
+    if (IsKnownValuesValid && all_of(KnownValues, IsaPred<UndefValue>)) {
       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");
-      IntegerType *CondTy = cast<IntegerType>(Cond->getType());
-      if (CondTy->getIntegerBitWidth() > 64 ||
-          !DL.fitsInLegalInteger(CondTy->getIntegerBitWidth()))
-        return false;
 
-      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 (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");
+        IntegerType *CondTy = cast<IntegerType>(Cond->getType());
+        if (CondTy->getIntegerBitWidth() > 64 ||
+            !DL.fitsInLegalInteger(CondTy->getIntegerBitWidth()))
+          return false;
+
+        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;
+      }
     }
   }
 
diff --git a/llvm/test/CodeGen/AArch64/arm64-ccmp.ll b/llvm/test/CodeGen/AArch64/arm64-ccmp.ll
index cad5df0d9655e..68ab8902767b3 100644
--- a/llvm/test/CodeGen/AArch64/arm64-ccmp.ll
+++ b/llvm/test/CodeGen/AArch64/arm64-ccmp.ll
@@ -430,12 +430,12 @@ declare i32 @foo()
 
 ; Test case distilled from 126.gcc.
 ; The phi in sw.bb.i.i gets multiple operands for the %entry predecessor.
-define void @build_modify_expr() nounwind ssp {
+define void @build_modify_expr(i32 %cond) nounwind ssp {
 ; CHECK-LABEL: build_modify_expr:
 ; CHECK:       ; %bb.0: ; %entry
 ; CHECK-NEXT:    ret
 entry:
-  switch i32 undef, label %sw.bb.i.i [
+  switch i32 %cond, label %sw.bb.i.i [
     i32 69, label %if.end85
     i32 70, label %if.end85
     i32 71, label %if.end85
diff --git a/llvm/test/CodeGen/Hexagon/vect/zext-v4i1.ll b/llvm/test/CodeGen/Hexagon/vect/zext-v4i1.ll
index 559bb68741e12..930cf8152b756 100644
--- a/llvm/test/CodeGen/Hexagon/vect/zext-v4i1.ll
+++ b/llvm/test/CodeGen/Hexagon/vect/zext-v4i1.ll
@@ -6,11 +6,11 @@
 target datalayout = "e-m:e-p:32:32:32-a:0-n16:32-i64:64:64-i32:32:32-i16:16:16-i1:8:8-f32:32:32-f64:64:64-v32:32:32-v64:64:64-v512:512:512-v1024:1024:1024-v2048:2048:2048"
 target triple = "hexagon"
 
-define i32 @fred(ptr %a0) #0 {
+define i32 @fred(ptr %a0, i32 %cond) #0 {
 ; CHECK-LABEL: fred:
 ; CHECK:       // %bb.0: // %b0
 ; CHECK-NEXT:    {
-; CHECK-NEXT:     if (p0) jump:nt .LBB0_2
+; CHECK-NEXT:     p0 = cmp.eq(r1,#5); if (!p0.new) jump:t .LBB0_2
 ; CHECK-NEXT:    }
 ; CHECK-NEXT:  // %bb.1: // %b2
 ; CHECK-NEXT:    {
@@ -40,7 +40,7 @@ define i32 @fred(ptr %a0) #0 {
 ; CHECK-NEXT:     jumpr r31
 ; CHECK-NEXT:    }
 b0:
-  switch i32 undef, label %b14 [
+  switch i32 %cond, label %b14 [
     i32 5, label %b2
     i32 3, label %b1
   ]
diff --git a/llvm/test/Transforms/SimplifyCFG/switch-on-const-select.ll b/llvm/test/Transforms/SimplifyCFG/switch-on-const.ll
similarity index 54%
rename from llvm/test/Transforms/SimplifyCFG/switch-on-const-select.ll
rename to llvm/test/Transforms/SimplifyCFG/switch-on-const.ll
index e8b58639c13dd..1ab1b5e8bd838 100644
--- a/llvm/test/Transforms/SimplifyCFG/switch-on-const-select.ll
+++ b/llvm/test/Transforms/SimplifyCFG/switch-on-const.ll
@@ -154,6 +154,132 @@ bees:
   unreachable
 }
 
+define void @pr165179(i1 %cond) {
+; CHECK-LABEL: @pr165179(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br i1 [[COND:%.*]], label [[IF_THEN:%.*]], label [[IF_ELSE:%.*]]
+; CHECK:       if.then:
+; CHECK-NEXT:    tail call void @bees.a() #[[ATTR0]]
+; CHECK-NEXT:    br label [[SWITCHBB:%.*]]
+; CHECK:       if.else:
+; CHECK-NEXT:    tail call void @bees.b() #[[ATTR0]]
+; CHECK-NEXT:    br label [[SWITCHBB]]
+; CHECK:       exit:
+; CHECK-NEXT:    tail call void @bees.a() #[[ATTR0]]
+; CHECK-NEXT:    ret void
+;
+entry:
+  br i1 %cond, label %if.then, label %if.else
+
+if.then:
+  tail call void @bees.a() nounwind
+  br label %switchbb
+
+if.else:
+  tail call void @bees.b() nounwind
+  br label %switchbb
+
+switchbb:
+  %cond1 = phi i32 [ 1, %if.else ], [ -1, %if.then ]
+  switch i32 %cond1, label %default [
+  i32 1, label %exit
+  i32 -1, label %exit
+  ]
+
+exit:
+  tail call void @bees.a() nounwind
+  ret void
+
+default:
+  tail call void @bees.b() nounwind
+  ret void
+}
+
+define void @switch_remove_dead_case_phi(i1 %cond1, i1 %cond2) {
+; CHECK-LABEL: @switch_remove_dead_case_phi(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br i1 [[COND1:%.*]], label [[IF_THEN:%.*]], label [[IF_ELSE:%.*]]
+; CHECK:       if.then:
+; CHECK-NEXT:    tail call void @bees.a() #[[ATTR0]]
+; CHECK-NEXT:    br i1 [[COND2:%.*]], label [[SWITCHBB:%.*]], label [[IF_ELSE]]
+; CHECK:       if.else:
+; CHECK-NEXT:    [[PHI:%.*]] = phi i32 [ 3, [[ENTRY:%.*]] ], [ -1, [[IF_THEN]] ]
+; CHECK-NEXT:    tail call void @bees.b() #[[ATTR0]]
+; CHECK-NEXT:    br label [[SWITCHBB]]
+; CHECK:       switchbb:
+; CHECK-NEXT:    [[COND:%.*]] = phi i32 [ [[PHI]], [[IF_ELSE]] ], [ 5, [[IF_THEN]] ]
+; CHECK-NEXT:    [[COND3:%.*]] = icmp eq i32 [[COND]], -1
+; CHECK-NEXT:    br i1 [[COND3]], label [[EXIT:%.*]], label [[DEFAULT:%.*]]
+; CHECK:       common.ret:
+; CHECK-NEXT:    ret void
+; CHECK:       exit:
+; CHECK-NEXT:    tail call void @bees.a() #[[ATTR0]]
+; CHECK-NEXT:    br label [[COMMON_RET:%.*]]
+; CHECK:       default:
+; CHECK-NEXT:    tail call void @bees.b() #[[ATTR0]]
+; CHECK-NEXT:    br label [[COMMON_RET]]
+;
+entry:
+  br i1 %cond1, label %if.then, label %if.else
+
+if.then:
+  tail call void @bees.a() nounwind
+  br i1 %cond2, label %switchbb, label %if.else
+
+if.else:
+  %phi = phi i32 [ 3, %entry ], [ -1, %if.then ]
+  tail call void @bees.b() nounwind
+  br label %switchbb
+
+switchbb:
+  %cond = phi i32 [ %phi, %if.else ], [ 5, %if.then ]
+  switch i32 %cond, label %default [
+  i32 1, label %exit
+  i32 -1, label %exit
+  ]
+
+exit:
+  tail call void @bees.a() nounwind
+  ret void
+
+default:
+  tail call void @bees.b() nounwind
+  ret void
+}
+
+define void @switch_remove_dead_case_select(i1 %cond1, i1 %cond2) {
+; CHECK-LABEL: @switch_remove_dead_case_select(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[X:%.*]] = select i1 [[COND1:%.*]], i32 -1, i32 3
+; CHECK-NEXT:    [[Y:%.*]] = select i1 [[COND2:%.*]], i32 [[X]], i32 5
+; CHECK-NEXT:    [[COND:%.*]] = icmp eq i32 [[Y]], -1
+; CHECK-NEXT:    br i1 [[COND]], label [[EXIT:%.*]], label [[DEFAULT:%.*]]
+; CHECK:       common.ret:
+; CHECK-NEXT:    ret void
+; CHECK:       exit:
+; CHECK-NEXT:    tail call void @bees.a() #[[ATTR0]]
+; CHECK-NEXT:    br label [[COMMON_RET:%.*]]
+; CHECK:       default:
+; CHECK-NEXT:    tail call void @bees.b() #[[ATTR0]]
+; CHECK-NEXT:    br label [[COMMON_RET]]
+;
+entry:
+  %x = select i1 %cond1, i32 -1, i32 3
+  %y = select i1 %cond2, i32 %x, i32 5
+  switch i32 %y, label %default [
+  i32 1, label %exit
+  i32 -1, label %exit
+  ]
+
+exit:
+  tail call void @bees.a() nounwind
+  ret void
+
+default:
+  tail call void @bees.b() nounwind
+  ret void
+}
+
 declare void @llvm.trap() nounwind noreturn
 declare void @bees.a() nounwind
 declare void @bees.b() nounwind
diff --git a/llvm/test/Transforms/SimplifyCFG/switch_mask.ll b/llvm/test/Transforms/SimplifyCFG/switch_mask.ll
index f8bcbc057a7ae..428c18fc18e3d 100644
--- a/llvm/test/Transforms/SimplifyCFG/switch_mask.ll
+++ b/llvm/test/Transforms/SimplifyCFG/switch_mask.ll
@@ -221,6 +221,7 @@ define i1 @pr88607() {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[COND:%.*]] = select i1 false, i32 4, i32 1
 ; CHECK-NEXT:    [[SPEC_SELECT:%.*]] = select i1 false, i32 2, i32 [[COND]]
+; CHECK-NEXT:    [[COND1:%.*]] = icmp eq i32 [[SPEC_SELECT]], 1
 ; CHECK-NEXT:    ret i1 false
 ;
 entry:
diff --git a/llvm/test/Transforms/SimplifyCFG/switch_undef.ll b/llvm/test/Transforms/SimplifyCFG/switch_undef.ll
index 88a729b7d941a..4de5ea948ed27 100644
--- a/llvm/test/Transforms/SimplifyCFG/switch_undef.ll
+++ b/llvm/test/Transforms/SimplifyCFG/switch_undef.ll
@@ -5,12 +5,11 @@
 define void @f6() #0 {
 ; CHECK-LABEL: @f6(
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    br label [[FOR_COND_I:%.*]]
-; CHECK:       for.cond.i:
+; CHECK-NEXT:    br label [[F1_EXIT_I:%.*]]
+; CHECK:       f1.exit.i:
 ; CHECK-NEXT:    [[TOBOOL7_I:%.*]] = icmp ne i16 1, 0
-; CHECK-NEXT:    br label [[FOR_COND_I]]
+; CHECK-NEXT:    br label [[F1_EXIT_I]]
 ;
-
 entry:
   br label %for.cond.i