diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index b46a1bc2abc6..3678327627b9 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -39321,6 +39321,7 @@ void X86TargetLowering::computeKnownBitsForTargetNode(const SDValue Op,
     }
     break;
   }
+  case X86ISD::FANDN:
   case X86ISD::ANDNP: {
     KnownBits Known2;
     Known = DAG.computeKnownBits(Op.getOperand(1), DemandedElts, Depth + 1);
diff --git a/llvm/unittests/Target/X86/CMakeLists.txt b/llvm/unittests/Target/X86/CMakeLists.txt
index b011681aa3b9..d4304438cfc9 100644
--- a/llvm/unittests/Target/X86/CMakeLists.txt
+++ b/llvm/unittests/Target/X86/CMakeLists.txt
@@ -24,4 +24,5 @@ set(LLVM_LINK_COMPONENTS
 add_llvm_unittest(X86Tests
   MachineSizeOptsTest.cpp
   TernlogTest.cpp
+  X86SelectionDAGTest.cpp
   )
diff --git a/llvm/unittests/Target/X86/X86SelectionDAGTest.cpp b/llvm/unittests/Target/X86/X86SelectionDAGTest.cpp
new file mode 100644
index 000000000000..b546908a4893
--- /dev/null
+++ b/llvm/unittests/Target/X86/X86SelectionDAGTest.cpp
@@ -0,0 +1,103 @@
+//===----------------------------------------------------------------------===//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "X86ISelLowering.h"
+#include "llvm/Analysis/MemoryLocation.h"
+#include "llvm/Analysis/OptimizationRemarkEmitter.h"
+#include "llvm/AsmParser/Parser.h"
+#include "llvm/CodeGen/MachineModuleInfo.h"
+#include "llvm/CodeGen/SelectionDAG.h"
+#include "llvm/CodeGen/TargetLowering.h"
+#include "llvm/IR/MDBuilder.h"
+#include "llvm/IR/Module.h"
+#include "llvm/MC/TargetRegistry.h"
+#include "llvm/Support/KnownBits.h"
+#include "llvm/Support/SourceMgr.h"
+#include "llvm/Support/TargetSelect.h"
+#include "llvm/Target/TargetMachine.h"
+#include "gtest/gtest.h"
+
+namespace llvm {
+
+class X86SelectionDAGTest : public testing::Test {
+protected:
+  const TargetSubtargetInfo *STI;
+
+  static void SetUpTestCase() {
+    LLVMInitializeX86TargetInfo();
+    LLVMInitializeX86Target();
+    LLVMInitializeX86TargetMC();
+  }
+
+  void SetUp() override {
+    StringRef Assembly = "define void @f() { ret void }";
+
+    Triple TargetTriple("x86_64--");
+    std::string Error;
+    const Target *T = TargetRegistry::lookupTarget("", TargetTriple, Error);
+
+    TargetOptions Options;
+    TM = std::unique_ptr<TargetMachine>(T->createTargetMachine(
+        TargetTriple, "x86-64-v4", "", Options, std::nullopt, std::nullopt,
+        CodeGenOptLevel::Aggressive));
+
+    SMDiagnostic SMError;
+    M = parseAssemblyString(Assembly, SMError, Context);
+    if (!M)
+      report_fatal_error(SMError.getMessage());
+    M->setDataLayout(TM->createDataLayout());
+
+    F = M->getFunction("f");
+    if (!F)
+      report_fatal_error("F?");
+
+    MachineModuleInfo MMI(TM.get());
+
+    STI = TM->getSubtargetImpl(*F);
+    MF = std::make_unique<MachineFunction>(*F, *TM, *STI, MMI.getContext(), 0);
+
+    DAG = std::make_unique<SelectionDAG>(*TM, CodeGenOptLevel::None);
+    if (!DAG)
+      report_fatal_error("DAG?");
+    OptimizationRemarkEmitter ORE(F);
+    DAG->init(*MF, ORE, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr,
+              MMI, nullptr);
+  }
+
+  LLVMContext Context;
+  std::unique_ptr<TargetMachine> TM;
+  std::unique_ptr<Module> M;
+  Function *F;
+  std::unique_ptr<MachineFunction> MF;
+  std::unique_ptr<SelectionDAG> DAG;
+};
+
+TEST_F(X86SelectionDAGTest, computeKnownBits_FANDN) {
+  SDLoc Loc;
+
+  auto SrcF32 = DAG->getCopyFromReg(DAG->getEntryNode(), Loc, 1, MVT::f32);
+  auto SignBitF32 = DAG->getConstantFP(-0.0f, Loc, MVT::f32);
+  auto OpF32 = DAG->getNode(X86ISD::FANDN, Loc, MVT::f32, SignBitF32, SrcF32);
+  KnownBits KnownF32 = DAG->computeKnownBits(OpF32);
+  EXPECT_TRUE(KnownF32.isNonNegative());
+
+  auto Src2xF64 = DAG->getCopyFromReg(DAG->getEntryNode(), Loc, 1, MVT::v2f64);
+  auto ZeroF64 = DAG->getConstantFP(+0.0f, Loc, MVT::f64);
+  auto SignBitF64 = DAG->getConstantFP(-0.0f, Loc, MVT::f64);
+  auto HiSign2xF64 =
+      DAG->getBuildVector(MVT::v2f64, Loc, {ZeroF64, SignBitF64});
+  auto Op2xF64 =
+      DAG->getNode(X86ISD::FANDN, Loc, MVT::v2f64, HiSign2xF64, Src2xF64);
+  KnownBits KnownAll2xF64 = DAG->computeKnownBits(Op2xF64);
+  KnownBits KnownLo2xF64 = DAG->computeKnownBits(Op2xF64, APInt(2, 1));
+  KnownBits KnownHi2xF64 = DAG->computeKnownBits(Op2xF64, APInt(2, 2));
+  EXPECT_FALSE(KnownAll2xF64.isNonNegative());
+  EXPECT_FALSE(KnownLo2xF64.isNonNegative());
+  EXPECT_TRUE(KnownHi2xF64.isNonNegative());
+}
+
+} // end namespace llvm
diff --git a/llvm/utils/gn/secondary/llvm/unittests/Target/X86/BUILD.gn b/llvm/utils/gn/secondary/llvm/unittests/Target/X86/BUILD.gn
index af2c6d38d951..e70ccd67b401 100644
--- a/llvm/utils/gn/secondary/llvm/unittests/Target/X86/BUILD.gn
+++ b/llvm/utils/gn/secondary/llvm/unittests/Target/X86/BUILD.gn
@@ -21,5 +21,6 @@ unittest("X86Tests") {
   sources = [
     "MachineSizeOptsTest.cpp",
     "TernlogTest.cpp",
+	"X86SelectionDAGTest.cpp",
   ]
 }