diff --git a/llvm/lib/MC/MCSubtargetInfo.cpp b/llvm/lib/MC/MCSubtargetInfo.cpp
--- a/llvm/lib/MC/MCSubtargetInfo.cpp
+++ b/llvm/lib/MC/MCSubtargetInfo.cpp
@@ -56,7 +56,7 @@
   }
 }
 
-static void ApplyFeatureFlag(FeatureBitset &Bits, StringRef Feature,
+static bool ApplyFeatureFlag(FeatureBitset &Bits, StringRef Feature,
                              ArrayRef<SubtargetFeatureKV> FeatureTable) {
   assert(SubtargetFeatures::hasFlag(Feature) &&
          "Feature flags should start with '+' or '-'");
@@ -78,10 +78,13 @@
       // For each feature that implies this, clear it.
       ClearImpliedBits(Bits, FeatureEntry->Value, FeatureTable);
     }
+    return true;
   } else {
     errs() << "'" << Feature << "' is not a recognized feature for this target"
            << " (ignoring feature)\n";
+    return false;
   }
+  return false;
 }
 
 /// Return the length of the longest entry in the table.
@@ -297,13 +300,19 @@
 
 bool MCSubtargetInfo::checkFeatures(StringRef FS) const {
   SubtargetFeatures T(FS);
-  FeatureBitset Set, All;
+  FeatureBitset Set, All, Empty;
   for (std::string F : T.getFeatures()) {
-    ::ApplyFeatureFlag(Set, F, ProcFeatures);
+    if (!::ApplyFeatureFlag(Set, F, ProcFeatures))
+      continue;
     if (F[0] == '-')
       F[0] = '+';
-    ::ApplyFeatureFlag(All, F, ProcFeatures);
+    (void)::ApplyFeatureFlag(All, F, ProcFeatures);
   }
+
+  // All input features are unknown features.
+  if (Set == Empty && All == Empty)
+    return false;
+
   return (FeatureBits & All) == Set;
 }
 
diff --git a/llvm/unittests/CodeGen/TargetOptionsTest.cpp b/llvm/unittests/CodeGen/TargetOptionsTest.cpp
--- a/llvm/unittests/CodeGen/TargetOptionsTest.cpp
+++ b/llvm/unittests/CodeGen/TargetOptionsTest.cpp
@@ -3,6 +3,7 @@
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/IR/LegacyPassManager.h"
 #include "llvm/InitializePasses.h"
+#include "llvm/MC/MCSubtargetInfo.h"
 #include "llvm/MC/TargetRegistry.h"
 #include "llvm/Support/TargetSelect.h"
 #include "llvm/Target/TargetMachine.h"
@@ -39,8 +40,8 @@
   TargetOptions Options;
   Options.EnableIPRA = EnableIPRA;
   return std::unique_ptr<TargetMachine>(
-      T->createTargetMachine("X86", "", "", Options, std::nullopt, std::nullopt,
-                             CodeGenOpt::Aggressive));
+      T->createTargetMachine("X86", "", "+avx", Options, std::nullopt,
+                             std::nullopt, CodeGenOpt::Aggressive));
 }
 
 typedef std::function<void(bool)> TargetOptionsTest;
@@ -61,6 +62,17 @@
   delete TPC;
 }
 
+static void targetFeaturesTest(StringRef F) {
+  std::unique_ptr<TargetMachine> TM = createTargetMachine(false);
+  // This test is designed for the X86 backend; stop if it is not available.
+  if (!TM)
+    GTEST_SKIP();
+
+  ASSERT_NE(TM->getMCSubtargetInfo(), nullptr);
+
+  ASSERT_FALSE(TM->getMCSubtargetInfo()->checkFeatures(F));
+}
+
 } // End of anonymous namespace.
 
 TEST(TargetOptionsTest, IPRASetToOff) {
@@ -71,6 +83,10 @@
   targetOptionsTest(true);
 }
 
+TEST(TargetOptionsTest, KnownFeature) { targetFeaturesTest("-avx"); }
+
+TEST(TargetOptionsTest, UnknownFeature) { targetFeaturesTest("+relax"); }
+
 int main(int argc, char **argv) {
   ::testing::InitGoogleTest(&argc, argv);
   initLLVM();