Index: llvm/lib/Support/RISCVISAInfo.cpp
===================================================================
--- llvm/lib/Support/RISCVISAInfo.cpp
+++ llvm/lib/Support/RISCVISAInfo.cpp
@@ -1101,6 +1101,8 @@
     std::string ExtName = Ext.first;
     if (ExtName == "i") // i is not recognized in clang -cc1
       continue;
+    if (!isSupportedExtension(ExtName))
+      continue;
     std::string Feature = isExperimentalExtension(ExtName)
                               ? "+experimental-" + ExtName
                               : "+" + ExtName;
Index: llvm/unittests/Support/RISCVISAInfoTest.cpp
===================================================================
--- llvm/unittests/Support/RISCVISAInfoTest.cpp
+++ llvm/unittests/Support/RISCVISAInfoTest.cpp
@@ -457,3 +457,10 @@
   EXPECT_THAT((*MaybeISAInfo2)->toFeatureVector(),
               ElementsAre("+e", "+experimental-zihintntl", "+xventanacondops"));
 }
+
+TEST(ToFeatureVector, UnsupportedExtensionsAreDropped) {
+  auto MaybeISAInfo =
+      RISCVISAInfo::parseNormalizedArchString("rv64i2p0_m2p0_xmadeup1p0");
+  ASSERT_THAT_EXPECTED(MaybeISAInfo, Succeeded());
+  EXPECT_THAT((*MaybeISAInfo)->toFeatureVector(), ElementsAre("+m"));
+}