diff --git a/llvm/lib/Support/RISCVISAInfo.cpp b/llvm/lib/Support/RISCVISAInfo.cpp
--- a/llvm/lib/Support/RISCVISAInfo.cpp
+++ b/llvm/lib/Support/RISCVISAInfo.cpp
@@ -804,6 +804,9 @@
                                  Desc.str().c_str(), Name.str().c_str());
       }
 
+      if (IgnoreUnknown && !isSupportedExtension(Ext))
+        continue;
+
       ISAInfo->addExtension(Name, Major, Minor);
       // Extension format is correct, keep parsing the extensions.
       // TODO: Save Type, Name, Major, Minor to avoid parsing them later.
diff --git a/llvm/unittests/Support/RISCVISAInfoTest.cpp b/llvm/unittests/Support/RISCVISAInfoTest.cpp
--- a/llvm/unittests/Support/RISCVISAInfoTest.cpp
+++ b/llvm/unittests/Support/RISCVISAInfoTest.cpp
@@ -230,7 +230,8 @@
 }
 
 TEST(ParseArchString, IgnoresUnrecognizedExtensionNamesWithIgnoreUnknown) {
-  for (StringRef Input : {"rv32ib"}) {
+  for (StringRef Input : {"rv32ib", "rv32i_zmadeup", "rv64i_smadeup",
+                          "rv32i_sxmadeup", "rv64i_xmadeup"}) {
     auto MaybeISAInfo = RISCVISAInfo::parseArchString(Input, true, false, true);
     ASSERT_THAT_EXPECTED(MaybeISAInfo, Succeeded());
     RISCVISAInfo &Info = **MaybeISAInfo;
@@ -238,13 +239,6 @@
     EXPECT_EQ(Exts.size(), 1UL);
     EXPECT_TRUE(Exts.at("i") == (RISCVExtensionInfo{"i", 2, 0}));
   }
-  // FIXME: These unrecognized extensions should be ignored just as in the
-  // case above. The below captures the current (incorrect) behaviour.
-  for (StringRef Input :
-       {"rv32i_zmadeup", "rv64i_smadeup", "rv32i_sxmadeup", "rv64i_xmadeup"}) {
-    auto MaybeISAInfo = RISCVISAInfo::parseArchString(Input, true, false, true);
-    EXPECT_THAT_EXPECTED(MaybeISAInfo, Failed());
-  }
 }
 
 TEST(ParseArchString, AcceptsVersionInLongOrShortForm) {