Index: llvm/lib/Support/RISCVISAInfo.cpp
===================================================================
--- llvm/lib/Support/RISCVISAInfo.cpp
+++ llvm/lib/Support/RISCVISAInfo.cpp
@@ -629,6 +629,10 @@
     break;
   }
 
+  if (Arch.back() == '_')
+    return createStringError(errc::invalid_argument,
+                             "extension name missing after separator '_'");
+
   // Skip rvxxx
   StringRef Exts = Arch.substr(5);
 
Index: llvm/unittests/Support/RISCVISAInfoTest.cpp
===================================================================
--- llvm/unittests/Support/RISCVISAInfoTest.cpp
+++ llvm/unittests/Support/RISCVISAInfoTest.cpp
@@ -317,16 +317,12 @@
   EXPECT_EQ(
       toString(RISCVISAInfo::parseArchString("rv64i__m", true).takeError()),
       "invalid standard user-level extension '_'");
-  EXPECT_EQ(
-      toString(RISCVISAInfo::parseArchString("rv32ezicsr_", true).takeError()),
-      "extension name missing after separator '_'");
-
-  // FIXME: Trailing underscores after single letter extensions are accepted,
-  // which is inconsistent.
-  ASSERT_THAT_EXPECTED(RISCVISAInfo::parseArchString("rv32i_", true),
-                       Succeeded());
-  ASSERT_THAT_EXPECTED(RISCVISAInfo::parseArchString("rv64im_", true),
-                       Succeeded());
+
+  for (StringRef Input :
+       {"rv32ezicsr__zifencei", "rv32i_", "rv32izicsr_", "rv64im_"}) {
+    EXPECT_EQ(toString(RISCVISAInfo::parseArchString(Input, true).takeError()),
+              "extension name missing after separator '_'");
+  }
 }
 
 TEST(ParseArchString, RejectsDuplicateExtensionNames) {