diff --git a/llvm/test/TableGen/MixedCasedMnemonic.td b/llvm/test/TableGen/MixedCasedMnemonic.td
--- a/llvm/test/TableGen/MixedCasedMnemonic.td
+++ b/llvm/test/TableGen/MixedCasedMnemonic.td
@@ -1,5 +1,6 @@
 // RUN: llvm-tblgen -gen-asm-matcher -I %p/../../include %s | FileCheck %s --check-prefix=MATCHER
 // RUN: llvm-tblgen -gen-asm-writer -I %p/../../include %s | FileCheck %s --check-prefix=WRITER
+// RUN: llvm-tblgen -gen-asm-matcher -I %p/../../include %s | FileCheck %s --check-prefix=ALIAS
 
 // Check that an instruction that uses mixed upper/lower case in its mnemonic
 // is printed as-is, and is parsed in its "canonicalized" lowercase form.
@@ -35,6 +36,9 @@
   let AsmString = "aInst";
 }
 
+def :MnemonicAlias<"Insta", "aInst">;
+def :MnemonicAlias<"InstB", "BInst">;
+
 // Check that the matcher lower()s the mnemonics it matches.
 // MATCHER: static const char *const MnemonicTable =
 // MATCHER-NEXT: "\005ainst\005binst";
@@ -53,3 +57,20 @@
 // WRITER-NEXT:   "aInst\0"
 // WRITER-NEXT: };
 
+// ALIAS: static void applyMnemonicAliases(StringRef &Mnemonic, const FeatureBitset &Features, unsigned VariantID) {
+// ALIAS-NEXT  switch (VariantID) {
+// ALIAS-NEXT  case 0:
+// ALIAS-NEXT      switch (Mnemonic.size()) {
+// ALIAS-NEXT      default: break;
+// ALIAS-NEXT      case 5:	 // 2 strings to match.
+// ALIAS-NEXT        if (memcmp(Mnemonic.data()+0, "inst", 4) != 0)
+// ALIAS-NEXT          break;
+// ALIAS-NEXT        switch (Mnemonic[4]) {
+// ALIAS-NEXT        default: break;
+// ALIAS-NEXT        case 'a':	 // 1 string to match.
+// ALIAS-NEXT          Mnemonic = "ainst";	 // "insta"
+// ALIAS-NEXT          return;
+// ALIAS-NEXT        case 'b':	 // 1 string to match.
+// ALIAS-NEXT          Mnemonic = "binst";	 // "instb"
+// ALIAS-NEXT          return;
+
diff --git a/llvm/utils/TableGen/AsmMatcherEmitter.cpp b/llvm/utils/TableGen/AsmMatcherEmitter.cpp
--- a/llvm/utils/TableGen/AsmMatcherEmitter.cpp
+++ b/llvm/utils/TableGen/AsmMatcherEmitter.cpp
@@ -2726,7 +2726,7 @@
     StringRef AsmVariantName = R->getValueAsString("AsmVariantName");
     if (AsmVariantName != AsmParserVariantName)
       continue;
-    AliasesFromMnemonic[std::string(R->getValueAsString("FromMnemonic"))]
+    AliasesFromMnemonic[R->getValueAsString("FromMnemonic").lower()]
         .push_back(R);
   }
   if (AliasesFromMnemonic.empty())
@@ -2768,7 +2768,7 @@
         MatchCode += "else ";
       MatchCode += "if (" + FeatureMask + ")\n";
       MatchCode += "  Mnemonic = \"";
-      MatchCode += R->getValueAsString("ToMnemonic");
+      MatchCode += R->getValueAsString("ToMnemonic").lower();
       MatchCode += "\";\n";
     }
 
@@ -2777,7 +2777,7 @@
       if (!MatchCode.empty())
         MatchCode += "else\n  ";
       MatchCode += "Mnemonic = \"";
-      MatchCode += R->getValueAsString("ToMnemonic");
+      MatchCode += R->getValueAsString("ToMnemonic").lower();
       MatchCode += "\";\n";
     }