Index: include/llvm/Target/Target.td
===================================================================
--- include/llvm/Target/Target.td
+++ include/llvm/Target/Target.td
@@ -605,6 +605,10 @@
   // match failure error message. By default, use a generic "invalid operand"
   // diagnostic. The target AsmParser maps these codes to text.
   string DiagnosticType = "";
+
+  /// Set to 1 if this operand is optional and not always required.  Optional
+  /// operands must be at the end of the operand list.
+  bit IsOptional = 0;
 }
 
 def ImmAsmOperand : AsmOperandClass {
Index: utils/TableGen/AsmMatcherEmitter.cpp
===================================================================
--- utils/TableGen/AsmMatcherEmitter.cpp
+++ utils/TableGen/AsmMatcherEmitter.cpp
@@ -199,6 +199,10 @@
 
   /// For custom match classes: the diagnostic kind for when the predicate fails.
   std::string DiagnosticType;
+
+  /// Is this operand optional and not always required.
+  bool IsOptional;
+
 public:
   /// isRegisterClass() - Check if this is a register class.
   bool isRegisterClass() const {
@@ -1058,6 +1062,7 @@
     Entry->RenderMethod = "<invalid>";
     Entry->ParserMethod = "";
     Entry->DiagnosticType = "";
+    Entry->IsOptional = false;
   }
 
   return Entry;
@@ -1193,6 +1198,7 @@
     CI->Registers = RS;
     // FIXME: diagnostic type.
     CI->DiagnosticType = "";
+    CI->IsOptional = false;
     RegisterSetClasses.insert(std::make_pair(RS, CI));
     ++Index;
   }
@@ -1307,6 +1313,10 @@
     if (StringInit *SI = dyn_cast<StringInit>(DiagnosticType))
       CI->DiagnosticType = SI->getValue();
 
+    Init *IsOptional = Rec->getValueInit("IsOptional");
+    if (BitInit *BI = dyn_cast<BitInit>(IsOptional))
+      CI->IsOptional = BI->getValue();
+
     ++Index;
   }
 }
@@ -2041,6 +2051,7 @@
      << "/// instruction matching.\n";
   OS << "enum MatchClassKind {\n";
   OS << "  InvalidMatchClass = 0,\n";
+  OS << "  OptionalMatchClass = 1,\n";
   for (const auto &CI : Infos) {
     OS << "  " << CI.Name << ", // ";
     if (CI.Kind == ClassInfo::Token) {
@@ -2132,6 +2143,8 @@
   SS << "    return false;\n";
   for (const auto &A : Infos) {
     std::vector<StringRef> SuperClasses;
+    if (A.IsOptional)
+      SuperClasses.push_back("OptionalMatchClass");
     for (const auto &B : Infos) {
       if (&A != &B && A.isSubsetOf(B))
         SuperClasses.push_back(B.Name);
@@ -2976,7 +2989,8 @@
   OS << "    for (unsigned i = SIndex; i != " << MaxNumOperands << "; ++i) {\n";
   OS << "      auto Formal = static_cast<MatchClassKind>(it->Classes[i]);\n";
   OS << "      if (i >= Operands.size()) {\n";
-  OS << "        OperandsValid = (Formal == " <<"InvalidMatchClass);\n";
+  OS << "        OperandsValid = (Formal == " <<"InvalidMatchClass) || " <<
+                                 "isSubclass(Formal, OptionalMatchClass);\n";
   OS << "        if (!OperandsValid) ErrorInfo = i;\n";
   OS << "        break;\n";
   OS << "      }\n";