Index: lib/Target/AArch64/AArch64InstrFormats.td
===================================================================
--- lib/Target/AArch64/AArch64InstrFormats.td
+++ lib/Target/AArch64/AArch64InstrFormats.td
@@ -234,6 +234,13 @@
 // Operand Definitions.
 //
 
+class AsmImmRange<int Low, int High> : AsmOperandClass {
+  let Name = "Imm" # Low # "_" # High;
+  let DiagnosticType = "InvalidImm" # Low # "_" # High;
+  let RenderMethod = "addImmOperands";
+  let PredicateMethod = "isImmInRange<" # Low # "," # High # ">";
+}
+
 // ADR[P] instruction labels.
 def AdrpOperand : AsmOperandClass {
   let Name = "AdrpLabel";
@@ -287,6 +294,10 @@
   let ParserMatchClass = UImm6Operand;
 }
 
+def uimm16 : Operand<i16>, ImmLeaf<i16, [{return Imm >= 0 && Imm < 65536;}]>{
+  let ParserMatchClass = AsmImmRange<0, 65535>;
+}
+
 def SImm9Operand : SImmOperand<9>;
 def simm9 : Operand<i64>, ImmLeaf<i64, [{ return Imm >= -256 && Imm < 256; }]> {
   let ParserMatchClass = SImm9Operand;
@@ -447,13 +458,6 @@
   let DecoderMethod = "DecodeSImm<4>";
 }
 
-class AsmImmRange<int Low, int High> : AsmOperandClass {
-  let Name = "Imm" # Low # "_" # High;
-  let DiagnosticType = "InvalidImm" # Low # "_" # High;
-  let RenderMethod = "addImmOperands";
-  let PredicateMethod = "isImmInRange<" # Low # "," # High # ">";
-}
-
 def Imm1_8Operand : AsmImmRange<1, 8>;
 def Imm1_16Operand : AsmImmRange<1, 16>;
 def Imm1_32Operand : AsmImmRange<1, 32>;
@@ -708,11 +712,10 @@
 }
 
 // imm0_65535 predicate - True if the immediate is in the range [0,65535].
-def Imm0_65535Operand : AsmImmRange<0, 65535>;
 def imm0_65535 : Operand<i32>, ImmLeaf<i32, [{
   return ((uint32_t)Imm) < 65536;
 }]> {
-  let ParserMatchClass = Imm0_65535Operand;
+  let ParserMatchClass = AsmImmRange<0, 65535>;
   let PrintMethod = "printImmHex";
 }
 
@@ -1937,7 +1940,7 @@
 //---
 
 def movimm32_imm : Operand<i32> {
-  let ParserMatchClass = Imm0_65535Operand;
+  let ParserMatchClass = AsmImmRange<0, 65535>;
   let EncoderMethod = "getMoveWideImmOpValue";
   let PrintMethod = "printImm";
 }
@@ -4084,6 +4087,20 @@
 
 let Predicates = [HasFPARMv8] in {
 
+//---
+// UDF : Permanently UNDEFINED instructions.  Format: Opc = 0x0000, 16 bit imm.
+//--
+let hasSideEffects = 1, isTrap = 1, mayLoad = 0, mayStore = 0 in {
+class UDFType<bits<16> opc, string asm>
+  : I<(outs), (ins uimm16:$imm),
+       asm, "\t$imm", "", []>,
+    Sched<[]> {
+  bits<16> imm;
+  let Inst{31-16} = opc;
+  let Inst{15-0} = imm;
+}
+}
+
 //---
 // Floating point to integer conversion
 //---
Index: lib/Target/AArch64/AArch64InstrInfo.td
===================================================================
--- lib/Target/AArch64/AArch64InstrInfo.td
+++ lib/Target/AArch64/AArch64InstrInfo.td
@@ -1618,6 +1618,8 @@
 def : InstAlias<"dcps2", (DCPS2 0)>;
 def : InstAlias<"dcps3", (DCPS3 0)>;
 
+def UDF : UDFType<0, "udf">;
+
 //===----------------------------------------------------------------------===//
 // Load instructions.
 //===----------------------------------------------------------------------===//
Index: test/MC/AArch64/udf.s
===================================================================
--- /dev/null
+++ test/MC/AArch64/udf.s
@@ -0,0 +1,9 @@
+# RUN: llvm-mc -assemble -show-encoding -triple=aarch64- %s | FileCheck %s
+# CHECK:  .text
+# CHECK-NEXT: udf #0       // encoding: [0x00,0x00,0x00,0x00]
+# CHECK-NEXT: udf #513     // encoding: [0x01,0x02,0x00,0x00]
+# CHECK-NEXT: udf #65535   // encoding: [0xff,0xff,0x00,0x00]
+.text
+udf 0
+udf 513
+udf 65535
\ No newline at end of file
Index: test/MC/AArch64/udf_not.s
===================================================================
--- /dev/null
+++ test/MC/AArch64/udf_not.s
@@ -0,0 +1,7 @@
+# RUN: not llvm-mc -assemble -show-encoding -triple=aarch64- %s 2>&1 | FileCheck %s
+udf 65536
+udf -1
+udf -768
+# CHECK:{{.*}} immediate must be an integer in range [0, 65535].
+# CHECK:{{.*}} immediate must be an integer in range [0, 65535].
+# CHECK:{{.*}} immediate must be an integer in range [0, 65535].
\ No newline at end of file
Index: test/MC/Disassembler/AArch64/udf.txt
===================================================================
--- /dev/null
+++ test/MC/Disassembler/AArch64/udf.txt
@@ -0,0 +1,18 @@
+# RUN: llvm-mc -arch aarch64 -disassemble -o - %s | FileCheck %s
+# RUN: llvm-mc -arch aarch64 -disassemble -o - %s | \
+# RUN: llvm-mc -assemble -filetype=obj -arch aarch64 -o - | \
+# RUN: llvm-objdump -r -d --triple=arm64- - | \
+# RUN: FileCheck %s -check-prefix=OBJ
+[0x00,0x00,0x00,0x00]
+[0x01,0x02,0x00,0x00]
+[0xff,0xff,0x00,0x00]
+# CHECK: .text
+# CHECK-NEXT: udf #0
+# CHECK-NEXT: udf #513
+# CHECK-NEXT: udf #65535
+
+#OBJ: Disassembly of section .text:
+#OBJ-NEXT: $x.0:
+#OBJ-NEXT:        0:	00 00 00 00 	udf	#0
+#OBJ-NEXT:        4:	01 02 00 00 	udf	#513
+#OBJ-NEXT:        8:  ff ff 00 00   udf #65535
\ No newline at end of file