Index: lib/Target/AArch64/AArch64InstrFormats.td
===================================================================
--- lib/Target/AArch64/AArch64InstrFormats.td
+++ lib/Target/AArch64/AArch64InstrFormats.td
@@ -4082,6 +4082,26 @@
   let Inst{1-0}   = ll;
 }
 
+//---
+// UDF : Permanently UNDEFINED instructions.  Format: Opc = 0x0000, 16 bit imm.
+//--
+def uimm16 : Operand<i16>, ImmLeaf<i16, [{
+  return ((uint32_t)Imm) < 65536;
+}]>{
+  let ParserMatchClass = Imm0_65535Operand;
+  let PrintMethod = "printImm";
+}
+
+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;
+}
+}
 let Predicates = [HasFPARMv8] in {
 
 //---
Index: lib/Target/AArch64/AArch64InstrInfo.td
===================================================================
--- lib/Target/AArch64/AArch64InstrInfo.td
+++ lib/Target/AArch64/AArch64InstrInfo.td
@@ -1594,6 +1594,8 @@
 def : InstAlias<"dcps2", (DCPS2 0)>;
 def : InstAlias<"dcps3", (DCPS3 0)>;
 
+def UDF : UDFType<0, "udf">;
+
 //===----------------------------------------------------------------------===//
 // Load instructions.
 //===----------------------------------------------------------------------===//
Index: test/MC/AArch64/udf-erros.s
===================================================================
--- /dev/null
+++ test/MC/AArch64/udf-erros.s
@@ -0,0 +1,10 @@
+# RUN: not llvm-mc -assemble -show-encoding -triple=aarch64- %s 2>&1 | FileCheck %s
+
+udf 65536
+# CHECK:{{.*}} immediate must be an integer in range [0, 65535].
+
+udf -1
+# CHECK:{{.*}} immediate must be an integer in range [0, 65535].
+
+udf -768
+# CHECK:{{.*}} immediate must be an integer in range [0, 65535].
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
+.text
+udf 0
+udf 258
+udf 65535
+# CHECK:  .text
+# CHECK-NEXT: udf #0      // encoding: [0x00,0x00,0x00,0x00]
+# CHECK-NEXT: udf #258      // encoding: [0x02,0x01,0x00,0x00]
+# CHECK-NEXT: udf #65535      // encoding: [0xff,0xff,0x00,0x00]
Index: test/MC/Disassembler/AArch64/udf.txt
===================================================================
--- /dev/null
+++ test/MC/Disassembler/AArch64/udf.txt
@@ -0,0 +1,19 @@
+# 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]
+[0x02,0x01,0x00,0x00]
+[0xff,0xff,0x00,0x00]
+
+# CHECK: .text
+# CHECK-NEXT: udf #0
+# CHECK-NEXT: udf #258
+# 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:	02 01 00 00 	udf	#258
+# OBJ-NEXT:        8:	ff ff 00 00 	udf	#65535