Index: llvm/trunk/lib/Target/RISCV/RISCVInstrInfo.td
===================================================================
--- llvm/trunk/lib/Target/RISCV/RISCVInstrInfo.td
+++ llvm/trunk/lib/Target/RISCV/RISCVInstrInfo.td
@@ -403,6 +403,15 @@
   let rd = 0;
   let imm12 = 1;
 }
+
+// This is a de facto standard (as set by GNU binutils) 32-bit unimplemented
+// instruction (i.e., it should always trap, if your implementation has invalid
+// instruction traps).
+def UNIMP : RVInstI<0b001, OPC_SYSTEM, (outs), (ins), "unimp", ""> {
+  let rs1 = 0;
+  let rd = 0;
+  let imm12 = 0b110000000000;
+}
 } // hasSideEffects = 1, mayLoad = 0, mayStore = 0
 
 def CSRRW : CSR_ir<0b001, "csrrw">;
Index: llvm/trunk/lib/Target/RISCV/RISCVInstrInfoC.td
===================================================================
--- llvm/trunk/lib/Target/RISCV/RISCVInstrInfoC.td
+++ llvm/trunk/lib/Target/RISCV/RISCVInstrInfoC.td
@@ -514,6 +514,13 @@
   let Inst{9-7}   = imm{8-6};
 }
 
+// The all zeros pattern isn't a valid RISC-V instruction. It's used by GNU
+// binutils as 16-bit instruction known to be unimplemented (i.e., trapping).
+let hasSideEffects = 1, mayLoad = 0, mayStore = 0 in
+def C_UNIMP : RVInst16<(outs), (ins), "c.unimp", "", [], InstFormatOther> {
+  let Inst{15-0} = 0;
+}
+
 } // Predicates = [HasStdExtC]
 
 //===----------------------------------------------------------------------===//
@@ -677,6 +684,7 @@
 def : CompressPat<(ADDI GPRNoX0:$rs1, GPRNoX0:$rs2, 0),
                   (C_MV GPRNoX0:$rs1, GPRNoX0:$rs2)>;
 def : CompressPat<(EBREAK), (C_EBREAK)>;
+def : CompressPat<(UNIMP), (C_UNIMP)>;
 def : CompressPat<(JALR X1, GPRNoX0:$rs1, 0),
                   (C_JALR GPRNoX0:$rs1)>;
 def : CompressPat<(ADD GPRNoX0:$rs1, GPRNoX0:$rs1, GPRNoX0:$rs2),
Index: llvm/trunk/test/MC/RISCV/compress-rv32i.s
===================================================================
--- llvm/trunk/test/MC/RISCV/compress-rv32i.s
+++ llvm/trunk/test/MC/RISCV/compress-rv32i.s
@@ -211,3 +211,9 @@
 # CHECK-INST: c.swsp zero, 252(sp)
 # CHECK: # encoding: [0x82,0xdf]
 sw zero, 252(sp)
+
+# CHECK-BYTES: 00 00
+# CHECK-ALIAS: unimp
+# CHECK-INST: c.unimp
+# CHECK: # encoding: [0x00,0x00]
+unimp
Index: llvm/trunk/test/MC/RISCV/rv32c-valid.s
===================================================================
--- llvm/trunk/test/MC/RISCV/rv32c-valid.s
+++ llvm/trunk/test/MC/RISCV/rv32c-valid.s
@@ -103,3 +103,6 @@
 # CHECK-ASM-AND-OBJ: c.lui s0, 1048575
 # CHECK-ASM: encoding: [0x7d,0x74]
 c.lui s0, 0xfffff
+# CHECK-ASM-AND-OBJ: c.unimp
+# CHECK-ASM: encoding: [0x00,0x00]
+c.unimp
Index: llvm/trunk/test/MC/RISCV/rv32i-valid.s
===================================================================
--- llvm/trunk/test/MC/RISCV/rv32i-valid.s
+++ llvm/trunk/test/MC/RISCV/rv32i-valid.s
@@ -248,6 +248,9 @@
 # CHECK-ASM-AND-OBJ: ebreak
 # CHECK-ASM: encoding: [0x73,0x00,0x10,0x00]
 ebreak
+# CHECK-ASM-AND-OBJ: unimp
+# CHECK-ASM: encoding: [0x73,0x10,0x00,0xc0]
+unimp
 
 # CHECK-ASM-AND-OBJ: csrrw t0, 4095, t1
 # CHECK-ASM: encoding: [0xf3,0x12,0xf3,0xff]