[RISCV] MC layer support for the instructions added in the privileged spec

Adds support for the instructions added in the RISC-V privileged ISA
(https://content.riscv.org/wp-content/uploads/2017/05/riscv-privileged-v1.10.pdf):
uret, sret, mret, wfi, and sfence.vma.

Note from the committer: I made very minor formatting changes prior to commit, 
which didn't seem worth creating another review round-trip for.

Differential Revision: https://reviews.llvm.org/D40383

Patch by David Craven.

llvm-svn: 320484

lib/Target/RISCV/RISCVInstrInfo.td

@@ -211,6 +211,11 @@ class ALUW_rr<bits<7> funct7, bits<3> funct3, string opcodestr>
     : RVInstR<funct7, funct3, OPC_OP_32, (outs GPR:$rd),
               (ins GPR:$rs1, GPR:$rs2), opcodestr, "$rd, $rs1, $rs2">;
 
+let hasSideEffects = 1, mayLoad = 0, mayStore = 0 in
+class Priv<string opcodestr, bits<7> funct7>
+    : RVInstR<funct7, 0b000, OPC_SYSTEM, (outs), (ins GPR:$rs1, GPR:$rs2),
+              opcodestr, "">;
+
 //===----------------------------------------------------------------------===//
 // Instructions
 //===----------------------------------------------------------------------===//
@@ -333,6 +338,43 @@ def SRLW  : ALUW_rr<0b0000000, 0b101, "srlw">;
 def SRAW  : ALUW_rr<0b0100000, 0b101, "sraw">;
 } // Predicates = [IsRV64]
 
+//===----------------------------------------------------------------------===//
+// Privileged instructions
+//===----------------------------------------------------------------------===//
+
+let isBarrier = 1, isReturn = 1, isTerminator = 1 in {
+def URET : Priv<"uret", 0b0000000> {
+  let rd = 0;
+  let rs1 = 0;
+  let rs2 = 0b00010;
+}
+
+def SRET : Priv<"sret", 0b0001000> {
+  let rd = 0;
+  let rs1 = 0;
+  let rs2 = 0b00010;
+}
+
+def MRET : Priv<"mret", 0b0011000> {
+  let rd = 0;
+  let rs1 = 0;
+  let rs2 = 0b00010;
+}
+} // isBarrier = 1, isReturn = 1, isTerminator = 1
+
+def WFI : Priv<"wfi", 0b0001000> {
+  let rd = 0;
+  let rs1 = 0;
+  let rs2 = 0b00101;
+}
+
+let hasSideEffects = 1, mayLoad = 0, mayStore = 0 in
+def SFENCE_VMA : RVInstR<0b0001001, 0b000, OPC_SYSTEM, (outs),
+                         (ins GPR:$rs1, GPR:$rs2),
+                         "sfence.vma", "$rs1, $rs2"> {
+  let rd = 0;
+}
+
 //===----------------------------------------------------------------------===//
 // Pseudo-instructions and codegen patterns
 //

test/MC/RISCV/priv-invalid.s

@@ -0,0 +1,7 @@
+# RUN: not llvm-mc -triple riscv32 < %s 2>&1 | FileCheck %s
+
+mret 0x10 # CHECK: :[[@LINE]]:6: error: invalid operand for instruction
+
+sfence.vma zero # CHECK: :[[@LINE]]:1: error: too few operands for instruction
+
+sfence.vma a0, 0x10 # CHECK: :[[@LINE]]:16: error: invalid operand for instruction

test/MC/RISCV/priv-valid.s

@@ -0,0 +1,32 @@
+# RUN: llvm-mc %s -triple=riscv32 -show-encoding \
+# RUN:     | FileCheck -check-prefixes=CHECK,CHECK-INST %s
+# RUN: llvm-mc %s -triple=riscv64 -show-encoding \
+# RUN:     | FileCheck -check-prefixes=CHECK,CHECK-INST %s
+# RUN: llvm-mc -filetype=obj -triple riscv32 < %s \
+# RUN:     | llvm-objdump -d - | FileCheck -check-prefix=CHECK-INST %s
+# RUN: llvm-mc -filetype=obj -triple riscv64 < %s \
+# RUN:     | llvm-objdump -d - | FileCheck -check-prefix=CHECK-INST %s
+
+# CHECK-INST: uret
+# CHECK: encoding: [0x73,0x00,0x20,0x00]
+uret
+
+# CHECK-INST: sret
+# CHECK: encoding: [0x73,0x00,0x20,0x10]
+sret
+
+# CHECK-INST: mret
+# CHECK: encoding: [0x73,0x00,0x20,0x30]
+mret
+
+# CHECK-INST: wfi
+# CHECK: encoding: [0x73,0x00,0x50,0x10]
+wfi
+
+# CHECK-INST: sfence.vma zero, zero
+# CHECK: encoding: [0x73,0x00,0x00,0x12]
+sfence.vma zero, zero
+
+# CHECK-INST: sfence.vma a0, a1
+# CHECK: encoding: [0x73,0x00,0xb5,0x12]
+sfence.vma a0, a1