[mips][microMIPS] Implement microMIPS 16-bit instructions registers including ZERO instead of S0

[jkolek]

Summary

Implement microMIPS 16-bit instructions register set: $0, $2-$7 and $17.

[sstankovic]

LGTM

[jkolek]

Closed by commit rL222652 (authored by @jkolek).

lib/Target/Mips/AsmParser/MipsAsmParser.cpp

     Inst.addOperand(MCOperand::CreateReg(getGPRMM16Reg()));
   }
 
+  void addGPRMM16AsmRegZeroOperands(MCInst &Inst, unsigned N) const {
+    assert(N == 1 && "Invalid number of operands!");
+    Inst.addOperand(MCOperand::CreateReg(getGPRMM16Reg()));
+  }
+
   /// Render the operand to an MCInst as a GPR64
   /// Asserts if the wrong number of operands are requested, or the operand
   /// is not a k_RegisterIndex compatible with RegKind_GPR

     return ((RegIdx.Index >= 2 && RegIdx.Index <= 7)
             || RegIdx.Index == 16 || RegIdx.Index == 17);
   }
+  bool isMM16AsmRegZero() const {
+    if (!(isRegIdx() && RegIdx.Kind))
+      return false;
+    return (RegIdx.Index == 0 ||
+            (RegIdx.Index >= 2 && RegIdx.Index <= 7) ||
+            RegIdx.Index == 17);
+  }
   bool isFGRAsmReg() const {
     // AFGR64 is $0-$15 but we handle this in getAFGR64()
     return isRegIdx() && RegIdx.Kind & RegKind_FGR && RegIdx.Index <= 31;

lib/Target/Mips/Disassembler/MipsDisassembler.cpp

                                                uint64_t Address,
                                                const void *Decoder);
 
+static DecodeStatus DecodeGPRMM16ZeroRegisterClass(MCInst &Inst,
+                                                   unsigned RegNo,
+                                                   uint64_t Address,
+                                                   const void *Decoder);
+
 static DecodeStatus DecodeGPR32RegisterClass(MCInst &Inst,
                                              unsigned RegNo,
                                              uint64_t Address,

   return MCDisassembler::Fail;
 }
 
+static DecodeStatus DecodeGPRMM16ZeroRegisterClass(MCInst &Inst,
+                                                   unsigned RegNo,
+                                                   uint64_t Address,
+                                                   const void *Decoder) {
+  return MCDisassembler::Fail;
+}
+
 static DecodeStatus DecodeGPR32RegisterClass(MCInst &Inst,
                                              unsigned RegNo,
                                              uint64_t Address,

lib/Target/Mips/MipsRegisterInfo.td

   // Callee save
   S0, S1)>;
 
+def GPRMM16Zero : RegisterClass<"Mips", [i32], 32, (add
+  // Reserved
+  ZERO,
+  // Return Values and Arguments
+  V0, V1, A0, A1, A2, A3,
+  // Callee save
+  S1)>;
+
 def GPR64 : RegisterClass<"Mips", [i64], 64, (add
 // Reserved
   ZERO_64, AT_64,

   let PredicateMethod = "isMM16AsmReg";
 }
 
+def GPRMM16AsmOperandZero : MipsAsmRegOperand {
+  let Name = "GPRMM16AsmRegZero";
+  let PredicateMethod = "isMM16AsmRegZero";
+}
+
 def ACC64DSPAsmOperand : MipsAsmRegOperand {
   let Name = "ACC64DSPAsmReg";
   let PredicateMethod = "isACCAsmReg";

   let ParserMatchClass = GPRMM16AsmOperand;
 }
 
+def GPRMM16OpndZero : RegisterOperand<GPRMM16Zero> {
+  let ParserMatchClass = GPRMM16AsmOperandZero;
+}
+
 def GPR64Opnd : RegisterOperand<GPR64> {
   let ParserMatchClass = GPR64AsmOperand;
 }