implement the null function for the Mips32 subzero compiler

src/IceInstMIPS32.h

 //===- subzero/src/IceInstMIPS32.h - MIPS32 machine instrs --*- C++ -*-===    //
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
 /// This file declares the InstMIPS32 and OperandMIPS32 classes and
 /// their subclasses.  This represents the machine instructions and
 /// operands used for MIPS32 code selection.
 ///
 //===----------------------------------------------------------------------===//
 
 #ifndef SUBZERO_SRC_ICEINSTMIPS32_H
 #define SUBZERO_SRC_ICEINSTMIPS32_H
 
 #include "IceDefs.h"
+#include "IceInst.h"
+#include "IceInstMIPS32.def"
+#include "IceOperand.h"
 
 namespace Ice {
 
 class TargetMIPS32;
-// Fill this in.
+
+// Base class for Mips instructions.
+class InstMIPS32 : public InstTarget {
+  InstMIPS32() = delete;
+  InstMIPS32(const InstMIPS32 &) = delete;
+  InstMIPS32 &operator=(const InstMIPS32 &) = delete;
+
+public:
+  enum InstKindMIPS32 { k__Start = Inst::Target, Ret };
+
+  static const char *getWidthString(Type Ty);
+
+  void dump(const Cfg *Func) const override;
+
+protected:
+  InstMIPS32(Cfg *Func, InstKindMIPS32 Kind, SizeT Maxsrcs, Variable *Dest)
+      : InstTarget(Func, static_cast<InstKind>(Kind), Maxsrcs, Dest) {}
+  ~InstMIPS32() override {}
+  static bool isClassof(const Inst *Inst, InstKindMIPS32 MyKind) {
+    return Inst->getKind() == static_cast<InstKind>(MyKind);
+  }
+};
+
+// Ret pseudo-instruction.  This is actually a "jr" instruction with
+// an "ra" register operand, but epilogue lowering will search for a Ret
+// instead of a generic "jr". This instruction also takes a Source
+// operand (for non-void returning functions) for liveness analysis, though
+// a FakeUse before the ret would do just as well.
+// This needs was take from the ARM port and needs to be scrubbed in the future
+// TODO(reed kotler)
+//
+class InstMIPS32Ret : public InstMIPS32 {
+
+  InstMIPS32Ret() = delete;
+  InstMIPS32Ret(const InstMIPS32Ret &) = delete;
+  InstMIPS32Ret &operator=(const InstMIPS32Ret &) = delete;
+
+public:
+  static InstMIPS32Ret *create(Cfg *Func, Variable *RA,
+                               Variable *Source = nullptr) {
+    return new (Func->allocate<InstMIPS32Ret>())
+        InstMIPS32Ret(Func, RA, Source);
+  }
+  void emit(const Cfg *Func) const override;
+  void emitIAS(const Cfg *Func) const override;
+  void dump(const Cfg *Func) const override;
+  static bool classof(const Inst *Inst) { return isClassof(Inst, Ret); }
+
+private:
+  InstMIPS32Ret(Cfg *Func, Variable *RA, Variable *Source);
+  ~InstMIPS32Ret() override {}
+};
 
 } // end of namespace Ice
 
 #endif // SUBZERO_SRC_ICEINSTMIPS32_H