On MIPS, keeps Object::null() in a register.

runtime/vm/assembler_mips.h

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #ifndef VM_ASSEMBLER_MIPS_H_
 #define VM_ASSEMBLER_MIPS_H_
 
 #ifndef VM_ASSEMBLER_H_
 #error Do not include assembler_mips.h directly; use assembler.h instead.
 #endif
@@ skipping 556 matching lines @@
   }
 
   void xor_(Register rd, Register rs, Register rt) {
     EmitRType(SPECIAL, rs, rt, rd, 0, XOR);
   }
 
   // Macros in alphabetical order.
 
   // Addition of rs and rt with the result placed in rd.
   // After, ro < 0 if there was signed overflow, ro >= 0 otherwise.
-  // rd and ro must not be TMP1 or TMP2.
+  // rd and ro must not be TMP1.
   // ro must be different from all the other registers.
-  void AdduDetectOverflow(Register rd, Register rs, Register rt, Register ro);
+  // If rd, rs, and rt are the same register, then a scratch register different
+  // from the other registers is needed.
+  void AdduDetectOverflow(Register rd, Register rs, Register rt, Register ro,
+                          Register scratch = kNoRegister);
 
   // ro must be different from rd and rs.
-  // rd and ro must not be TMP1 or TMP2
+  // rd and ro must not be TMP1.
+  // If rd and rs are the same, a scratch register different from the other
+  // registers is needed.
   void AddImmediateDetectOverflow(Register rd, Register rs, int32_t imm,
-                                  Register ro) {
+                                  Register ro, Register scratch = kNoRegister) {
     LoadImmediate(rd, imm);
-    AdduDetectOverflow(rd, rs, rd, ro);
+    AdduDetectOverflow(rd, rs, rd, ro, scratch);
   }
 
   // Subtraction of rt from rs (rs - rt) with the result placed in rd.
   // After, ro < 0 if there was signed overflow, ro >= 0 otherwise.
   // None of rd, rs, rt, or ro may be TMP1.
   // ro must be different from the other registers.
   void SubuDetectOverflow(Register rd, Register rs, Register rt, Register ro);
 
   // ro must be different from rd and rs.
   // None of rd, rs, rt, or ro may be TMP1.
@@ skipping 244 matching lines @@
   void StoreIntoObjectNoBarrier(Register object,
                                 const Address& dest,
                                 const Object& value);
 
   void CallRuntime(const RuntimeEntry& entry);
 
   // Set up a Dart frame on entry with a frame pointer and PC information to
   // enable easy access to the RawInstruction object of code corresponding
   // to this frame.
   void EnterDartFrame(intptr_t frame_size);
-  void LeaveDartFrame();
+  void LeaveDartFrameAndReturn();
 
  private:
   AssemblerBuffer buffer_;
   GrowableObjectArray& object_pool_;  // Objects and patchable jump targets.
   int prologue_offset_;
 
   bool delay_slot_available_;
   bool in_delay_slot_;
 
   int32_t AddObject(const Object& obj);
@@ skipping 142 matching lines @@
                                   Register value,
                                   Label* no_update);
 
   DISALLOW_ALLOCATION();
   DISALLOW_COPY_AND_ASSIGN(Assembler);
 };
 
 }  // namespace dart
 
 #endif  // VM_ASSEMBLER_MIPS_H_