Adds macros to the MIPS assembler for detecting overflow.

runtime/vm/assembler_mips.cc

Index: runtime/vm/assembler_mips.cc
===================================================================
--- runtime/vm/assembler_mips.cc	(revision 20999)
+++ runtime/vm/assembler_mips.cc	(working copy)
@@ -79,6 +79,43 @@
 }
 
 
+void Assembler::AdduDetectOverflow(Register rd, Register rs, Register rt,
+                                   Register ro, Register scratch) {
+  ASSERT(rd != ro);
+  ASSERT(rd != scratch);
+  ASSERT(ro != scratch);
+  ASSERT(ro != rs);
+  ASSERT(ro != rt);
+
+  if ((rs == rt) && (rd == rs)) {
+    ASSERT(rd != TMP);
+    ASSERT(ro != TMP);
+    ASSERT(scratch != TMP);
+    mov(TMP, rt);
+    rt = TMP;
+  }
+
+  if (rd == rs) {
+    mov(scratch, rs);  // Preserve rs.
+    addu(rd, rs, rt);  // rs is overwritten.
+    xor_(scratch, rd, scratch);  // Original rs.
+    xor_(ro, rd, rt);
+    and_(ro, ro, scratch);
+  } else if (rd == rt) {
+    mov(scratch, rt);  // Preserve rt.
+    addu(rd, rs, rt);  // rt is overwritten.
+    xor_(scratch, rd, scratch);  // Original rt.
+    xor_(ro, rd, rs);
+    and_(ro, ro, scratch);
+  } else {
+    addu(rd, rs, rt);
+    xor_(ro, rd, rs);
+    xor_(scratch, rd, rt);
+    and_(ro, scratch, ro);
+  }
+}
+
+
 void Assembler::LoadObject(Register rd, const Object& object) {
   // Smi's and VM heap objects are never relocated; do not use object pool.
   if (object.IsSmi()) {