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 +79,49 @@
 }
 
 
+void Assembler::AdduDetectOverflow(Register rd, Register rs, Register rt,
+                                   Register ro) {
+  ASSERT(rd != ro);
+  ASSERT(rd != TMP);
+  ASSERT(ro != TMP);
+  ASSERT(ro != rs);
+  ASSERT(ro != rt);
+
+  if ((rs == rt) && (rd == rs)) {
+    ASSERT(rd != T9);

[regis, 2013/04/05 23:23:45]

This macro should take a scratch register argument that would be used here
instead of implicitly using T9: ASSERT(scratch != kNoRegister)
The caller is free to pass kNoRegister if no scratch register will be required.

[zra, 2013/04/06 00:04:19]

I thought it might be clearer to require a scratch register and to use that
instead of TMP, and then to use TMP instead of T9 in the case that rd = rs = rt.
I think it might be too hard to remember to provide a scratch register only in
this one weird case.

+    ASSERT(ro != T9);
+    mov(T9, rt);
+    rt = T9;
+  }
+
+  if (rd == rs) {
+    mov(TMP, rs);  // Preserve rs.
+    addu(rd, rs, rt);  // rs is overwritten.
+    xor_(TMP, rd, TMP);  // Original rs.
+    xor_(ro, rd, rt);
+    and_(ro, ro, TMP);
+  } else if (rd == rt) {
+    mov(TMP, rt);  // Preserve rt.
+    addu(rd, rs, rt);  // rt is overwritten.
+    xor_(TMP, rd, TMP);  // Original rt.
+    xor_(ro, rd, rs);
+    and_(ro, ro, TMP);
+  } else {
+    addu(rd, rs, rt);
+    xor_(ro, rd, rs);
+    xor_(TMP, rd, rt);
+    and_(ro, TMP, ro);
+  }
+}
+
+
+void Assembler::AddiuDetectOverflow(Register rt, Register rs,
+                                    const Immediate& imm, Register ro) {

[regis, 2013/04/05 23:23:45]

ditto, scratch argument instead of T8.

[zra, 2013/04/06 00:04:19]

I decided to remove this function. Otherwise it would need multiple scratch
registers, and it's probably more readable at call sites just to load the
immediate by hand.

+  addiu(T8, ZR, imm);
+  AdduDetectOverflow(rt, rs, T8, 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()) {