MIPS: Use BOVC/BNVC for overflow checking on r6.

src/mips/macro-assembler-mips.h

Index: src/mips/macro-assembler-mips.h
diff --git a/src/mips/macro-assembler-mips.h b/src/mips/macro-assembler-mips.h
index 8890be8131cf5eb9678ca9fe14945020952ce2a8..6c2961bcf340a1fcc8667bc7717921f3d962eec1 100644
--- a/src/mips/macro-assembler-mips.h
+++ b/src/mips/macro-assembler-mips.h
@@ -1187,15 +1187,16 @@ class MacroAssembler: public Assembler {
   // Usage: first call the appropriate arithmetic function, then call one of the
   // jump functions with the overflow_dst register as the second parameter.
 
-  void AdduAndCheckForOverflow(Register dst,
-                               Register left,
-                               Register right,
-                               Register overflow_dst,
-                               Register scratch = at);
+  void AdduAndCheckForOverflow(Register dst, Register left, Register right,
+                               Label* overflow_label, Label* no_overflow_label,
+                               Register scratch = at,
+                               BranchDelaySlot bd = PROTECT);

[paul.l..., 2015/11/13 02:16:20]

I don't see any existing usages of BranchDelaySlot in the BranchOn[No]Overflow.
So ;et's get rid of it. (We can't implement it properly with compact branches
anyway.)

[balazs.kilvady, 2015/11/20 19:28:08]

Done.

 
   void AdduAndCheckForOverflow(Register dst, Register left,
-                               const Operand& right, Register overflow_dst,
-                               Register scratch = at);
+                               const Operand& right, Label* overflow_label,
+                               Label* no_overflow_label, Register scratch = at,
+                               BranchDelaySlot bd = PROTECT);
+
 
   void SubuAndCheckForOverflow(Register dst,

[paul.l..., 2015/11/13 02:16:20]

I'm assuming that you will refactor the SubuAndCheck... tfor consistency, even
though we do not have the equivalent of BOVC/BNVC for subtract....)

[balazs.kilvady, 2015/11/20 19:28:08]

I thought we could negate one of the arguments of subu and then we could use
bovc/bnvc but in that case MIN_INT would cause one more overflow :) and checking
it is expensive. I am thinking about it.

                                Register left,