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

src/mips/macro-assembler-mips.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_MIPS_MACRO_ASSEMBLER_MIPS_H_
 #define V8_MIPS_MACRO_ASSEMBLER_MIPS_H_
 
 #include "src/assembler.h"
 #include "src/globals.h"
 #include "src/mips/assembler-mips.h"
@@ skipping 1169 matching lines @@
   // untagged value afterwards.
   void SmiToDoubleFPURegister(Register smi,
                               FPURegister value,
                               Register scratch1);
 
   // -------------------------------------------------------------------------
   // Overflow handling functions.
   // 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,
                                Register right,
                                Register overflow_dst,
                                Register scratch = at);
 
   void SubuAndCheckForOverflow(Register dst, Register left,
                                const Operand& right, Register overflow_dst,
                                Register scratch = at);
 
   void BranchOnOverflow(Label* label,

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

Once you refactor SubuAnd...(), please remove all these BranchOn[No]Overlfow.

                         Register overflow_check,
                         BranchDelaySlot bd = PROTECT) {
     Branch(label, lt, overflow_check, Operand(zero_reg), bd);
   }
 
   void BranchOnNoOverflow(Label* label,
                           Register overflow_check,
                           BranchDelaySlot bd = PROTECT) {
     Branch(label, ge, overflow_check, Operand(zero_reg), bd);
   }
 
   void RetOnOverflow(Register overflow_check, BranchDelaySlot bd = PROTECT) {

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

Looks like we have no users of these RetOn... ops. Please remove them.

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

Done.

     Ret(lt, overflow_check, Operand(zero_reg), bd);
   }
 
   void RetOnNoOverflow(Register overflow_check, BranchDelaySlot bd = PROTECT) {
     Ret(ge, overflow_check, Operand(zero_reg), bd);
   }
 
   // -------------------------------------------------------------------------
   // Runtime calls.
 
@@ skipping 529 matching lines @@
 #define __FILE_LINE__ __FILE__ ":" CODE_COVERAGE_TOSTRING(__LINE__)
 #define ACCESS_MASM(masm) masm->stop(__FILE_LINE__); masm->
 #else
 #define ACCESS_MASM(masm) masm->
 #endif
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_MIPS_MACRO_ASSEMBLER_MIPS_H_