[Interpreter] Adds an optimization to remove redundant Ldar/Star.

test/unittests/interpreter/bytecode-array-builder-unittest.cc

 // Copyright 2014 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.
 
 #include "src/v8.h"
 
 #include "src/interpreter/bytecode-array-builder.h"
 #include "src/interpreter/bytecode-array-iterator.h"
 #include "test/unittests/test-utils.h"
 
@@ skipping 21 matching lines @@
   // Emit constant loads.
   builder.LoadLiteral(Smi::FromInt(0))
       .LoadLiteral(Smi::FromInt(8))
       .LoadLiteral(Smi::FromInt(10000000))
       .LoadUndefined()
       .LoadNull()
       .LoadTheHole()
       .LoadTrue()
       .LoadFalse();
 
-  // Emit accumulator transfers.
+  // Emit accumulator transfers. Stores followed by loads to the same register
+  // are not generated. Hence, a dummy instruction in between.
   Register reg(0);
-  builder.LoadAccumulatorWithRegister(reg).StoreAccumulatorInRegister(reg);
+  builder.LoadAccumulatorWithRegister(reg)
+      .LoadNull()
+      .StoreAccumulatorInRegister(reg);
 
   // Emit global load / store operations.
   builder.LoadGlobal(0, 1, LanguageMode::SLOPPY, TypeofMode::NOT_INSIDE_TYPEOF)
       .LoadGlobal(0, 1, LanguageMode::STRICT, TypeofMode::NOT_INSIDE_TYPEOF)
       .LoadGlobal(0, 1, LanguageMode::SLOPPY, TypeofMode::INSIDE_TYPEOF)
       .LoadGlobal(0, 1, LanguageMode::STRICT, TypeofMode::INSIDE_TYPEOF)
       .StoreGlobal(0, 1, LanguageMode::SLOPPY)
       .StoreGlobal(0, 1, LanguageMode::STRICT);
 
   // Emit wide global load / store operations.
@@ skipping 643 matching lines @@
 
   CHECK_EQ(iterator.current_bytecode(), Bytecode::kReturn);
   iterator.Advance();
   CHECK(iterator.done());
 }
 
 
 }  // namespace interpreter
 }  // namespace internal
 }  // namespace v8