Convert String array index/length hash to BitField

src/x64/macro-assembler-x64.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_X64_MACRO_ASSEMBLER_X64_H_
 #define V8_X64_MACRO_ASSEMBLER_X64_H_
 
 #include "assembler.h"
 #include "frames.h"
 #include "globals.h"
@@ skipping 996 matching lines @@
   void LoadUint32(XMMRegister dst, Register src);
 
   void LoadInstanceDescriptors(Register map, Register descriptors);
   void EnumLength(Register dst, Register map);
   void NumberOfOwnDescriptors(Register dst, Register map);
 
   template<typename Field>
   void DecodeField(Register reg) {
     static const int shift = Field::kShift;
     static const int mask = Field::kMask >> Field::kShift;
-    shrp(reg, Immediate(shift));
+    if (shift != 0) {
+      shrp(reg, Immediate(shift));
+    }
     andp(reg, Immediate(mask));
   }
 
+  template<typename Field>
+  void DecodeFieldToSmi(Register reg) {
+    andp(reg, Immediate(Field::kMask));
+    shlp(reg, Immediate(kSmiShift - Field::kShift));
+  }
+
   // Abort execution if argument is not a number, enabled via --debug-code.
   void AssertNumber(Register object);
 
   // Abort execution if argument is a smi, enabled via --debug-code.
   void AssertNotSmi(Register object);
 
   // Abort execution if argument is not a smi, enabled via --debug-code.
   void AssertSmi(Register object);
   void AssertSmi(const Operand& object);
 
@@ skipping 564 matching lines @@
     masm->popfq();                                                           \
   }                                                                          \
   masm->
 #else
 #define ACCESS_MASM(masm) masm->
 #endif
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_MACRO_ASSEMBLER_X64_H_