Convert String array index/length hash to BitField

src/arm/macro-assembler-arm.h

Index: src/arm/macro-assembler-arm.h
diff --git a/src/arm/macro-assembler-arm.h b/src/arm/macro-assembler-arm.h
index 4000478eb6359b458df80138eef19284b11107a4..5f77be6a4293feb82e6bde281ae0c5769b4c97bf 100644
--- a/src/arm/macro-assembler-arm.h
+++ b/src/arm/macro-assembler-arm.h
@@ -1357,17 +1357,33 @@ class MacroAssembler: public Assembler {
 
   template<typename Field>
   void DecodeField(Register dst, Register src) {
+    Ubfx(dst, src, Field::kShift, Field::kSize);
+  }
+
+  template<typename Field>
+  void DecodeField(Register reg) {
+    DecodeField<Field>(reg, reg);
+  }
+
+  template<typename Field>
+  void DecodeFieldToSmi(Register dst, Register src) {
     static const int shift = Field::kShift;
-    static const int mask = Field::kMask >> shift;
-    static const int size = Field::kSize;
-    mov(dst, Operand(src, LSR, shift));
-    if (shift + size != 32) {
+    static const int mask = Field::kMask >> shift << kSmiTagSize;
+    STATIC_ASSERT((mask & (0x80000000u >> (kSmiTagSize - 1))) == 0);
+    STATIC_ASSERT(kSmiTag == 0);
+    if (shift < kSmiTagSize) {
+      mov(dst, Operand(src, LSL, kSmiTagSize - shift));
+      and_(dst, dst, Operand(mask));
+    } else if (shift > kSmiTagSize) {
+      mov(dst, Operand(src, LSR, shift - kSmiTagSize));
       and_(dst, dst, Operand(mask));
+    } else {
+      and_(dst, src, Operand(mask));
     }
   }
 
   template<typename Field>
-  void DecodeField(Register reg) {
+  void DecodeFieldToSmi(Register reg) {
     DecodeField<Field>(reg, reg);
   }