Convert String array index/length hash to BitField

src/ia32/macro-assembler-ia32.h

Index: src/ia32/macro-assembler-ia32.h
diff --git a/src/ia32/macro-assembler-ia32.h b/src/ia32/macro-assembler-ia32.h
index 41c0ea6355c42384ea398e571401aa8cf5e31261..fc6a5a0d3cc4aa486b43942214651d3b7aa5addb 100644
--- a/src/ia32/macro-assembler-ia32.h
+++ b/src/ia32/macro-assembler-ia32.h
@@ -497,9 +497,26 @@ class MacroAssembler: public Assembler {
   void DecodeField(Register reg) {
     static const int shift = Field::kShift;
     static const int mask = Field::kMask >> Field::kShift;
-    sar(reg, shift);
+    if (shift != 0) {
+      sar(reg, shift);
+    }
     and_(reg, Immediate(mask));
   }
+
+  template<typename Field>
+  void DecodeFieldToSmi(Register reg) {
+    static const int shift = Field::kShift;
+    static const int mask = (Field::kMask >> Field::kShift) << kSmiTagSize;
+    STATIC_ASSERT((mask & (0x80000000u >> (kSmiTagSize - 1))) == 0);
+    STATIC_ASSERT(kSmiTag == 0);
+    if (shift < kSmiTagSize) {
+      shl(reg, kSmiTagSize - shift);
+    } else if (shift > kSmiTagSize) {
+      sar(reg, shift - kSmiTagSize);
+    }
+    and_(reg, Immediate(mask));
+  }
+
   void LoadPowerOf2(XMMRegister dst, Register scratch, int power);
 
   // Abort execution if argument is not a number, enabled via --debug-code.