MIPS: Convert String array index/length hash to BitField.

src/mips/macro-assembler-mips.h

Index: src/mips/macro-assembler-mips.h
diff --git a/src/mips/macro-assembler-mips.h b/src/mips/macro-assembler-mips.h
index 88967db9c488c049dd0c9aba288485b2d48f4c46..dcf895bfcb11a210f2fb73c0a708c09e7f66106b 100644
--- a/src/mips/macro-assembler-mips.h
+++ b/src/mips/macro-assembler-mips.h
@@ -1485,17 +1485,33 @@ const Operand& rt = Operand(zero_reg), BranchDelaySlot bd = PROTECT
 
   template<typename Field>
   void DecodeField(Register dst, Register src) {
+    Ext(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;
-    srl(dst, src, 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) {
+      sll(dst, src, kSmiTagSize - shift);
       And(dst, dst, Operand(mask));
+    } else if (shift > kSmiTagSize) {
+      srl(dst, src, 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);
   }