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

Port r21533 (736c779)

BUG=
R=plind44@gmail.com

Review URL: https://codereview.chromium.org/297233007

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21539 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/mips/macro-assembler-mips.cc

@@ -3999,19 +3999,14 @@ bool MacroAssembler::AllowThisStubCall(CodeStub* stub) {
 }
 
 
-void MacroAssembler::IndexFromHash(Register hash,
-                                   Register index) {
+void MacroAssembler::IndexFromHash(Register hash, Register index) {
   // If the hash field contains an array index pick it out. The assert checks
   // that the constants for the maximum number of digits for an array index
   // cached in the hash field and the number of bits reserved for it does not
   // conflict.
   ASSERT(TenToThe(String::kMaxCachedArrayIndexLength) <
          (1 << String::kArrayIndexValueBits));
-  // We want the smi-tagged index in key.  kArrayIndexValueMask has zeros in
-  // the low kHashShift bits.
-  STATIC_ASSERT(kSmiTag == 0);
-  Ext(hash, hash, String::kHashShift, String::kArrayIndexValueBits);
-  sll(index, hash, kSmiTagSize);
+  DecodeFieldToSmi<String::ArrayIndexValueBits>(index, hash);
 }
 
 

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);
   }