MIPS: Fix big-endian after r21774/r21803.

Fix big-endian ordering of InstanceType and BitField by always loading
the pair as a 16-bit value, even in the API accessor. Clean up some
assertions.

R=danno@chromium.org, mtbrandy@gmail.com

Review URL: https://codereview.chromium.org/334403003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21906 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

include/v8.h

@@ -5523,7 +5523,8 @@ class Internals {
   // These values match non-compiler-dependent values defined within
   // the implementation of v8.
   static const int kHeapObjectMapOffset = 0;
-  static const int kMapInstanceTypeOffset = 1 * kApiPointerSize + kApiIntSize;
+  static const int kMapInstanceTypeAndBitFieldOffset =
+      1 * kApiPointerSize + kApiIntSize;
   static const int kStringResourceOffset = 3 * kApiPointerSize;
 
   static const int kOddballKindOffset = 3 * kApiPointerSize;
@@ -5601,7 +5602,9 @@ class Internals {
   V8_INLINE static int GetInstanceType(const internal::Object* obj) {
     typedef internal::Object O;
     O* map = ReadField<O*>(obj, kHeapObjectMapOffset);
-    return ReadField<uint8_t>(map, kMapInstanceTypeOffset);
+    // Map::InstanceType is defined so that it will always be loaded into
+    // the LS 8 bits of one 16-bit word, regardless of endianess.
+    return ReadField<uint16_t>(map, kMapInstanceTypeAndBitFieldOffset) & 0xff;
   }
 
   V8_INLINE static int GetOddballKind(const internal::Object* obj) {

src/hydrogen-instructions.h

@@ -6094,10 +6094,12 @@ class HObjectAccess V8_FINAL {
   }
 
   static HObjectAccess ForMapInstanceTypeAndBitField() {
-    STATIC_ASSERT((Map::kInstanceTypeOffset & 1) == 0);
-    STATIC_ASSERT(Map::kBitFieldOffset == Map::kInstanceTypeOffset + 1);
+    STATIC_ASSERT((Map::kInstanceTypeAndBitFieldOffset & 1) == 0);
+    // Ensure the two fields share one 16-bit word, endian-independent.
+    STATIC_ASSERT((Map::kBitFieldOffset & ~1) ==
+                  (Map::kInstanceTypeOffset & ~1));
     return HObjectAccess(kInobject,
-                         Map::kInstanceTypeOffset,
+                         Map::kInstanceTypeAndBitFieldOffset,
                          Representation::UInteger16());
   }
 

src/objects.h

@@ -6704,17 +6704,20 @@ class Map: public HeapObject {
 #if V8_TARGET_LITTLE_ENDIAN
   // Order instance type and bit field together such that they can be loaded
   // together as a 16-bit word with instance type in the lower 8 bits regardless
-  // of endianess.
+  // of endianess. Also provide endian-independent offset to that 16-bit word.
   static const int kInstanceTypeOffset = kInstanceAttributesOffset + 0;
   static const int kBitFieldOffset = kInstanceAttributesOffset + 1;
 #else
   static const int kBitFieldOffset = kInstanceAttributesOffset + 0;
   static const int kInstanceTypeOffset = kInstanceAttributesOffset + 1;
 #endif
+  static const int kInstanceTypeAndBitFieldOffset =
+      kInstanceAttributesOffset + 0;
   static const int kBitField2Offset = kInstanceAttributesOffset + 2;
   static const int kUnusedPropertyFieldsOffset = kInstanceAttributesOffset + 3;
 
-  STATIC_ASSERT(kInstanceTypeOffset == Internals::kMapInstanceTypeOffset);
+  STATIC_ASSERT(kInstanceTypeAndBitFieldOffset ==
+                Internals::kMapInstanceTypeAndBitFieldOffset);
 
   // Bit positions for bit field.
   static const int kHasNonInstancePrototype = 0;