Implement WASM big-endian support

src/utils.h

Index: src/utils.h
diff --git a/src/utils.h b/src/utils.h
index 9a601416ffd837e48209a3bf5eec80d70d024346..9108fe90d46eaa4089886a11f0a3b86474bc2da4 100644
--- a/src/utils.h
+++ b/src/utils.h
@@ -1548,6 +1548,33 @@ static inline void WriteUnalignedUInt32(void* p, uint32_t value) {
   WriteUnalignedValue(p, value);
 }
 
+template <typename V>
+static inline V ReadLittleEndianValue(const void* p) {

[titzer, 2016/06/06 07:44:00]

Is it possible to unify these routines with those in wasm/decoder.h and
wasm/encoder.h?

[ivica.bogosavljevic, 2016/06/13 14:59:02]

I suggest we keep ReadLittleEndianValue and WriteLittleEndianValue, and modify
read_ and write methods in encoder.h and decoder.h to use  them. 

+#if defined(V8_TARGET_LITTLE_ENDIAN)
+  return ReadUnalignedValue(p);
+#elif defined(V8_TARGET_BIG_ENDIAN)
+  V ret = 0;
+  const byte* src = reinterpret_cast<const byte*>(p);
+  byte* dst = reinterpret_cast<byte*>(&ret);

[titzer, 2016/06/06 07:44:00]

That kind of type punning is undefined behavior in C++, so it will unfortunately
break one day. A memmove will work.

[ivica.bogosavljevic, 2016/06/13 14:59:02]

For big-endian implementation, we need to swap bytes within word. memmove
doesn't do this. We could implement the same using byte reading and shifting,
but this doesn't works for floats. At the end, we need to convert the void *
pointer to byte * pointer, I don't see any other way to do this so it works with
both integers and floats

+  for (size_t i = 0; i < sizeof(V); i++) {
+    dst[i] = src[sizeof(V) - i - 1];
+  }
+  return ret;
+#endif  // V8_TARGET_LITTLE_ENDIAN
+}
+
+template <typename V>
+static inline void WriteLittleEndianValue(void* p, V value) {
+#if defined(V8_TARGET_LITTLE_ENDIAN)
+  WriteUnalignedValue(p, value);
+#elif defined(V8_TARGET_BIG_ENDIAN)
+  byte* src = reinterpret_cast<byte*>(&value);
+  byte* dst = reinterpret_cast<byte*>(p);
+  for (size_t i = 0; i < sizeof(V); i++) {
+    dst[i] = src[sizeof(V) - i - 1];
+  }
+#endif  // V8_TARGET_LITTLE_ENDIAN
+}
 }  // namespace internal
 }  // namespace v8