MIPS: Fix unaligned read/write operations in wasm.

src/utils.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_UTILS_H_
 #define V8_UTILS_H_
 
 #include <limits.h>
 #include <stdlib.h>
 #include <string.h>
@@ skipping 1768 matching lines @@
     uint16_t h;
     uint8_t b[2];
   } c;
   c.h = value;
   uint8_t* ptr = reinterpret_cast<uint8_t*>(p);
   *ptr = c.b[0];
   *(ptr + 1) = c.b[1];
 #endif  // V8_TARGET_ARCH_MIPS || V8_TARGET_ARCH_MIPS64
 }
 
+
+static inline void WriteUnalignedUInt32(void* p, uint32_t value) {

[titzer, 2016/01/19 16:00:55]

Would it be enough to just do a memmove(p, &value, sizeof(uint32_t)) ?

[ivica.bogosavljevic, 2016/01/20 10:32:57]

I would like to point out that the MIPS specific case should be possibly
extended to all architectures. 
Even though the code *(reinterpret_cast<uint32_t*>(p)) = value; works on other
architectures, accessing unaligned memory using wrong instructions can generate
Unaligned memory access on ARM which are then fixed in kernel, or is just slower
(X86-X64).

[akos.palfi.imgtec, 2016/01/21 21:00:22]

Thanks, great idea. I've removed the unions and have done some cleanup on the
way.

[akos.palfi.imgtec, 2016/01/21 21:07:35]

I think other archs support unaligned fixups in hardware and it should be
cheaper than doing a memmove. However, it would worth a deeper investigation. If
there's no objection, I'll leave the other archs as is, for now.

+#if !(V8_TARGET_ARCH_MIPS || V8_TARGET_ARCH_MIPS64)
+  *(reinterpret_cast<uint32_t*>(p)) = value;
+#else   // V8_TARGET_ARCH_MIPS || V8_TARGET_ARCH_MIPS64
+  // Prevent compiler from using store-half (mips sh) on (possibly)
+  // non-16-bit aligned address.
+  union conversion {
+    uint32_t h;
+    uint8_t b[4];
+  } c;
+  c.h = value;
+  uint8_t* ptr = reinterpret_cast<uint8_t*>(p);
+  *ptr = c.b[0];
+  *(ptr + 1) = c.b[1];
+  *(ptr + 2) = c.b[2];
+  *(ptr + 3) = c.b[3];
+#endif  // V8_TARGET_ARCH_MIPS || V8_TARGET_ARCH_MIPS64
+}
+
+
+template <typename V>
+static inline V ReadUnalignedValue(const void* p) {
+#if !(V8_TARGET_ARCH_MIPS || V8_TARGET_ARCH_MIPS64)
+  return *reinterpret_cast<const V*>(p);
+#else
+  union conversion {
+    V d;
+    uint8_t u[sizeof(V)];
+  } c;
+  const uint8_t* ptr = reinterpret_cast<const uint8_t*>(p);
+  for (int i = 0; i < sizeof(V); i++) {
+    c.u[i] = *(ptr + i);
+  }
+  return c.d;
+#endif  // V8_TARGET_ARCH_MIPS || V8_TARGET_ARCH_MIPS64
+}
+
+
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_UTILS_H_