Make UNALIGNED_LOAD16/32 on ARMv7 go through an explicitly unaligned struct,

to avoid the compiler coalescing multiple loads into a single load instruction
(which only work for aligned accesses).

A typical example where GCC would coalesce:

  uint8* p = ...;
  uint32 a = UNALIGNED_LOAD32(p);
  uint32 b = UNALIGNED_LOAD32(p + 4);
  uint32 c = a | b;

snappy-stubs-internal.h

@@ -116,6 +116,15 @@ static const int64 kint64max = static_cast<int64>(0x7FFFFFFFFFFFFFFFLL);
 // sub-architectures.
 //
 // This is a mess, but there's not much we can do about it.
+//
+// To further complicate matters, only LDR instructions (single reads) are
+// allowed to be unaligned, not LDRD (two reads) or LDM (many reads). Unless we
+// explicitly tell the compiler that these accesses can be unaligned, it can and
+// will combine accesses. On armcc, the way to signal this is done by accessing
+// through the type (uint32 __packed *), but GCC has no such attribute
+// (it ignores __attribute__((packed)) on individual variables). However,
+// we can tell it that a _struct_ is unaligned, which has the same effect,
+// so we do that.
 
 #elif defined(__arm__) && \
       !defined(__ARM_ARCH_4__) && \
@@ -131,11 +140,33 @@ static const int64 kint64max = static_cast<int64>(0x7FFFFFFFFFFFFFFFLL);
       !defined(__ARM_ARCH_6ZK__) && \
       !defined(__ARM_ARCH_6T2__)
 
-#define UNALIGNED_LOAD16(_p) (*reinterpret_cast<const uint16 *>(_p))
-#define UNALIGNED_LOAD32(_p) (*reinterpret_cast<const uint32 *>(_p))
+namespace base {
+namespace internal {
 
-#define UNALIGNED_STORE16(_p, _val) (*reinterpret_cast<uint16 *>(_p) = (_val))
-#define UNALIGNED_STORE32(_p, _val) (*reinterpret_cast<uint32 *>(_p) = (_val))
+struct Unaligned16Struct {
+  uint16 value;
+  uint8 dummy;  // To make the size non-power-of-two.
+} ATTRIBUTE_PACKED;
+
+struct Unaligned32Struct {
+  uint32 value;
+  uint8 dummy;  // To make the size non-power-of-two.
+} ATTRIBUTE_PACKED;
+
+}  // namespace internal
+}  // namespace base
+
+#define UNALIGNED_LOAD16(_p) \
+    ((reinterpret_cast<const ::base::internal::Unaligned16Struct *>(_p))->value)
+#define UNALIGNED_LOAD32(_p) \
+    ((reinterpret_cast<const ::base::internal::Unaligned32Struct *>(_p))->value)
+
+#define UNALIGNED_STORE16(_p, _val) \
+    ((reinterpret_cast<::base::internal::Unaligned16Struct *>(_p))->value = \
+         (_val))
+#define UNALIGNED_STORE32(_p, _val) \
+    ((reinterpret_cast<::base::internal::Unaligned32Struct *>(_p))->value = \
+         (_val))
 
 // TODO(user): NEON supports unaligned 64-bit loads and stores.
 // See if that would be more efficient on platforms supporting it,