Fix undefined type-punning when loading/storing words

Despite common practice, type-punning integer types out of buffers is
undefined in C and C++. It's both a strict aliasing violation on access,
and if the pointer isn't aligned, an alignment violation on cast. Being
undefined, the compiler is allowed to arbitrarily miscompile the code
when we rely on it.

Instead, the two legal ways to pull a uint32_t out of a buffer are to
either use memcpy, or load byte by byte and use shifts. In both cases,
a good compiler should be smart enough to recognize what we're doing and
generate reasonable code. Since there was already fallback code for the
latter (for a middle-endian architecture?), I went ahead and switched to
that.

This change is needed to fix UBSan violations in Chromium.

src/store_bytes.h

@@ -27,15 +27,8 @@ inline size_t StoreU32(uint8_t* dst, size_t offset, uint32_t x) {
 }
 
 inline size_t Store16(uint8_t* dst, size_t offset, int x) {
-#if defined(WOFF_LITTLE_ENDIAN)
-  *reinterpret_cast<uint16_t*>(dst + offset) =
-      ((x & 0xFF) << 8) | ((x & 0xFF00) >> 8);
-#elif defined(WOFF_BIG_ENDIAN)
-  *reinterpret_cast<uint16_t*>(dst + offset) = static_cast<uint16_t>(x);
-#else
   dst[offset] = x >> 8;
   dst[offset + 1] = x;
-#endif
   return offset + 2;
 }
 
@@ -47,17 +40,8 @@ inline void StoreU32(uint32_t val, size_t* offset, uint8_t* dst) {
 }
 
 inline void Store16(int val, size_t* offset, uint8_t* dst) {
-#if defined(WOFF_LITTLE_ENDIAN)
-  *reinterpret_cast<uint16_t*>(dst + *offset) =
-      ((val & 0xFF) << 8) | ((val & 0xFF00) >> 8);
-  *offset += 2;
-#elif defined(WOFF_BIG_ENDIAN)
-  *reinterpret_cast<uint16_t*>(dst + *offset) = static_cast<uint16_t>(val);
-  *offset += 2;
-#else
   dst[(*offset)++] = val >> 8;
   dst[(*offset)++] = val;
-#endif
 }
 
 inline void StoreBytes(const uint8_t* data, size_t len,

src/woff2_common.cc

@@ -19,16 +19,8 @@ uint32_t ComputeULongSum(const uint8_t* buf, size_t size) {
   uint32_t checksum = 0;
   size_t aligned_size = size & ~3;
   for (size_t i = 0; i < aligned_size; i += 4) {
-#if defined(WOFF_LITTLE_ENDIAN)
-    uint32_t v = *reinterpret_cast<const uint32_t*>(buf + i);
-    checksum += (((v & 0xFF) << 24) | ((v & 0xFF00) << 8) |
-      ((v & 0xFF0000) >> 8) | ((v & 0xFF000000) >> 24));
-#elif defined(WOFF_BIG_ENDIAN)
-    checksum += *reinterpret_cast<const uint32_t*>(buf + i);
-#else
-    checksum += (buf[i] << 24) | (buf[i + 1] << 16) |
-      (buf[i + 2] << 8) | buf[i + 3];
-#endif
+    checksum +=
+        (buf[i] << 24) | (buf[i + 1] << 16) | (buf[i + 2] << 8) | buf[i + 3];
   }
 
   // treat size not aligned on 4 as if it were padded to 4 with 0's