Slim Skia down to just one murmur3 implementation.

src/core/SkChecksum.h

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkChecksum_DEFINED
 #define SkChecksum_DEFINED
 
@@ skipping 44 matching lines @@
      * Calculate 32-bit Murmur hash (murmur3).
      * This should take 2-3x longer than SkChecksum::Compute, but is a considerably better hash.
      * See en.wikipedia.org/wiki/MurmurHash.
      *
      *  @param data Memory address of the data block to be processed. Must be 32-bit aligned.
      *  @param size Size of the data block in bytes. Must be a multiple of 4.
      *  @param seed Initial hash seed. (optional)
      *  @return hash result
      */
     static uint32_t Murmur3(const uint32_t* data, size_t bytes, uint32_t seed=0) {
+        // Use may_alias to remind the compiler we're intentionally violating strict aliasing,
+        // and so not to apply strict-aliasing-based optimizations.
+        typedef uint32_t SK_ATTRIBUTE(may_alias) aliased_uint32_t;
+        const aliased_uint32_t* safe_data = (const aliased_uint32_t*)data;
+
         SkASSERTF(SkIsAlign4(bytes), "Expected 4-byte multiple, got %zu", bytes);
         const size_t words = bytes/4;
 
+
         uint32_t hash = seed;
         for (size_t i = 0; i < words; i++) {
-            uint32_t k = data[i];
+            uint32_t k = safe_data[i];
             k *= 0xcc9e2d51;
             k = (k << 15) | (k >> 17);
             k *= 0x1b873593;
 
             hash ^= k;
             hash = (hash << 13) | (hash >> 19);
             hash *= 5;
             hash += 0xe6546b64;
         }
         hash ^= bytes;
         return Mix(hash);
     }
 
     /**
      *  Compute a 32-bit checksum for a given data block
      *
      *  WARNING: this algorithm is tuned for efficiency, not backward/forward
      *  compatibility.  It may change at any time, so a checksum generated with
      *  one version of the Skia code may not match a checksum generated with
      *  a different version of the Skia code.
      *
      *  @param data Memory address of the data block to be processed. Must be
      *              32-bit aligned.
      *  @param size Size of the data block in bytes. Must be a multiple of 4.
      *  @return checksum result
      */
     static uint32_t Compute(const uint32_t* data, size_t size) {
+        // Use may_alias to remind the compiler we're intentionally violating strict aliasing,
+        // and so not to apply strict-aliasing-based optimizations.
+        typedef uint32_t SK_ATTRIBUTE(may_alias) aliased_uint32_t;
+        const aliased_uint32_t* safe_data = (const aliased_uint32_t*)data;
+
         SkASSERT(SkIsAlign4(size));
 
         /*
          *  We want to let the compiler use 32bit or 64bit addressing and math
          *  so we use uintptr_t as our magic type. This makes the code a little
          *  more obscure (we can't hard-code 32 or 64 anywhere, but have to use
          *  sizeof()).
          */
         uintptr_t result = 0;
-        const uintptr_t* ptr = reinterpret_cast<const uintptr_t*>(data);
+        const uintptr_t* ptr = reinterpret_cast<const uintptr_t*>(safe_data);
 
         /*
          *  count the number of quad element chunks. This takes into account
          *  if we're on a 32bit or 64bit arch, since we use sizeof(uintptr_t)
          *  to compute how much to shift-down the size.
          */
         size_t n4 = size / (sizeof(uintptr_t) << 2);
         for (size_t i = 0; i < n4; ++i) {
             result = Mash(result, *ptr++);
             result = Mash(result, *ptr++);
             result = Mash(result, *ptr++);
             result = Mash(result, *ptr++);
         }
         size &= ((sizeof(uintptr_t) << 2) - 1);
 
-        data = reinterpret_cast<const uint32_t*>(ptr);
-        const uint32_t* stop = data + (size >> 2);
-        while (data < stop) {
-            result = Mash(result, *data++);
+        safe_data = reinterpret_cast<const aliased_uint32_t*>(ptr);
+        const aliased_uint32_t* stop = safe_data + (size >> 2);
+        while (safe_data < stop) {
+            result = Mash(result, *safe_data++);
         }
 
         /*
          *  smash us down to 32bits if we were 64. Note that when uintptr_t is
          *  32bits, this code-path should go away, but I still got a warning
          *  when I wrote
          *      result ^= result >> 32;
          *  since >>32 is undefined for 32bit ints, hence the wacky HALFBITS
          *  define.
          */
         if (8 == sizeof(result)) {
             result ^= result >> HALFBITS;
         }
         return static_cast<uint32_t>(result);
     }
 };
 
 #endif