Switch uses of SkChecksum::Compute to Murmur3.

SkChecksum::Compute is a very, very poorly distributed hash function.
This replaces all remaining uses with Murmur3.

The only interesting stuff is in src/gpu.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/1d024a3c909ae5cefa5e8b339e2b52dc73ee85ac

Review URL: https://codereview.chromium.org/1436973003

1 files changed, 0 insertions, 89 deletions

diff --git a/include/private/SkChecksum.h b/include/private/SkChecksum.h
index 4526416fc1..6289a444ae 100644
--- a/include/private/SkChecksum.h
+++ b/include/private/SkChecksum.h
@@ -12,31 +12,7 @@
 #include "SkTLogic.h"
 #include "SkTypes.h"
 
-/**
- *  Computes a 32bit checksum from a blob of 32bit aligned data. This is meant
- *  to be very very fast, as it is used internally by the font cache, in
- *  conjuction with the entire raw key. This algorithm does not generate
- *  unique values as well as others (e.g. MD5) but it performs much faster.
- *  Skia's use cases can survive non-unique values (since the entire key is
- *  always available). Clients should only be used in circumstances where speed
- *  over uniqueness is at a premium.
- */
 class SkChecksum : SkNoncopyable {
-private:
-    /*
-     *  Our Rotate and Mash helpers are meant to automatically do the right
-     *  thing depending if sizeof(uintptr_t) is 4 or 8.
-     */
-    enum {
-        ROTR = 17,
-        ROTL = sizeof(uintptr_t) * 8 - ROTR,
-        HALFBITS = sizeof(uintptr_t) * 4
-    };
-
-    static inline uintptr_t Mash(uintptr_t total, uintptr_t value) {
-        return ((total >> ROTR) | (total << ROTL)) ^ value;
-    }
-
 public:
     /**
      * uint32_t -> uint32_t hash, useful for when you're about to trucate this hash but you
@@ -68,7 +44,6 @@ public:
 
     /**
      * 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.
@@ -77,70 +52,6 @@ public:
      *  @return hash result
      */
     static uint32_t Murmur3(const void* data, size_t bytes, uint32_t seed=0);
-
-    /**
-     *  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*>(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);
-
-        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);
-    }
 };
 
 // SkGoodHash should usually be your first choice in hashing data.