From fd8ed69447f2a126f7006a1a95356f15feca3797 Mon Sep 17 00:00:00 2001 From: mtklein Date: Tue, 28 Jul 2015 09:54:52 -0700 Subject: Move SkTHash.h to include/private. include/views/SkOSWindow_Win.h includes it. To move SkTHash.h to include/private, SkChecksum.h needs to go there too. To move SkChecksum.h to include/private, SkTLogic needs to go there too. This adds a bunch of -Iinclude/private to tools.gyp I missed in the last CL. No public API changes. TBR=reed@google.com BUG=skia:4126 Review URL: https://codereview.chromium.org/1260613006 --- include/private/SkChecksum.h | 198 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 198 insertions(+) create mode 100644 include/private/SkChecksum.h (limited to 'include/private/SkChecksum.h') diff --git a/include/private/SkChecksum.h b/include/private/SkChecksum.h new file mode 100644 index 0000000000..8eb1766ec0 --- /dev/null +++ b/include/private/SkChecksum.h @@ -0,0 +1,198 @@ +/* + * 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 + +#include "SkString.h" +#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 + * suspect its low bits aren't well mixed. + * + * This is the Murmur3 finalizer. + */ + static uint32_t Mix(uint32_t hash) { + hash ^= hash >> 16; + hash *= 0x85ebca6b; + hash ^= hash >> 13; + hash *= 0xc2b2ae35; + hash ^= hash >> 16; + return hash; + } + + /** + * uint32_t -> uint32_t hash, useful for when you're about to trucate this hash but you + * suspect its low bits aren't well mixed. + * + * This version is 2-lines cheaper than Mix, but seems to be sufficient for the font cache. + */ + static uint32_t CheapMix(uint32_t hash) { + hash ^= hash >> 16; + hash *= 0x85ebca6b; + hash ^= hash >> 16; + return hash; + } + + /** + * 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. + * @param size Size of the data block in bytes. + * @param seed Initial hash seed. (optional) + * @return hash result + */ + static uint32_t Murmur3(const void* 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; + typedef uint8_t SK_ATTRIBUTE(may_alias) aliased_uint8_t; + + // Handle 4 bytes at a time while possible. + const aliased_uint32_t* safe_data = (const aliased_uint32_t*)data; + const size_t words = bytes/4; + uint32_t hash = seed; + for (size_t i = 0; i < words; 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; + } + + // Handle last 0-3 bytes. + const aliased_uint8_t* safe_tail = (const uint8_t*)(safe_data + words); + uint32_t k = 0; + switch (bytes & 3) { + case 3: k ^= safe_tail[2] << 16; + case 2: k ^= safe_tail[1] << 8; + case 1: k ^= safe_tail[0] << 0; + k *= 0xcc9e2d51; + k = (k << 15) | (k >> 17); + k *= 0x1b873593; + hash ^= k; + } + + 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(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(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(result); + } +}; + +// SkGoodHash should usually be your first choice in hashing data. +// It should be both reasonably fast and high quality. + +template +uint32_t SkGoodHash(const K& k) { + if (sizeof(K) == 4) { + return SkChecksum::Mix(*(const uint32_t*)&k); + } + return SkChecksum::Murmur3(&k, sizeof(K)); +} + +inline uint32_t SkGoodHash(const SkString& k) { + return SkChecksum::Murmur3(k.c_str(), k.size()); +} + +#endif -- cgit v1.2.3