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Switch uses of SkChecksum::Compute to Murmur3.

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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
This commit is contained in:
mtklein 2015-11-16 09:08:21 -08:00 committed by Commit bot
parent e004bfc0a5
commit 540e95483d
6 changed files with 8 additions and 107 deletions
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9
bench/ChecksumBench.cpp
View File

@ -13,7 +13,6 @@
#include "SkTemplates.h"
enum ChecksumType {
kChecksum_ChecksumType,
kMD5_ChecksumType,
kSHA1_ChecksumType,
kMurmur3_ChecksumType,
@ -42,7 +41,6 @@ public:
protected:
const char* onGetName() override {
switch (fType) {
case kChecksum_ChecksumType: return "compute_checksum";
case kMD5_ChecksumType: return "compute_md5";
case kSHA1_ChecksumType: return "compute_sha1";
case kMurmur3_ChecksumType: return "compute_murmur3";
@ -53,12 +51,6 @@ protected:
void onDraw(int loops, SkCanvas*) override {
switch (fType) {
case kChecksum_ChecksumType: {
for (int i = 0; i < loops; i++) {
volatile uint32_t result = SkChecksum::Compute(fData, sizeof(fData));
sk_ignore_unused_variable(result);
}
} break;
case kMD5_ChecksumType: {
for (int i = 0; i < loops; i++) {
SkMD5 md5;
@ -91,7 +83,6 @@ private:
///////////////////////////////////////////////////////////////////////////////
DEF_BENCH( return new ComputeChecksumBench(kChecksum_ChecksumType); )
DEF_BENCH( return new ComputeChecksumBench(kMD5_ChecksumType); )
DEF_BENCH( return new ComputeChecksumBench(kSHA1_ChecksumType); )
DEF_BENCH( return new ComputeChecksumBench(kMurmur3_ChecksumType); )

89
include/private/SkChecksum.h
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@ -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.

4
src/core/SkPictureFlat.h
View File

@ -179,7 +179,7 @@ class SkFlatData;
class SkFlatController : public SkRefCnt {
public:
SkFlatController(uint32_t writeBufferFlags = 0);
virtual ~SkFlatController();
@ -357,7 +357,7 @@ private:
fIndex = index;
fFlatSize = size;
fTopBot[0] = SK_ScalarNaN; // Mark as unwritten.
fChecksum = SkChecksum::Compute((uint32_t*)this->data(), size);
fChecksum = SkChecksum::Murmur3(this->data(), size);
}
int fIndex;

3
src/gpu/GrProgramDesc.h
View File

@ -107,8 +107,7 @@ protected:
*(this->atOffset<uint32_t, GrProgramDesc::kLengthOffset>()) = SkToU32(keyLength);
uint32_t* checksum = this->atOffset<uint32_t, GrProgramDesc::kChecksumOffset>();
*checksum = 0;
*checksum = SkChecksum::Compute(reinterpret_cast<uint32_t*>(fKey.begin()), keyLength);
*checksum = SkChecksum::Murmur3(fKey.begin(), keyLength);
}
// The key, stored in fKey, is composed of four parts:

8
src/gpu/GrResourceCache.cpp
View File

@ -42,7 +42,7 @@ GrUniqueKey::Domain GrUniqueKey::GenerateDomain() {
}
uint32_t GrResourceKeyHash(const uint32_t* data, size_t size) {
return SkChecksum::Compute(data, size);
return SkChecksum::Murmur3(data, size);
}
//////////////////////////////////////////////////////////////////////////////
@ -564,7 +564,7 @@ uint32_t GrResourceCache::getNextTimestamp() {
int currP = 0;
int currNP = 0;
while (currP < sortedPurgeableResources.count() &&
currNP < fNonpurgeableResources.count()) {
currNP < fNonpurgeableResources.count()) {
uint32_t tsP = sortedPurgeableResources[currP]->cacheAccess().timestamp();
uint32_t tsNP = fNonpurgeableResources[currNP]->cacheAccess().timestamp();
SkASSERT(tsP != tsNP);
@ -596,10 +596,10 @@ uint32_t GrResourceCache::getNextTimestamp() {
// count should be the next timestamp we return.
SkASSERT(fTimestamp == SkToU32(count));
// The historical timestamps of flushes are now invalid.
this->resetFlushTimestamps();
}
}
}
return fTimestamp++;
}

2
tests/ChecksumTest.cpp
View File

@ -18,7 +18,7 @@ static uint32_t murmur_noseed(const uint32_t* d, size_t l) { return SkChecksum::
DEF_TEST(Checksum, r) {
// Algorithms to test. They're currently all uint32_t(const uint32_t*, size_t).
typedef uint32_t(*algorithmProc)(const uint32_t*, size_t);
const algorithmProc kAlgorithms[] = { &SkChecksum::Compute, &murmur_noseed };
const algorithmProc kAlgorithms[] = { &murmur_noseed };
// Put 128 random bytes into two identical buffers. Any multiple of 4 will do.
const size_t kBytes = SkAlign4(128);