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Cleanup of ZopfliFindLongestMatch -- gives ~8% overall performance boost #110

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Cleanup of ZopfliFindLongestMatch -- gives ~8% overall performance boost #110

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b5a1ea2
Cleanup of ZopfliFindLongestMatch -- gives ~8% overall performance boost
jthlim Apr 30, 2016
8ffda7e
Revert order of comparisons change due to this causing regression wit…
jthlim May 1, 2016
84114af
Simplification of initial dist setup too
jthlim May 1, 2016
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58 changes: 31 additions & 27 deletions src/zopfli/lz77.c
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Original file line number Diff line number Diff line change
Expand Up @@ -408,9 +408,9 @@ void ZopfliFindLongestMatch(ZopfliBlockState* s, const ZopfliHash* h,
const unsigned char* array,
size_t pos, size_t size, size_t limit,
unsigned short* sublen, unsigned short* distance, unsigned short* length) {
unsigned short hpos = pos & ZOPFLI_WINDOW_MASK, p, pp;
unsigned short bestdist = 0;
unsigned short bestlength = 1;
unsigned int p, pp;
unsigned int bestdist = 0;
unsigned int bestlength = 1;
const unsigned char* scan;
const unsigned char* match;
const unsigned char* arrayend;
Expand All @@ -421,6 +421,10 @@ void ZopfliFindLongestMatch(ZopfliBlockState* s, const ZopfliHash* h,

unsigned dist = 0; /* Not unsigned short on purpose. */

#ifdef ZOPFLI_HASH_SAME
const unsigned int same0 = h->same[pos & ZOPFLI_WINDOW_MASK];
#endif

int* hhead = h->head;
unsigned short* hprev = h->prev;
int* hhashval = h->hashval;
Expand All @@ -437,17 +441,18 @@ void ZopfliFindLongestMatch(ZopfliBlockState* s, const ZopfliHash* h,
assert(limit >= ZOPFLI_MIN_MATCH);
assert(pos < size);

if (size - pos < ZOPFLI_MIN_MATCH) {
if (pos + limit > size) {
limit = size - pos;
}

if (limit < ZOPFLI_MIN_MATCH) {
/* The rest of the code assumes there are at least ZOPFLI_MIN_MATCH bytes to
try. */
*length = 0;
*distance = 0;
return;
}

if (pos + limit > size) {
limit = size - pos;
}
arrayend = &array[pos] + limit;
arrayend_safe = arrayend - 8;

Expand All @@ -456,13 +461,12 @@ void ZopfliFindLongestMatch(ZopfliBlockState* s, const ZopfliHash* h,
pp = hhead[hval]; /* During the whole loop, p == hprev[pp]. */
p = hprev[pp];

assert(pp == hpos);
assert(pp == (pos & ZOPFLI_WINDOW_MASK));

dist = p < pp ? pp - p : ((ZOPFLI_WINDOW_SIZE - p) + pp);
dist = (pp - p) & ZOPFLI_WINDOW_MASK;

/* Go through all distances. */
while (dist < ZOPFLI_WINDOW_SIZE) {
unsigned short currentlength = 0;

assert(p < ZOPFLI_WINDOW_SIZE);
assert(p == hprev[pp]);
Expand All @@ -475,40 +479,40 @@ void ZopfliFindLongestMatch(ZopfliBlockState* s, const ZopfliHash* h,
match = &array[pos - dist];

/* Testing the byte at position bestlength first, goes slightly faster. */
if (pos + bestlength >= size
|| *(scan + bestlength) == *(match + bestlength)) {
if (*(scan + bestlength) == *(match + bestlength)) {

unsigned int currentlength;

#ifdef ZOPFLI_HASH_SAME
unsigned short same0 = h->same[pos & ZOPFLI_WINDOW_MASK];
if (same0 > 2 && *scan == *match) {
unsigned short same1 = h->same[(pos - dist) & ZOPFLI_WINDOW_MASK];
unsigned short same = same0 < same1 ? same0 : same1;
unsigned int same1 = h->same[(pos - dist) & ZOPFLI_WINDOW_MASK];
unsigned int same = same0 < same1 ? same0 : same1;
if (same > limit) same = limit;
scan += same;
match += same;
}
#endif
scan = GetMatch(scan, match, arrayend, arrayend_safe);
currentlength = scan - &array[pos]; /* The found length. */
}

if (currentlength > bestlength) {
if (sublen) {
unsigned short j;
for (j = bestlength + 1; j <= currentlength; j++) {
sublen[j] = dist;
if (currentlength > bestlength) {
if (sublen) {
size_t j;
for (j = bestlength + 1; j <= currentlength; j++) {
sublen[j] = dist;
}
}
bestdist = dist;
bestlength = currentlength;
if (currentlength >= limit) break;
}
bestdist = dist;
bestlength = currentlength;
if (currentlength >= limit) break;
}
}


#ifdef ZOPFLI_HASH_SAME_HASH
/* Switch to the other hash once this will be more efficient. */
if (hhead != h->head2 && bestlength >= h->same[hpos] &&
if (hhead != h->head2 && bestlength >= same0 &&
h->val2 == h->hashval2[p]) {
/* Now use the hash that encodes the length and first byte. */
hhead = h->head2;
Expand All @@ -522,7 +526,7 @@ void ZopfliFindLongestMatch(ZopfliBlockState* s, const ZopfliHash* h,
p = hprev[p];
if (p == pp) break; /* Uninited prev value. */

dist += p < pp ? pp - p : ((ZOPFLI_WINDOW_SIZE - p) + pp);
dist += (pp - p) & ZOPFLI_WINDOW_MASK;

#if ZOPFLI_MAX_CHAIN_HITS < ZOPFLI_WINDOW_SIZE
chain_counter--;
Expand Down