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git/diffcore-delta.c
Linus Torvalds eb4d0e3f45 optimize diffcore-delta by sorting hash entries.
Here's a test-patch. I don't guarantee anything, except that when I did
the timings I also did a "wc" on the result, and they matched..

Before:
	[torvalds@woody linux]$ time git diff -l0 --stat -C v2.6.22.. | wc
	   7104   28574  438020

	real    0m10.526s
	user    0m10.401s
	sys     0m0.136s

After:
	[torvalds@woody linux]$ time ~/git/git diff -l0 --stat -C v2.6.22.. | wc
	   7104   28574  438020

	real    0m8.876s
	user    0m8.761s
	sys     0m0.128s

but the diff is fairly simple, so if somebody will go over it and say
whether it's likely to be *correct* too, that 15% may well be worth it.

[ Side note, without rename detection, that diff takes just under three
  seconds for me, so in that sense the improvement to the rename detection
  itself is larger than the overall 15% - it brings the cost of just
  rename detection from 7.5s to 5.9s, which would be on the order of just
  over a 20% performance improvement. ]

Hmm. The patch depends on half-way subtle issues like the fact that the
hashtables are guaranteed to not be full => we're guaranteed to have zero
counts at the end => we don't need to do any steenking iterator count in
the loop. A few comments might in order.

		Linus
2007-10-04 00:05:36 -07:00

224 lines
5.3 KiB
C

#include "cache.h"
#include "diff.h"
#include "diffcore.h"
/*
* Idea here is very simple.
*
* Almost all data we are interested in are text, but sometimes we have
* to deal with binary data. So we cut them into chunks delimited by
* LF byte, or 64-byte sequence, whichever comes first, and hash them.
*
* For those chunks, if the source buffer has more instances of it
* than the destination buffer, that means the difference are the
* number of bytes not copied from source to destination. If the
* counts are the same, everything was copied from source to
* destination. If the destination has more, everything was copied,
* and destination added more.
*
* We are doing an approximation so we do not really have to waste
* memory by actually storing the sequence. We just hash them into
* somewhere around 2^16 hashbuckets and count the occurrences.
*/
/* Wild guess at the initial hash size */
#define INITIAL_HASH_SIZE 9
/* We leave more room in smaller hash but do not let it
* grow to have unused hole too much.
*/
#define INITIAL_FREE(sz_log2) ((1<<(sz_log2))*(sz_log2-3)/(sz_log2))
/* A prime rather carefully chosen between 2^16..2^17, so that
* HASHBASE < INITIAL_FREE(17). We want to keep the maximum hashtable
* size under the current 2<<17 maximum, which can hold this many
* different values before overflowing to hashtable of size 2<<18.
*/
#define HASHBASE 107927
struct spanhash {
unsigned int hashval;
unsigned int cnt;
};
struct spanhash_top {
int alloc_log2;
int free;
struct spanhash data[FLEX_ARRAY];
};
static struct spanhash_top *spanhash_rehash(struct spanhash_top *orig)
{
struct spanhash_top *new;
int i;
int osz = 1 << orig->alloc_log2;
int sz = osz << 1;
new = xmalloc(sizeof(*orig) + sizeof(struct spanhash) * sz);
new->alloc_log2 = orig->alloc_log2 + 1;
new->free = INITIAL_FREE(new->alloc_log2);
memset(new->data, 0, sizeof(struct spanhash) * sz);
for (i = 0; i < osz; i++) {
struct spanhash *o = &(orig->data[i]);
int bucket;
if (!o->cnt)
continue;
bucket = o->hashval & (sz - 1);
while (1) {
struct spanhash *h = &(new->data[bucket++]);
if (!h->cnt) {
h->hashval = o->hashval;
h->cnt = o->cnt;
new->free--;
break;
}
if (sz <= bucket)
bucket = 0;
}
}
free(orig);
return new;
}
static struct spanhash_top *add_spanhash(struct spanhash_top *top,
unsigned int hashval, int cnt)
{
int bucket, lim;
struct spanhash *h;
lim = (1 << top->alloc_log2);
bucket = hashval & (lim - 1);
while (1) {
h = &(top->data[bucket++]);
if (!h->cnt) {
h->hashval = hashval;
h->cnt = cnt;
top->free--;
if (top->free < 0)
return spanhash_rehash(top);
return top;
}
if (h->hashval == hashval) {
h->cnt += cnt;
return top;
}
if (lim <= bucket)
bucket = 0;
}
}
static int spanhash_cmp(const void *a_, const void *b_)
{
const struct spanhash *a = a_;
const struct spanhash *b = b_;
/* A count of zero compares at the end.. */
if (!a->cnt)
return !b->cnt ? 0 : 1;
if (!b->cnt)
return -1;
return a->hashval < b->hashval ? -1 :
a->hashval > b->hashval ? 1 : 0;
}
static struct spanhash_top *hash_chars(struct diff_filespec *one)
{
int i, n;
unsigned int accum1, accum2, hashval;
struct spanhash_top *hash;
unsigned char *buf = one->data;
unsigned int sz = one->size;
int is_text = !diff_filespec_is_binary(one);
i = INITIAL_HASH_SIZE;
hash = xmalloc(sizeof(*hash) + sizeof(struct spanhash) * (1<<i));
hash->alloc_log2 = i;
hash->free = INITIAL_FREE(i);
memset(hash->data, 0, sizeof(struct spanhash) * (1<<i));
n = 0;
accum1 = accum2 = 0;
while (sz) {
unsigned int c = *buf++;
unsigned int old_1 = accum1;
sz--;
/* Ignore CR in CRLF sequence if text */
if (is_text && c == '\r' && sz && *buf == '\n')
continue;
accum1 = (accum1 << 7) ^ (accum2 >> 25);
accum2 = (accum2 << 7) ^ (old_1 >> 25);
accum1 += c;
if (++n < 64 && c != '\n')
continue;
hashval = (accum1 + accum2 * 0x61) % HASHBASE;
hash = add_spanhash(hash, hashval, n);
n = 0;
accum1 = accum2 = 0;
}
qsort(hash->data,
1ul << hash->alloc_log2,
sizeof(hash->data[0]),
spanhash_cmp);
return hash;
}
int diffcore_count_changes(struct diff_filespec *src,
struct diff_filespec *dst,
void **src_count_p,
void **dst_count_p,
unsigned long delta_limit,
unsigned long *src_copied,
unsigned long *literal_added)
{
struct spanhash *s, *d;
struct spanhash_top *src_count, *dst_count;
unsigned long sc, la;
src_count = dst_count = NULL;
if (src_count_p)
src_count = *src_count_p;
if (!src_count) {
src_count = hash_chars(src);
if (src_count_p)
*src_count_p = src_count;
}
if (dst_count_p)
dst_count = *dst_count_p;
if (!dst_count) {
dst_count = hash_chars(dst);
if (dst_count_p)
*dst_count_p = dst_count;
}
sc = la = 0;
s = src_count->data;
d = dst_count->data;
for (;;) {
unsigned dst_cnt, src_cnt;
if (!s->cnt)
break; /* we checked all in src */
while (d->cnt) {
if (d->hashval >= s->hashval)
break;
d++;
}
src_cnt = s->cnt;
dst_cnt = d->hashval == s->hashval ? d->cnt : 0;
if (src_cnt < dst_cnt) {
la += dst_cnt - src_cnt;
sc += src_cnt;
}
else
sc += dst_cnt;
s++;
}
if (!src_count_p)
free(src_count);
if (!dst_count_p)
free(dst_count);
*src_copied = sc;
*literal_added = la;
return 0;
}