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Improve performance of mbfl_name2encoding() by using perfect hashing (p…
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…hp#12707)

mbfl_name2encoding() uses a linear loop through the encodings, comparing
the name one by one, which is very slow. For the benchmark [1] just
looking up the name takes about 50% of run-time.

By using perfect hashing instead, we no longer have to loop over the
list, and the number of string comparisons is reduced to just a single
one. The perfect hashing table is generated using GNU gperf and amended
manually to fit in with mbstring and manually changed to  reduce the
cache size.

[1] php#12684 (comment)
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@nielsdos
nielsdos authored Nov 17, 2023
1 parent 931a8b0 commit 7658220
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2 changes: 2 additions & 0 deletions UPGRADING
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Original file line number Diff line number Diff line change
Expand Up @@ -227,3 +227,5 @@ PHP 8.4 UPGRADE NOTES
* mb_strcut() is much faster now for UTF-8 and UTF-16 strings.

* get_browser() is much faster now, up to 1.5x - 2.5x for some test cases.

* Looking up mbstring encoding names is much faster now.
205 changes: 202 additions & 3 deletions ext/mbstring/libmbfl/mbfl/mbfl_encoding.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -144,13 +144,212 @@ static const mbfl_encoding *mbfl_encoding_ptr_list[] = {
NULL
};

/* The following perfect hashing table was amended from gperf, and hashing code was generated using gperf.
* The table was amended to refer to the table above such that it is lighter for the data cache.
* Command used: gperf encodings.txt --readonly-tables --null-strings --ignore-case
* The encodings.txt contains all the contents of the name fields of the mbfl_encoding_ptr_list table. */

static const int8_t mbfl_encoding_ptr_list_after_hashing[187] = {
-1, -1, -1,
65,
23,
9,
-1,
60,
36,
-1, -1,
58,
42,
-1, -1,
18,
27,
77,
26,
40,
72,
12,
10,
2,
31,
-1, -1,
75,
74,
33,
45,
-1,
67,
13,
-1,
51,
53,
11,
1,
-1,
48,
56,
-1,
38,
20,
46,
54,
-1,
14,
24,
44,
39,
43,
-1,
30,
49,
57,
76,
-1, -1,
68,
73,
7,
16,
-1,
35,
66,
-1, -1, -1,
47,
55,
-1, -1, -1,
63,
15,
8,
17,
-1,
21,
70,
-1,
29,
5,
6,
61,
-1, -1,
71,
52,
3,
37,
-1, -1,
28,
-1, -1, -1,
32,
50,
34,
-1, -1, -1,
62,
-1, -1, -1, -1, -1, -1, -1, -1, -1,
59,
0,
-1, -1, -1, -1,
22,
-1, -1, -1, -1, -1, -1, -1, -1, -1,
25,
41,
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
19,
-1, -1, -1,
4,
-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
69,
-1, -1, -1, -1,
64,
};

static unsigned int mbfl_name2encoding_perfect_hash(const char *str, size_t len)
{
static const unsigned char asso_values[] =
{
187, 187, 187, 187, 187, 187, 187, 187, 187, 187,
187, 187, 187, 187, 187, 187, 187, 187, 187, 187,
187, 187, 187, 187, 187, 187, 187, 187, 187, 187,
187, 187, 187, 187, 187, 187, 187, 187, 187, 187,
187, 187, 187, 187, 187, 0, 187, 187, 5, 20,
0, 15, 40, 10, 25, 70, 5, 60, 187, 187,
187, 187, 187, 187, 187, 75, 5, 0, 20, 5,
0, 75, 5, 0, 40, 75, 20, 0, 0, 0,
35, 45, 50, 0, 75, 0, 187, 0, 187, 187,
0, 187, 187, 187, 187, 187, 187, 75, 5, 0,
20, 5, 0, 75, 5, 0, 40, 75, 20, 0,
0, 0, 35, 45, 50, 0, 75, 0, 187, 0,
187, 187, 0, 187, 187, 187, 187, 187, 187, 187,
187, 187, 187, 187, 187, 187, 187, 187, 187, 187,
187, 187, 187, 187, 187, 187, 187, 187, 187, 187,
187, 187, 187, 187, 187, 187, 187, 187, 187, 187,
187, 187, 187, 187, 187, 187, 187, 187, 187, 187,
187, 187, 187, 187, 187, 187, 187, 187, 187, 187,
187, 187, 187, 187, 187, 187, 187, 187, 187, 187,
187, 187, 187, 187, 187, 187, 187, 187, 187, 187,
187, 187, 187, 187, 187, 187, 187, 187, 187, 187,
187, 187, 187, 187, 187, 187, 187, 187, 187, 187,
187, 187, 187, 187, 187, 187, 187, 187, 187, 187,
187, 187, 187, 187, 187, 187, 187, 187, 187, 187,
187, 187, 187, 187, 187, 187, 187, 187, 187, 187,
187, 187, 187, 187, 187, 187
};
unsigned int hval = len;

switch (hval)
{
default:
hval += asso_values[(unsigned char)str[6]];
ZEND_FALLTHROUGH;
case 6:
hval += asso_values[(unsigned char)str[5]];
ZEND_FALLTHROUGH;
case 5:
hval += asso_values[(unsigned char)str[4]];
ZEND_FALLTHROUGH;
case 4:
case 3:
hval += asso_values[(unsigned char)str[2]];
ZEND_FALLTHROUGH;
case 2:
case 1:
hval += asso_values[(unsigned char)str[0]];
break;
}
return hval + asso_values[(unsigned char)str[len - 1]];
}

#define NAME_HASH_MIN_NAME_LENGTH 2
#define NAME_HASH_MAX_NAME_LENGTH 23

const mbfl_encoding *mbfl_name2encoding(const char *name)
{
const mbfl_encoding **encoding;
const mbfl_encoding *const *encoding;

/* Sanity check perfect hash for name.
* Never enable this in production, this is only a development-time sanity check! */
#if ZEND_DEBUG && 0
for (encoding = mbfl_encoding_ptr_list; *encoding; encoding++) {
if (strcasecmp((*encoding)->name, name) == 0) {
return *encoding;
size_t name_length = strlen((*encoding)->name);
if (!(name_length <= NAME_HASH_MAX_NAME_LENGTH && name_length >= NAME_HASH_MIN_NAME_LENGTH)) {
fprintf(stderr, "name length is not satisfying bound check: %zu %s\n", name_length, (*encoding)->name);
abort();
}
unsigned int key = mbfl_name2encoding_perfect_hash((*encoding)->name, name_length);
if (mbfl_encoding_ptr_list[mbfl_encoding_ptr_list_after_hashing[key]] != *encoding) {
fprintf(stderr, "mbfl_name2encoding_perfect_hash: key %u %s mismatch\n", key, (*encoding)->name);
abort();
}
}
#endif

/* Use perfect hash lookup for name */
size_t name_len = strlen(name);
if (name_len <= NAME_HASH_MAX_NAME_LENGTH && name_len >= NAME_HASH_MIN_NAME_LENGTH) {
unsigned int key = mbfl_name2encoding_perfect_hash(name, name_len);
if (key <= 186) {
int8_t offset = mbfl_encoding_ptr_list_after_hashing[key];
if (offset >= 0) {
encoding = mbfl_encoding_ptr_list + offset;
if (strcasecmp((*encoding)->name, name) == 0) {
return *encoding;
}
}
}
}

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