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Speed up string-lessp for multibyte strings

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Improve comparison speed when both arguments are multibyte strings,
at least one of them containing a non-ASCII character.  (All-ASCII
multibyte strings are already fast.)
The speed-up is about 2× for strings of 10 chars, 10× for strings of
100 chars.

* src/fns.c (Fstring_lessp): Quickly skip the common prefix by
comparing words.
This commit is contained in:
Mattias Engdegård 2022-10-06 17:46:02 +02:00
parent 6b4c17dec0
commit def6fa4246
1 changed files with 39 additions and 12 deletions
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51
src/fns.c
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@ -454,23 +454,50 @@ Symbols are also allowed; their print names are used instead. */)
&& (!STRING_MULTIBYTE (string2) || SCHARS (string2) == SBYTES (string2)))
{
/* Each argument is either unibyte or all-ASCII multibyte:
we can compare bytewise.
(Arbitrary multibyte strings cannot be compared bytewise because
that would give a different order for raw bytes 80..FF.) */
we can compare bytewise. */
int d = memcmp (SSDATA (string1), SSDATA (string2), n);
return d < 0 || (d == 0 && n < SCHARS (string2)) ? Qt : Qnil;
}
else if (STRING_MULTIBYTE (string1) && STRING_MULTIBYTE (string2))
{
ptrdiff_t i1 = 0, i1_byte = 0, i2 = 0, i2_byte = 0;
while (i1 < n)
{
int c1 = fetch_string_char_advance_no_check (string1, &i1, &i1_byte);
int c2 = fetch_string_char_advance_no_check (string2, &i2, &i2_byte);
if (c1 != c2)
return c1 < c2 ? Qt : Qnil;
}
return i1 < SCHARS (string2) ? Qt : Qnil;
/* Two arbitrary multibyte strings: we cannot use memcmp because
the encoding for raw bytes would sort those between U+007F and U+0080
which isn't where we want them.
Instead, we skip the longest common prefix and look at
what follows. */
ptrdiff_t nb1 = SBYTES (string1);
ptrdiff_t nb2 = SBYTES (string2);
ptrdiff_t nb = min (nb1, nb2);
/* First compare entire machine words. (String data is allocated
with word alignment.) */
typedef size_t word_t;
int ws = sizeof (word_t);
const word_t *w1 = (const word_t *) SDATA (string1);
const word_t *w2 = (const word_t *) SDATA (string2);
ptrdiff_t b = 0;
while (b < nb - ws + 1 && w1[b / ws] == w2[b / ws])
b += ws;
/* Scan forward to the differing byte (at most ws-1 bytes). */
while (b < nb && SREF (string1, b) == SREF (string2, b))
b++;
if (b >= nb)
/* One string is a prefix of the other. */
return b < nb2 ? Qt : Qnil;
/* Now back up to the start of the differing characters:
it's the last byte not having the bit pattern 10xxxxxx. */
while ((SREF (string1, b) & 0xc0) == 0x80)
b--;
/* Compare the differing characters. */
ptrdiff_t i1 = 0, i2 = 0;
ptrdiff_t i1_byte = b, i2_byte = b;
int c1 = fetch_string_char_advance_no_check (string1, &i1, &i1_byte);
int c2 = fetch_string_char_advance_no_check (string2, &i2, &i2_byte);
return c1 < c2 ? Qt : Qnil;
}
else if (STRING_MULTIBYTE (string1))
{