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master def6fa4246 2/2: Speed up string-lessp for multibyte strings
From: |
Mattias Engdegård |
Subject: |
master def6fa4246 2/2: Speed up string-lessp for multibyte strings |
Date: |
Fri, 7 Oct 2022 07:59:52 -0400 (EDT) |
branch: master
commit def6fa4246502befa174aa6409166b0967621f7b
Author: Mattias Engdegård <mattiase@acm.org>
Commit: Mattias Engdegård <mattiase@acm.org>
Speed up string-lessp for multibyte strings
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.
---
src/fns.c | 51 +++++++++++++++++++++++++++++++++++++++------------
1 file changed, 39 insertions(+), 12 deletions(-)
diff --git a/src/fns.c b/src/fns.c
index 22e66d3653..bc4915eb25 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -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))
{