On 07/07/2010 03:44 PM, Pádraig Brady wrote:
Subject: [PATCH] unistr/u8-strchr: speed up searching for ASCII characters
* lib/unistr/u8-strchr.c (u8_strchr): Use strchr() for
the single byte case as it was measured to be 50% faster
than the existing code on x86 linux. Also add a comment
on why not to use memmem() for the moment for the multibyte case.
If p is surely a valid UTF-8 string, you can do better in general like
this. Say [q, q+q_len) points to an UTF-8 representation of uc:
for (; p = strchr (p, *q) && memcmp (p+1, q+1, q_len-1); p += q_len)
;
return p;
That's because once the first byte has matched, the length of the UTF-8
character is known to be q_len. It's better than memmem if the startup
cost of strchr is low enough (of course memcmp has to be
inlined/unrolled/unswitched to get decent performance).
Does the argument of u8_strchr have this guarantee? If not, the above
code can read arbitrary memory.
Paolo
---
ChangeLog | 4 ++++
lib/unistr/u8-strchr.c | 19 +++++++------------
2 files changed, 11 insertions(+), 12 deletions(-)
diff --git a/ChangeLog b/ChangeLog
index afcae28..8ca0bd7 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+2010-07-07 Pádraig Brady<p...@draigbrady.com>
+
+ * lib/unistr/u8-strchr.c (u8_strchr): Use strchr() as it's faster
+
2010-07-04 Bruno Haible<br...@clisp.org>
fsusage: Clarify which code applies to which platforms.
diff --git a/lib/unistr/u8-strchr.c b/lib/unistr/u8-strchr.c
index 3be14c7..3dbd3ca 100644
--- a/lib/unistr/u8-strchr.c
+++ b/lib/unistr/u8-strchr.c
@@ -21,25 +21,20 @@
/* Specification. */
#include "unistr.h"
+#include<string.h>
+
uint8_t *
u8_strchr (const uint8_t *s, ucs4_t uc)
{
uint8_t c[6];
if (uc< 0x80)
- {
- uint8_t c0 = uc;
-
- for (;; s++)
- {
- if (*s == c0)
- break;
- if (*s == 0)
- goto notfound;
- }
- return (uint8_t *) s;
- }
+ return strchr (s, uc);
else
+ /* The following is equivalent to:
+ return memmem (s, strlen(s), c, csize);
+ but faster for long S with matching UC near the start,
+ and also memmem is sometimes buggy and inefficient. */
switch (u8_uctomb_aux (c, uc, 6))
{
case 2:
