[2/4] Improve generic strspn performance
Commit Message
On 29-03-2016 10:02, Wilco Dijkstra wrote:
> Adhemerval Zanella wrote:
>
>> + if (accept[0] == '\0')
>> + return 0;
>> + if (accept[1] == '\0')
>> + {
>
> GCC doesn't get the static branch prediction correct for the 2nd if,
> so it would be useful to add __glibc_unlikely given that single-character
> accepts are rare.
>
>> + s = (unsigned char *) ((size_t)(s) & ~3);
>> + unsigned int c0, c1, c2, c3;
>> + do {
>> + s += 4;
>> + c0 = p[s[0]];
>> + c1 = p[s[1]];
>> + c2 = p[s[2]];
>> + c3 = p[s[3]];
>> + } while ((c0 && c1 && c2 && c3) == 1);
>
> That should use '&' rather than '&&' and '!= 0' similar to how I did it in strcspn.
> This will use 3 AND(S) instructions and a single branch.
>
>> +
>> + size_t count = s - (unsigned char *) str;
>> + return (c0 && c1) == 0 ? count - !c0 + 1 : count - !c2 + 3;
>
> Again, c0 & c1 is better and allows CSE with the while expression above.
> Also -!c0 +1 is equivalent to c0, -!c2 + 3 is equivalent to c2 + 2 - this is simpler
> and faster.
>
> Otherwise it looks good, and thanks for doing this one too!
>
> Cheers,
> Wilco
>
Thanks for the review. I think this version fixes the points you noted:
@@ -25,23 +25,49 @@
/* Return the length of the maximum initial segment
of S which contains only characters in ACCEPT. */
size_t
-STRSPN (const char *s, const char *accept)
+STRSPN (const char *str, const char *accept)
{
- const char *p;
- const char *a;
- size_t count = 0;
-
- for (p = s; *p != '\0'; ++p)
- {
- for (a = accept; *a != '\0'; ++a)
- if (*p == *a)
- break;
- if (*a == '\0')
- return count;
- else
- ++count;
+ if (accept[0] == '\0')
+ return 0;
+ if (__glibc_unlikely (accept[1] == '\0'))
+ {
+ const char *a = str;
+ for (; *str == *accept; str++);
+ return str - a;
}
- return count;
+ /* Use multiple small memsets to enable inlining on most targets. */
+ unsigned char table[256];
+ unsigned char *p = memset (table, 0, 64);
+ memset (p + 64, 0, 64);
+ memset (p + 128, 0, 64);
+ memset (p + 192, 0, 64);
+
+ unsigned char *s = (unsigned char*) accept;
+ /* Different from strcspn it does not add the NULL on the table
+ so can avoid check if str[i] is NULL, since table['\0'] will
+ be 0 and thus stopping the loop check. */
+ do
+ p[*s++] = 1;
+ while (*s);
+
+ s = (unsigned char*) str;
+ if (!p[s[0]]) return 0;
+ if (!p[s[1]]) return 1;
+ if (!p[s[2]]) return 2;
+ if (!p[s[3]]) return 3;
+
+ s = (unsigned char *) ((size_t)(s) & ~3);
+ unsigned int c0, c1, c2, c3;
+ do {
+ s += 4;
+ c0 = p[s[0]];
+ c1 = p[s[1]];
+ c2 = p[s[2]];
+ c3 = p[s[3]];
+ } while ((c0 & c1 & c2 & c3) != 0);
+
+ size_t count = s - (unsigned char *) str;
+ return (c0 & c1) == 0 ? count + c0 : count + c2 + 2;
}
libc_hidden_builtin_def (strspn)