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MINOR: ist: add ist_find_ctl()

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This new function looks for the first control character in a string (a
char whose value is between 0x00 and 0x1F included) and returns it, or
NULL if there is none. It is optimized for quickly evicting non-matching
strings and scans ~0.43 bytes per cycle. It can be used as an accelerator
when it's needed to look up several of these characters (e.g. CR/LF/NUL).
This commit is contained in:
Willy Tarreau 2019-11-22 15:58:53 +01:00
parent 2e945c8ee7
commit 8f3ce06f14
1 changed files with 41 additions and 0 deletions
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41
include/common/ist.h
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@ -606,6 +606,47 @@ static inline char *istchr(const struct ist ist, char chr)
return s - 1;
}
/* Returns a pointer to the first control character found in <ist>, or NULL if
* none is present. A control character is defined as a byte whose value is
* between 0x00 and 0x1F included. The function is optimized for strings having
* no CTL chars by processing up to sizeof(long) bytes at once on architectures
* supporting efficient unaligned accesses. Despite this it is not very fast
* (~0.43 byte/cycle) and should mostly be used on low match probability when
* it can save a call to a much slower function.
*/
static inline const char *ist_find_ctl(const struct ist ist)
{
const union { unsigned long v; } __attribute__((packed)) *u;
const char *curr = (void *)ist.ptr - sizeof(long);
const char *last = curr + ist.len;
unsigned long l1, l2;
do {
curr += sizeof(long);
if (curr > last)
break;
u = (void *)curr;
/* subtract 0x202020...20 to the value to generate a carry in
* the lower byte if the byte contains a lower value. If we
* generate a bit 7 that was not there, it means the byte was
* within 0x00..0x1F.
*/
l2 = u->v;
l1 = ~l2 & ((~0UL / 255) * 0x80); /* 0x808080...80 */
l2 -= (~0UL / 255) * 0x20; /* 0x202020...20 */
} while ((l1 & l2) == 0);
last += sizeof(long);
if (__builtin_expect(curr < last, 0)) {
do {
if ((uint8_t)*curr < 0x20)
return curr;
curr++;
} while (curr < last);
}
return NULL;
}
/* looks for first occurrence of character <chr> in string <ist> and returns
* the tail of the string starting with this character, or (ist.end,0) if not
* found.