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[PATCH 07/11] Improve generic strnlen
- From: Richard Henderson <rth at twiddle dot net>
- To: libc-alpha at sourceware dot org
- Date: Fri, 16 Dec 2016 22:57:25 -0800
- Subject: [PATCH 07/11] Improve generic strnlen
- Authentication-results: sourceware.org; auth=none
- References: <20161217065729.28561-1-rth@twiddle.net>
* string/strnlen.c: Use haszero.h, whichzero.h.
---
string/strnlen.c | 125 +++++++++----------------------------------------------
1 file changed, 19 insertions(+), 106 deletions(-)
diff --git a/string/strnlen.c b/string/strnlen.c
index b2b0664..9927b9d 100644
--- a/string/strnlen.c
+++ b/string/strnlen.c
@@ -21,7 +21,9 @@
not, see <http://www.gnu.org/licenses/>. */
#include <string.h>
-#include <stdlib.h>
+#include <stdint.h>
+#include <haszero.h>
+#include <whichzero.h>
/* Find the length of S, but scan at most MAXLEN characters. If no
'\0' terminator is found in that many characters, return MAXLEN. */
@@ -33,11 +35,12 @@
size_t
__strnlen (const char *str, size_t maxlen)
{
- const char *char_ptr, *end_ptr = str + maxlen;
+ const char *char_ptr = str, *end_ptr = str + maxlen;
const unsigned long int *longword_ptr;
- unsigned long int longword, himagic, lomagic;
+ unsigned long int longword;
+ size_t i, align;
- if (maxlen == 0)
+ if (__glibc_unlikely (maxlen == 0))
return 0;
if (__glibc_unlikely (end_ptr < str))
@@ -45,116 +48,26 @@ __strnlen (const char *str, size_t maxlen)
/* Handle the first few characters by reading one character at a time.
Do this until CHAR_PTR is aligned on a longword boundary. */
- for (char_ptr = str; ((unsigned long int) char_ptr
- & (sizeof (longword) - 1)) != 0;
- ++char_ptr)
+ align = -(uintptr_t)char_ptr % sizeof(longword);
+ for (i = 0; i < align; ++i, ++char_ptr)
if (*char_ptr == '\0')
- {
- if (char_ptr > end_ptr)
- char_ptr = end_ptr;
- return char_ptr - str;
- }
+ goto found;
- /* All these elucidatory comments refer to 4-byte longwords,
- but the theory applies equally well to 8-byte longwords. */
+ longword_ptr = (const unsigned long int *) char_ptr;
+ char_ptr = end_ptr;
- longword_ptr = (unsigned long int *) char_ptr;
-
- /* Bits 31, 24, 16, and 8 of this number are zero. Call these bits
- the "holes." Note that there is a hole just to the left of
- each byte, with an extra at the end:
-
- bits: 01111110 11111110 11111110 11111111
- bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD
-
- The 1-bits make sure that carries propagate to the next 0-bit.
- The 0-bits provide holes for carries to fall into. */
- himagic = 0x80808080L;
- lomagic = 0x01010101L;
- if (sizeof (longword) > 4)
+ while (longword_ptr < (const unsigned long int *) end_ptr)
{
- /* 64-bit version of the magic. */
- /* Do the shift in two steps to avoid a warning if long has 32 bits. */
- himagic = ((himagic << 16) << 16) | himagic;
- lomagic = ((lomagic << 16) << 16) | lomagic;
- }
- if (sizeof (longword) > 8)
- abort ();
-
- /* Instead of the traditional loop which tests each character,
- we will test a longword at a time. The tricky part is testing
- if *any of the four* bytes in the longword in question are zero. */
- while (longword_ptr < (unsigned long int *) end_ptr)
- {
- /* We tentatively exit the loop if adding MAGIC_BITS to
- LONGWORD fails to change any of the hole bits of LONGWORD.
-
- 1) Is this safe? Will it catch all the zero bytes?
- Suppose there is a byte with all zeros. Any carry bits
- propagating from its left will fall into the hole at its
- least significant bit and stop. Since there will be no
- carry from its most significant bit, the LSB of the
- byte to the left will be unchanged, and the zero will be
- detected.
-
- 2) Is this worthwhile? Will it ignore everything except
- zero bytes? Suppose every byte of LONGWORD has a bit set
- somewhere. There will be a carry into bit 8. If bit 8
- is set, this will carry into bit 16. If bit 8 is clear,
- one of bits 9-15 must be set, so there will be a carry
- into bit 16. Similarly, there will be a carry into bit
- 24. If one of bits 24-30 is set, there will be a carry
- into bit 31, so all of the hole bits will be changed.
-
- The one misfire occurs when bits 24-30 are clear and bit
- 31 is set; in this case, the hole at bit 31 is not
- changed. If we had access to the processor carry flag,
- we could close this loophole by putting the fourth hole
- at bit 32!
-
- So it ignores everything except 128's, when they're aligned
- properly. */
-
- longword = *longword_ptr++;
-
- if ((longword - lomagic) & himagic)
+ longword = *longword_ptr;
+ if (haszero (longword))
{
- /* Which of the bytes was the zero? If none of them were, it was
- a misfire; continue the search. */
-
- const char *cp = (const char *) (longword_ptr - 1);
-
- char_ptr = cp;
- if (cp[0] == 0)
- break;
- char_ptr = cp + 1;
- if (cp[1] == 0)
- break;
- char_ptr = cp + 2;
- if (cp[2] == 0)
- break;
- char_ptr = cp + 3;
- if (cp[3] == 0)
- break;
- if (sizeof (longword) > 4)
- {
- char_ptr = cp + 4;
- if (cp[4] == 0)
- break;
- char_ptr = cp + 5;
- if (cp[5] == 0)
- break;
- char_ptr = cp + 6;
- if (cp[6] == 0)
- break;
- char_ptr = cp + 7;
- if (cp[7] == 0)
- break;
- }
+ char_ptr = (const char *) longword_ptr + whichzero (longword);
+ break;
}
- char_ptr = end_ptr;
+ longword_ptr++;
}
+ found:
if (char_ptr > end_ptr)
char_ptr = end_ptr;
return char_ptr - str;
--
2.9.3