git.saurik.com Git - apple/xnu.git/blob - libsyscall/wrappers/string/strlen.c

1 /*-

4 *

5 * Redistribution and use in source and binary forms, with or without

6 * modification, are permitted provided that the following conditions

7 * are met:

8 * 1. Redistributions of source code must retain the above copyright

9 * notice, this list of conditions and the following disclaimer.

10 * 2. Redistributions in binary form must reproduce the above copyright

11 * notice, this list of conditions and the following disclaimer in the

12 * documentation and/or other materials provided with the distribution.

13 *

14 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND

15 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE

16 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE

17 * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE

18 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL

19 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS

20 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)

21 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT

22 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY

23 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF

24 * SUCH DAMAGE.

25 */

27 #include "string.h"

28 #include <limits.h>

30 /*

31 * Portable strlen() for 32-bit and 64-bit systems.

32 *

33 * Rationale: it is generally much more efficient to do word length

34 * operations and avoid branches on modern computer systems, as

35 * compared to byte-length operations with a lot of branches.

36 *

37 * The expression:

38 *

39 * ((x - 0x01....01) & ~x & 0x80....80)

40 *

41 * would evaluate to a non-zero value iff any of the bytes in the

42 * original word is zero. However, we can further reduce ~1/3 of

43 * time if we consider that strlen() usually operate on 7-bit ASCII

44 * by employing the following expression, which allows false positive

45 * when high bit of 1 and use the tail case to catch these case:

46 *

47 * ((x - 0x01....01) & 0x80....80)

48 *

49 * This is more than 5.2 times as fast as the raw implementation on

50 * Intel T7300 under long mode for strings longer than word length.

51 */

53 /* Magic numbers for the algorithm */

54 #if LONG_BIT == 32

55 static const unsigned long mask01 = 0x01010101;

56 static const unsigned long mask80 = 0x80808080;

57 #elif LONG_BIT == 64

58 static const unsigned long mask01 = 0x0101010101010101;

59 static const unsigned long mask80 = 0x8080808080808080;

60 #else

61 #error Unsupported word size

62 #endif

64 #define LONGPTR_MASK (sizeof(long) - 1)

66 /*

67 * Helper macro to return string length if we caught the zero

68 * byte.

69 */

70 #define testbyte(x) \

71 do { \

72 if (p[x] == '\0') \

73 return (p - str + x); \

74 } while (0)

 __attribute__((visibility("hidden")))

77 size_t

 _libkernel_strlen(const char *str)

79 {

80 const char *p;

81 const unsigned long *lp;

83 /* Skip the first few bytes until we have an aligned p */

         for (p = str; (uintptr_t)p & LONGPTR_MASK; p++) {

                 if (*p == '\0') {

86 return p - str;

87 }

88 }

90 /* Scan the rest of the string using word sized operation */

         for (lp = (const unsigned long *)p;; lp++) {

                 if ((*lp - mask01) & mask80) {

93 p = (const char *)(lp);

94 testbyte(0);

95 testbyte(1);

96 testbyte(2);

97 testbyte(3);

98 #if (LONG_BIT >= 64)

99 testbyte(4);

100 testbyte(5);

101 testbyte(6);

102 testbyte(7);

103 #endif

104 }

105 }

106

107 /* NOTREACHED */

108 return 0;

109 }