xf.li | bdd93d5 | 2023-05-12 07:10:14 -0700 | [diff] [blame] | 1 | /* Copyright (C) 1991-2016 Free Software Foundation, Inc. |
| 2 | This file is part of the GNU C Library. |
| 3 | Written by Torbjorn Granlund (tege@sics.se), |
| 4 | with help from Dan Sahlin (dan@sics.se); |
| 5 | commentary by Jim Blandy (jimb@ai.mit.edu). |
| 6 | |
| 7 | The GNU C Library is free software; you can redistribute it and/or |
| 8 | modify it under the terms of the GNU Lesser General Public |
| 9 | License as published by the Free Software Foundation; either |
| 10 | version 2.1 of the License, or (at your option) any later version. |
| 11 | |
| 12 | The GNU C Library is distributed in the hope that it will be useful, |
| 13 | but WITHOUT ANY WARRANTY; without even the implied warranty of |
| 14 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
| 15 | Lesser General Public License for more details. |
| 16 | |
| 17 | You should have received a copy of the GNU Lesser General Public |
| 18 | License along with the GNU C Library; if not, see |
| 19 | <http://www.gnu.org/licenses/>. */ |
| 20 | |
| 21 | #include <string.h> |
| 22 | #include <stdlib.h> |
| 23 | |
| 24 | #undef strlen |
| 25 | |
| 26 | #ifndef STRLEN |
| 27 | # define STRLEN strlen |
| 28 | #endif |
| 29 | |
| 30 | /* Return the length of the null-terminated string STR. Scan for |
| 31 | the null terminator quickly by testing four bytes at a time. */ |
| 32 | size_t |
| 33 | STRLEN (const char *str) |
| 34 | { |
| 35 | const char *char_ptr; |
| 36 | const unsigned long int *longword_ptr; |
| 37 | unsigned long int longword, himagic, lomagic; |
| 38 | |
| 39 | /* Handle the first few characters by reading one character at a time. |
| 40 | Do this until CHAR_PTR is aligned on a longword boundary. */ |
| 41 | for (char_ptr = str; ((unsigned long int) char_ptr |
| 42 | & (sizeof (longword) - 1)) != 0; |
| 43 | ++char_ptr) |
| 44 | if (*char_ptr == '\0') |
| 45 | return char_ptr - str; |
| 46 | |
| 47 | /* All these elucidatory comments refer to 4-byte longwords, |
| 48 | but the theory applies equally well to 8-byte longwords. */ |
| 49 | |
| 50 | longword_ptr = (unsigned long int *) char_ptr; |
| 51 | |
| 52 | /* Bits 31, 24, 16, and 8 of this number are zero. Call these bits |
| 53 | the "holes." Note that there is a hole just to the left of |
| 54 | each byte, with an extra at the end: |
| 55 | |
| 56 | bits: 01111110 11111110 11111110 11111111 |
| 57 | bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD |
| 58 | |
| 59 | The 1-bits make sure that carries propagate to the next 0-bit. |
| 60 | The 0-bits provide holes for carries to fall into. */ |
| 61 | himagic = 0x80808080L; |
| 62 | lomagic = 0x01010101L; |
| 63 | if (sizeof (longword) > 4) |
| 64 | { |
| 65 | /* 64-bit version of the magic. */ |
| 66 | /* Do the shift in two steps to avoid a warning if long has 32 bits. */ |
| 67 | himagic = ((himagic << 16) << 16) | himagic; |
| 68 | lomagic = ((lomagic << 16) << 16) | lomagic; |
| 69 | } |
| 70 | if (sizeof (longword) > 8) |
| 71 | abort (); |
| 72 | |
| 73 | /* Instead of the traditional loop which tests each character, |
| 74 | we will test a longword at a time. The tricky part is testing |
| 75 | if *any of the four* bytes in the longword in question are zero. */ |
| 76 | for (;;) |
| 77 | { |
| 78 | longword = *longword_ptr++; |
| 79 | |
| 80 | if (((longword - lomagic) & ~longword & himagic) != 0) |
| 81 | { |
| 82 | /* Which of the bytes was the zero? If none of them were, it was |
| 83 | a misfire; continue the search. */ |
| 84 | |
| 85 | const char *cp = (const char *) (longword_ptr - 1); |
| 86 | |
| 87 | if (cp[0] == 0) |
| 88 | return cp - str; |
| 89 | if (cp[1] == 0) |
| 90 | return cp - str + 1; |
| 91 | if (cp[2] == 0) |
| 92 | return cp - str + 2; |
| 93 | if (cp[3] == 0) |
| 94 | return cp - str + 3; |
| 95 | if (sizeof (longword) > 4) |
| 96 | { |
| 97 | if (cp[4] == 0) |
| 98 | return cp - str + 4; |
| 99 | if (cp[5] == 0) |
| 100 | return cp - str + 5; |
| 101 | if (cp[6] == 0) |
| 102 | return cp - str + 6; |
| 103 | if (cp[7] == 0) |
| 104 | return cp - str + 7; |
| 105 | } |
| 106 | } |
| 107 | } |
| 108 | } |
| 109 | libc_hidden_builtin_def (strlen) |