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Diffstat (limited to 'sysdeps/generic/strchr.c')
| -rw-r--r-- | sysdeps/generic/strchr.c | 183 |
1 files changed, 183 insertions, 0 deletions
diff --git a/sysdeps/generic/strchr.c b/sysdeps/generic/strchr.c new file mode 100644 index 0000000000..b9c8393f8e --- /dev/null +++ b/sysdeps/generic/strchr.c @@ -0,0 +1,183 @@ +/* Copyright (C) 1991, 1993, 1994, 1995 Free Software Foundation, Inc. + Based on strlen implemention by Torbjorn Granlund (tege@sics.se), + with help from Dan Sahlin (dan@sics.se) and + bug fix and commentary by Jim Blandy (jimb@ai.mit.edu); + adaptation to strchr suggested by Dick Karpinski (dick@cca.ucsf.edu), + and implemented by Roland McGrath (roland@ai.mit.edu). + +The GNU C Library is free software; you can redistribute it and/or +modify it under the terms of the GNU Library General Public License as +published by the Free Software Foundation; either version 2 of the +License, or (at your option) any later version. + +The GNU C Library is distributed in the hope that it will be useful, +but WITHOUT ANY WARRANTY; without even the implied warranty of +MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU +Library General Public License for more details. + +You should have received a copy of the GNU Library General Public +License along with the GNU C Library; see the file COPYING.LIB. If +not, write to the Free Software Foundation, Inc., 675 Mass Ave, +Cambridge, MA 02139, USA. */ + +#include <ansidecl.h> +#include <string.h> + + +/* Find the first ocurrence of C in S. */ + +char * +DEFUN(strchr, (s, c), CONST char *s AND int c) +{ + CONST unsigned char *char_ptr; + CONST unsigned long int *longword_ptr; + unsigned long int longword, magic_bits, charmask; + + c = (unsigned char) c; + + /* 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 = s; ((unsigned long int) char_ptr + & (sizeof (longword) - 1)) != 0; + ++char_ptr) + if (*char_ptr == c) + return (PTR) char_ptr; + else if (*char_ptr == '\0') + return NULL; + + /* All these elucidatory comments refer to 4-byte longwords, + but the theory applies equally well to 8-byte longwords. */ + + 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. */ + switch (sizeof (longword)) + { + case 4: magic_bits = 0x7efefeffL; break; + case 8: magic_bits = (0x7efefefeL << 32) | 0xfefefeffL; break; + default: + abort (); + } + + /* Set up a longword, each of whose bytes is C. */ + charmask = c | (c << 8); + charmask |= charmask << 16; + if (sizeof (longword) > 4) + charmask |= charmask << 32; + 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. */ + for (;;) + { + /* 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. + + 3) But wait! Aren't we looking for C as well as zero? + Good point. So what we do is XOR LONGWORD with a longword, + each of whose bytes is C. This turns each byte that is C + into a zero. */ + + longword = *longword_ptr++; + + /* Add MAGIC_BITS to LONGWORD. */ + if ((((longword + magic_bits) + + /* Set those bits that were unchanged by the addition. */ + ^ ~longword) + + /* Look at only the hole bits. If any of the hole bits + are unchanged, most likely one of the bytes was a + zero. */ + & ~magic_bits) != 0 || + + /* That caught zeroes. Now test for C. */ + ((((longword ^ charmask) + magic_bits) ^ ~(longword ^ charmask)) + & ~magic_bits) != 0) + { + /* Which of the bytes was C or zero? + If none of them were, it was a misfire; continue the search. */ + + CONST unsigned char *cp = (CONST unsigned char *) (longword_ptr - 1); + + if (*cp == c) + return (char *) cp; + else if (*cp == '\0') + return NULL; + if (*++cp == c) + return (char *) cp; + else if (*cp == '\0') + return NULL; + if (*++cp == c) + return (char *) cp; + else if (*cp == '\0') + return NULL; + if (*++cp == c) + return (char *) cp; + else if (*cp == '\0') + return NULL; + if (sizeof (longword) > 4) + { + if (*++cp == c) + return (char *) cp; + else if (*cp == '\0') + return NULL; + if (*++cp == c) + return (char *) cp; + else if (*cp == '\0') + return NULL; + if (*++cp == c) + return (char *) cp; + else if (*cp == '\0') + return NULL; + if (*++cp == c) + return (char *) cp; + else if (*cp == '\0') + return NULL; + } + } + } + + return NULL; +} + +#ifdef weak_alias +#undef index +weak_alias (strchr, index) +#endif |