~linaro-toolchain-dev/cortex-strings/trunk : revision 28

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Free Software Foundation, Inc.

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This file is part of the GNU C Library.

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Based on strlen implementation by Torbjorn Granlund (tege@sics.se),

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with help from Dan Sahlin (dan@sics.se) and

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bug fix and commentary by Jim Blandy (jimb@ai.mit.edu);

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adaptation to strchr suggested by Dick Karpinski (dick@cca.ucsf.edu),

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and implemented by Roland McGrath (roland@ai.mit.edu).

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The GNU C Library is free software; you can redistribute it and/or

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modify it under the terms of the GNU Lesser General Public

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License as published by the Free Software Foundation; either

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version 2.1 of the License, or (at your option) any later version.

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The GNU C Library is distributed in the hope that it will be useful,

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but WITHOUT ANY WARRANTY; without even the implied warranty of

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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU

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Lesser General Public License for more details.

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You should have received a copy of the GNU Lesser General Public

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License along with the GNU C Library; if not, write to the Free

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Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA

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02111-1307 USA. */

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#include <string.h>

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#include <stdlib.h>

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#undef strchr

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# define reg_char char

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/* Find the first occurrence of C in S. */

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char *

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strchr (s, c_in)

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const char *s;

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int c_in;

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{

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const unsigned char *char_ptr;

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const unsigned long int *longword_ptr;

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unsigned long int longword, magic_bits, charmask;

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unsigned reg_char c;

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c = (unsigned char) c_in;

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/* Handle the first few characters by reading one character at a time.

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Do this until CHAR_PTR is aligned on a longword boundary. */

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for (char_ptr = (const unsigned char *) s;

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((unsigned long int) char_ptr & (sizeof (longword) - 1)) != 0;

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++char_ptr)

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if (*char_ptr == c)

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return (void *) char_ptr;

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else if (*char_ptr == '\0')

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return NULL;

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/* All these elucidatory comments refer to 4-byte longwords,

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but the theory applies equally well to 8-byte longwords. */

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longword_ptr = (unsigned long int *) char_ptr;

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/* Bits 31, 24, 16, and 8 of this number are zero. Call these bits

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the "holes." Note that there is a hole just to the left of

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each byte, with an extra at the end:

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bits: 01111110 11111110 11111110 11111111

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bytes: AAAAAAAA BBBBBBBB CCCCCCCC DDDDDDDD

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The 1-bits make sure that carries propagate to the next 0-bit.

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The 0-bits provide holes for carries to fall into. */

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switch (sizeof (longword))

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{

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case 4: magic_bits = 0x7efefeffL; break;

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case 8: magic_bits = ((0x7efefefeL << 16) << 16) | 0xfefefeffL; break;

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default:

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abort ();

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}

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/* Set up a longword, each of whose bytes is C. */

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charmask = c | (c << 8);

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charmask |= charmask << 16;

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if (sizeof (longword) > 4)

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/* Do the shift in two steps to avoid a warning if long has 32 bits. */

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charmask |= (charmask << 16) << 16;

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if (sizeof (longword) > 8)

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abort ();

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/* Instead of the traditional loop which tests each character,

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we will test a longword at a time. The tricky part is testing

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if *any of the four* bytes in the longword in question are zero. */

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for (;;)

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{

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/* We tentatively exit the loop if adding MAGIC_BITS to

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LONGWORD fails to change any of the hole bits of LONGWORD.

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1) Is this safe? Will it catch all the zero bytes?

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Suppose there is a byte with all zeros. Any carry bits

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propagating from its left will fall into the hole at its

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least significant bit and stop. Since there will be no

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carry from its most significant bit, the LSB of the

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byte to the left will be unchanged, and the zero will be

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detected.

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2) Is this worthwhile? Will it ignore everything except

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zero bytes? Suppose every byte of LONGWORD has a bit set

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somewhere. There will be a carry into bit 8. If bit 8

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is set, this will carry into bit 16. If bit 8 is clear,

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one of bits 9-15 must be set, so there will be a carry

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into bit 16. Similarly, there will be a carry into bit

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24. If one of bits 24-30 is set, there will be a carry

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into bit 31, so all of the hole bits will be changed.

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The one misfire occurs when bits 24-30 are clear and bit

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31 is set; in this case, the hole at bit 31 is not

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changed. If we had access to the processor carry flag,

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we could close this loophole by putting the fourth hole

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at bit 32!

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So it ignores everything except 128's, when they're aligned

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properly.

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3) But wait! Aren't we looking for C as well as zero?

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Good point. So what we do is XOR LONGWORD with a longword,

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each of whose bytes is C. This turns each byte that is C

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into a zero. */

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longword = *longword_ptr++;

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/* Add MAGIC_BITS to LONGWORD. */

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if ((((longword + magic_bits)

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/* Set those bits that were unchanged by the addition. */

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^ ~longword)

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/* Look at only the hole bits. If any of the hole bits

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are unchanged, most likely one of the bytes was a

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zero. */

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& ~magic_bits) != 0 ||

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/* That caught zeroes. Now test for C. */

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((((longword ^ charmask) + magic_bits) ^ ~(longword ^ charmask))

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& ~magic_bits) != 0)

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{

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/* Which of the bytes was C or zero?

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If none of them were, it was a misfire; continue the search. */

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const unsigned char *cp = (const unsigned char *) (longword_ptr - 1);

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if (*cp == c)

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return (char *) cp;

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else if (*cp == '\0')

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return NULL;

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if (*++cp == c)

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return (char *) cp;

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else if (*cp == '\0')

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return NULL;

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if (*++cp == c)

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return (char *) cp;

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else if (*cp == '\0')

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return NULL;

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if (*++cp == c)

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return (char *) cp;

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else if (*cp == '\0')

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return NULL;

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if (sizeof (longword) > 4)

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{

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if (*++cp == c)

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return (char *) cp;

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else if (*cp == '\0')

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return NULL;

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if (*++cp == c)

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return (char *) cp;

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else if (*cp == '\0')

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return NULL;

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if (*++cp == c)

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return (char *) cp;

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else if (*cp == '\0')

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return NULL;

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if (*++cp == c)

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return (char *) cp;

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else if (*cp == '\0')

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return NULL;

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}

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}

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}

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return NULL;

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}