minix/common/lib/libc/arch/powerpc/string/strlen.S

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/* $NetBSD: strlen.S,v 1.6 2011/01/15 07:31:12 matt Exp $ */
/*-
* Copyright (C) 2001 Martin J. Laubach <mjl@NetBSD.org>
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* 3. The name of the author may not be used to endorse or promote products
* derived from this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
* IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
* OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
* IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
* NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
/*----------------------------------------------------------------------*/
#include <machine/asm.h>
__RCSID("$NetBSD: strlen.S,v 1.6 2011/01/15 07:31:12 matt Exp $");
/*----------------------------------------------------------------------*/
/* The algorithm here uses the following techniques:
1) Given a word 'x', we can test to see if it contains any 0 bytes
by subtracting 0x01010101, and seeing if any of the high bits of each
byte changed from 0 to 1. This works because the least significant
0 byte must have had no incoming carry (otherwise it's not the least
significant), so it is 0x00 - 0x01 == 0xff. For all other
byte values, either they have the high bit set initially, or when
1 is subtracted you get a value in the range 0x00-0x7f, none of which
have their high bit set. The expression here is
(x + 0xfefefeff) & ~(x | 0x7f7f7f7f), which gives 0x00000000 when
there were no 0x00 bytes in the word.
2) Given a word 'x', we can test to see _which_ byte was zero by
calculating ~(((x & 0x7f7f7f7f) + 0x7f7f7f7f) | x | 0x7f7f7f7f).
This produces 0x80 in each byte that was zero, and 0x00 in all
the other bytes. The '| 0x7f7f7f7f' clears the low 7 bits in each
byte, and the '| x' part ensures that bytes with the high bit set
produce 0x00. The addition will carry into the high bit of each byte
iff that byte had one of its low 7 bits set. We can then just see
which was the most significant bit set and divide by 8 to find how
many to add to the index.
This is from the book 'The PowerPC Compiler Writer's Guide',
by Steve Hoxey, Faraydon Karim, Bill Hay and Hank Warren.
*/
/*----------------------------------------------------------------------*/
.text
.align 4
ENTRY(strlen)
/* Setup constants */
lis %r10, 0x7f7f
lis %r9, 0xfefe
ori %r10, %r10, 0x7f7f
ori %r9, %r9, 0xfeff
/* Mask out leading bytes on non aligned strings */
rlwinm. %r8, %r3, 3, 27, 28 /* leading bits to mask */
#ifdef _LP64
clrrdi %r5, %r3, 2 /* clear low 2 addr bits */
#else
clrrwi %r5, %r3, 2 /* clear low 2 addr bits */
#endif
li %r0, -1
beq+ 3f /* skip alignment if already */
/* aligned */
srw %r0, %r0, %r8 /* make 0000...1111 mask */
lwz %r7, 0(%r5)
nor %r0, %r0, %r0 /* invert mask */
or %r7, %r7, %r0 /* make leading bytes != 0 */
b 2f
3: subi %r5, %r5, 4
1: lwzu %r7, 4(%r5) /* fetch data word */
2: nor %r0, %r7, %r10 /* do step 1 */
add %r6, %r7, %r9
and. %r0, %r0, %r6
beq+ 1b /* no NUL bytes here */
and %r8, %r7, %r10 /* ok, a NUL is somewhere */
or %r7, %r7, %r10 /* do step 2 to find out */
add %r0, %r8, %r10 /* where */
nor %r8, %r7, %r0
cntlzw %r0, %r8 /* offset from this word */
srwi %r4, %r0, 3
add %r4, %r5, %r4 /* r4 contains end pointer */
/* NOTE: Keep it so this function returns the end pointer
in r4, so we can it use from other str* calls (strcat
comes to mind */
subf %r3, %r3, %r4
blr
END(strlen)
/*----------------------------------------------------------------------*/