/* $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) /*----------------------------------------------------------------------*/
