/* $NetBSD: strlen.S,v 1.1 2014/09/03 19:34:25 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 1.1 2014/09/03 19:34:25 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. */ /*----------------------------------------------------------------------*/ ENTRY(strlen) l.or r12, r3, r0 /* save start */ /* Setup constants */ l.movhi r13, 0x7f7f l.movhi r15, 0xfefe l.ori r13, r13, 0x7f7f l.ori r15, r15, 0xfeff 1: l.andi r7, r12, 3 /* get low bits of start */ l.sfeqi r7, 0 /* all clear? */ l.bf 3f /* yes, skip alignment */ l.nop /* -- delay slot -- */ l.sub r12, r12, r7 /* word align start */ l.lwz r8, 0(r12) /* load data */ l.addi r6, r0, -1 /* r6 = 0xffffffff */ l.slli r5, r7, 3 /* bits to bytes */ l.srl r6, r6, r5 /* clear low (MSB) bytes */ l.xori r6, r6, -1 /* complement */ l.or r8, r8, r6 /* merge with loaded word */ l.j 4f /* and process */ l.nop /* -- delay-slot -- */ 2: l.addi r12, r12, 4 /* advance to next word */ 3: l.lwz r8, 0(r12) /* fetch data word */ // Step 1: (x + 0xfefefeff) & ~(x | 0x7f7f7f7f) 4: l.or r7, r8, r13 /* t0 = x | 0x7f7f7f7f */ l.xori r6, r7, -1 /* t1 = ~t0 */ l.add r5, r8, r15 /* t2 = x + 0xfefefeff */ l.and r4, r7, r5 /* t3 = t1 & t2 */ l.sfeqi r4, 0 l.bf 2b /* no NUL bytes here */ l.nop /* -- delay slot -- */ // Step 2: ~(((x & 0x7f7f7f7f) + 0x7f7f7f7f) | x | 0x7f7f7f7f) l.and r7, r8, r13 /* t0 = x & 0x7f7f7f7f */ l.or r6, r8, r13 /* t1 = x | 0x7f7f7f7f */ l.add r5, r7, r13 /* t2 = t0 + 0x7f7f7f7f */ l.or r4, r5, r6 /* t3 = t2 | t1 */ l.xori r4, r4, -1 /* t3 = ~t3 */ l.fl1 r5, r4 /* find last bit set */ l.ori r6, r0, 32 /* bits per word */ l.sub r7, r6, r5 /* cvt to leading zeros */ l.srli r8, r7, 3 /* shift to byte count */ Ldone: l.add r12, r12, r8 /* r12 contains end pointer */ /* NOTE: Keep it so this function returns the end pointer in r12, so we can it use from other str* calls (strcat comes to mind */ l.sub r11, r12, r3 /* length = end - start */ l.jr lr /* return */ l.nop /* -- delay slot -- */ END(strlen) /*----------------------------------------------------------------------*/
