| /* strlen(str) -- determine the length of the string STR. |
| Optimized for Intel 80x86, x>=4. |
| Copyright (C) 1991-2016 Free Software Foundation, Inc. |
| Contributed by Ulrich Drepper <drepper@gnu.ai.mit.edu>. |
| This file is part of the GNU C Library. |
| |
| The GNU C Library is free software; you can redistribute it and/or |
| modify it under the terms of the GNU Lesser General Public |
| License as published by the Free Software Foundation; either |
| version 2.1 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 |
| Lesser General Public License for more details. |
| |
| You should have received a copy of the GNU Lesser General Public |
| License along with the GNU C Library; if not, see |
| <http://www.gnu.org/licenses/>. */ |
| |
| #include <sysdep.h> |
| #include "asm-syntax.h" |
| |
| #define PARMS 4 /* no space for saved regs */ |
| #define STR PARMS |
| |
| .text |
| ENTRY (strlen) |
| |
| movl STR(%esp), %ecx |
| movl %ecx, %eax /* duplicate it */ |
| |
| andl $3, %ecx /* mask alignment bits */ |
| jz L(1) /* aligned => start loop */ |
| cmpb %ch, (%eax) /* is byte NUL? */ |
| je L(2) /* yes => return */ |
| incl %eax /* increment pointer */ |
| |
| xorl $3, %ecx /* was alignment = 3? */ |
| jz L(1) /* yes => now it is aligned and start loop */ |
| cmpb %ch, (%eax) /* is byte NUL? */ |
| je L(2) /* yes => return */ |
| addl $1, %eax /* increment pointer */ |
| |
| subl $1, %ecx /* was alignment = 2? */ |
| jz L(1) /* yes => now it is aligned and start loop */ |
| cmpb %ch, (%eax) /* is byte NUL? */ |
| je L(2) /* yes => return */ |
| |
| /* Don't change the above `addl $1,%eax' and `subl $1, %ecx' into `incl %eax' |
| and `decl %ecx' resp. The additional two byte per instruction make the |
| label 4 to be aligned on a 16 byte boundary with nops. |
| |
| The following `sub $15, %eax' is part of this trick, too. Together with |
| the next instruction (`addl $16, %eax') it is in fact a `incl %eax', just |
| as expected from the algorithm. But doing so has the advantage that |
| no jump to label 1 is necessary and so the pipeline is not flushed. */ |
| |
| subl $15, %eax /* effectively +1 */ |
| |
| |
| L(4): addl $16, %eax /* adjust pointer for full loop */ |
| |
| L(1): movl (%eax), %ecx /* get word (= 4 bytes) in question */ |
| movl $0xfefefeff, %edx /* magic value */ |
| addl %ecx, %edx /* add the magic value to the word. We get |
| carry bits reported for each byte which |
| is *not* 0 */ |
| jnc L(3) /* highest byte is NUL => return pointer */ |
| xorl %ecx, %edx /* (word+magic)^word */ |
| orl $0xfefefeff, %edx /* set all non-carry bits */ |
| incl %edx /* add 1: if one carry bit was *not* set |
| the addition will not result in 0. */ |
| jnz L(3) /* found NUL => return pointer */ |
| |
| movl 4(%eax), %ecx /* get word (= 4 bytes) in question */ |
| movl $0xfefefeff, %edx /* magic value */ |
| addl %ecx, %edx /* add the magic value to the word. We get |
| carry bits reported for each byte which |
| is *not* 0 */ |
| jnc L(5) /* highest byte is NUL => return pointer */ |
| xorl %ecx, %edx /* (word+magic)^word */ |
| orl $0xfefefeff, %edx /* set all non-carry bits */ |
| incl %edx /* add 1: if one carry bit was *not* set |
| the addition will not result in 0. */ |
| jnz L(5) /* found NUL => return pointer */ |
| |
| movl 8(%eax), %ecx /* get word (= 4 bytes) in question */ |
| movl $0xfefefeff, %edx /* magic value */ |
| addl %ecx, %edx /* add the magic value to the word. We get |
| carry bits reported for each byte which |
| is *not* 0 */ |
| jnc L(6) /* highest byte is NUL => return pointer */ |
| xorl %ecx, %edx /* (word+magic)^word */ |
| orl $0xfefefeff, %edx /* set all non-carry bits */ |
| incl %edx /* add 1: if one carry bit was *not* set |
| the addition will not result in 0. */ |
| jnz L(6) /* found NUL => return pointer */ |
| |
| movl 12(%eax), %ecx /* get word (= 4 bytes) in question */ |
| movl $0xfefefeff, %edx /* magic value */ |
| addl %ecx, %edx /* add the magic value to the word. We get |
| carry bits reported for each byte which |
| is *not* 0 */ |
| jnc L(7) /* highest byte is NUL => return pointer */ |
| xorl %ecx, %edx /* (word+magic)^word */ |
| orl $0xfefefeff, %edx /* set all non-carry bits */ |
| incl %edx /* add 1: if one carry bit was *not* set |
| the addition will not result in 0. */ |
| jz L(4) /* no NUL found => continue loop */ |
| |
| L(7): addl $4, %eax /* adjust pointer */ |
| L(6): addl $4, %eax |
| L(5): addl $4, %eax |
| |
| L(3): testb %cl, %cl /* is first byte NUL? */ |
| jz L(2) /* yes => return */ |
| incl %eax /* increment pointer */ |
| |
| testb %ch, %ch /* is second byte NUL? */ |
| jz L(2) /* yes => return */ |
| incl %eax /* increment pointer */ |
| |
| testl $0xff0000, %ecx /* is third byte NUL? */ |
| jz L(2) /* yes => return pointer */ |
| incl %eax /* increment pointer */ |
| |
| L(2): subl STR(%esp), %eax /* compute difference to string start */ |
| |
| ret |
| END (strlen) |
| libc_hidden_builtin_def (strlen) |