diff options
Diffstat (limited to 'sysdeps/i386/strlen.S')
| -rw-r--r-- | sysdeps/i386/strlen.S | 132 |
1 files changed, 132 insertions, 0 deletions
diff --git a/sysdeps/i386/strlen.S b/sysdeps/i386/strlen.S new file mode 100644 index 0000000000..fa0e0fe6e3 --- /dev/null +++ b/sysdeps/i386/strlen.S @@ -0,0 +1,132 @@ +/* strlen(str) -- determine the length of the string STR. + Optimized for Intel 80x86, x>=4. + Copyright (C) 1991-2015 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) |