| /* strlen -- Compute length of NUL terminated string. |
| Highly optimized version for ix86, x>=5. |
| Copyright (C) 1995-2014 Free Software Foundation, Inc. |
| This file is part of the GNU C Library. |
| Contributed by Ulrich Drepper, <drepper@gnu.ai.mit.edu>. |
| |
| 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" |
| |
| /* This version is especially optimized for the i586 (and following?) |
| processors. This is mainly done by using the two pipelines. The |
| version optimized for i486 is weak in this aspect because to get |
| as much parallelism we have to execute some *more* instructions. |
| |
| The code below is structured to reflect the pairing of the instructions |
| as *I think* it is. I have no processor data book to verify this. |
| If you find something you think is incorrect let me know. */ |
| |
| |
| /* The magic value which is used throughout in the whole code. */ |
| #define magic 0xfefefeff |
| |
| #define PARMS 4 /* no space for saved regs */ |
| #define STR PARMS |
| |
| .text |
| ENTRY (strlen) |
| |
| movl STR(%esp), %eax |
| movl $3, %edx /* load mask (= 3) */ |
| |
| andl %eax, %edx /* separate last two bits of address */ |
| |
| jz L(1) /* aligned => start loop */ |
| jp L(0) /* exactly two bits set */ |
| |
| cmpb %dh, (%eax) /* is byte NUL? */ |
| je L(2) /* yes => return */ |
| |
| incl %eax /* increment pointer */ |
| cmpb %dh, (%eax) /* is byte NUL? */ |
| |
| je L(2) /* yes => return */ |
| |
| incl %eax /* increment pointer */ |
| xorl $2, %edx |
| |
| jz L(1) |
| |
| L(0): cmpb %dh, (%eax) /* is byte NUL? */ |
| je L(2) /* yes => return */ |
| |
| incl %eax /* increment pointer */ |
| xorl %edx, %edx /* We need %edx == 0 for later */ |
| |
| /* We exit the loop if adding MAGIC_BITS to LONGWORD fails to |
| change any of the hole bits of LONGWORD. |
| |
| 1) Is this safe? Will it catch all the zero bytes? |
| Suppose there is a byte with all zeros. Any carry bits |
| propagating from its left will fall into the hole at its |
| least significant bit and stop. Since there will be no |
| carry from its most significant bit, the LSB of the |
| byte to the left will be unchanged, and the zero will be |
| detected. |
| |
| 2) Is this worthwhile? Will it ignore everything except |
| zero bytes? Suppose every byte of LONGWORD has a bit set |
| somewhere. There will be a carry into bit 8. If bit 8 |
| is set, this will carry into bit 16. If bit 8 is clear, |
| one of bits 9-15 must be set, so there will be a carry |
| into bit 16. Similarly, there will be a carry into bit |
| 24. If one of bits 24-31 is set, there will be a carry |
| into bit 32 (=carry flag), so all of the hole bits will |
| be changed. |
| |
| Note: %edx == 0 in any case here. */ |
| |
| L(1): |
| movl (%eax), %ecx /* get word (= 4 bytes) in question */ |
| addl $4, %eax /* adjust pointer for *next* word */ |
| |
| subl %ecx, %edx /* first step to negate word */ |
| addl $magic, %ecx /* add magic word */ |
| |
| decl %edx /* complete negation of word */ |
| jnc L(3) /* previous addl caused overflow? */ |
| |
| xorl %ecx, %edx /* (word+magic)^word */ |
| |
| andl $~magic, %edx /* any of the carry flags set? */ |
| |
| jne L(3) /* yes => determine byte */ |
| |
| |
| movl (%eax), %ecx /* get word (= 4 bytes) in question */ |
| addl $4, %eax /* adjust pointer for *next* word */ |
| |
| subl %ecx, %edx /* first step to negate word */ |
| addl $magic, %ecx /* add magic word */ |
| |
| decl %edx /* complete negation of word */ |
| jnc L(3) /* previous addl caused overflow? */ |
| |
| xorl %ecx, %edx /* (word+magic)^word */ |
| |
| andl $~magic, %edx /* any of the carry flags set? */ |
| |
| jne L(3) /* yes => determine byte */ |
| |
| |
| movl (%eax), %ecx /* get word (= 4 bytes) in question */ |
| addl $4, %eax /* adjust pointer for *next* word */ |
| |
| subl %ecx, %edx /* first step to negate word */ |
| addl $magic, %ecx /* add magic word */ |
| |
| decl %edx /* complete negation of word */ |
| jnc L(3) /* previous addl caused overflow? */ |
| |
| xorl %ecx, %edx /* (word+magic)^word */ |
| |
| andl $~magic, %edx /* any of the carry flags set? */ |
| |
| jne L(3) /* yes => determine byte */ |
| |
| |
| movl (%eax), %ecx /* get word (= 4 bytes) in question */ |
| addl $4, %eax /* adjust pointer for *next* word */ |
| |
| subl %ecx, %edx /* first step to negate word */ |
| addl $magic, %ecx /* add magic word */ |
| |
| decl %edx /* complete negation of word */ |
| jnc L(3) /* previous addl caused overflow? */ |
| |
| xorl %ecx, %edx /* (word+magic)^word */ |
| |
| andl $~magic, %edx /* any of the carry flags set? */ |
| |
| je L(1) /* no => start loop again */ |
| |
| |
| L(3): subl $4, %eax /* correct too early pointer increment */ |
| subl $magic, %ecx |
| |
| cmpb $0, %cl /* lowest byte NUL? */ |
| jz L(2) /* yes => return */ |
| |
| inc %eax /* increment pointer */ |
| testb %ch, %ch /* second byte NUL? */ |
| |
| jz L(2) /* yes => return */ |
| |
| shrl $16, %ecx /* make upper bytes accessible */ |
| incl %eax /* increment pointer */ |
| |
| cmpb $0, %cl /* is third byte NUL? */ |
| jz L(2) /* yes => return */ |
| |
| incl %eax /* increment pointer */ |
| |
| L(2): subl STR(%esp), %eax /* now compute the length as difference |
| between start and terminating NUL |
| character */ |
| ret |
| END (strlen) |
| libc_hidden_builtin_def (strlen) |