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Diffstat (limited to 'sysdeps/i386/i586/strlen.S')
| -rw-r--r-- | sysdeps/i386/i586/strlen.S | 185 |
1 files changed, 185 insertions, 0 deletions
diff --git a/sysdeps/i386/i586/strlen.S b/sysdeps/i386/i586/strlen.S new file mode 100644 index 0000000000..b807ed4b4f --- /dev/null +++ b/sysdeps/i386/i586/strlen.S @@ -0,0 +1,185 @@ +/* strlen -- Compute length og NUL terminated string. +Highly optimized version for ix86, x>=5. +Copyright (C) 1995 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 Library General Public License as +published by the Free Software Foundation; either version 2 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 +Library General Public License for more details. + +You should have received a copy of the GNU Library General Public +License along with the GNU C Library; see the file COPYING.LIB. If +not, write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330, +Boston, MA 02111-1307, USA. */ + +#include <sysdep.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 executs 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 + +/* + INPUT PARAMETERS: + str (sp + 4) +*/ + + .text +ENTRY(strlen) + movl 4(%esp), %eax /* get string pointer */ + + movl %eax, %ecx /* duplicate it */ + andl $3, %ecx /* mask alignment bits */ + + jz L11 /* aligned => start loop */ + + cmpb %ch, (%eax) /* is byte NUL? */ + je L2 /* yes => return */ + + incl %eax /* increment pointer */ + cmpl $3, %ecx /* was alignment = 3? */ + + je L11 /* yes => now it is aligned and start loop */ + + cmpb %ch, (%eax) /* is byte NUL? */ + je L2 /* yes => return */ + + incl %eax /* increment pointer */ + cmpl $2, %ecx /* was alignment = 2? */ + + je L11 /* yes => now it is aligned and start loop */ + + cmpb %ch, (%eax) /* is byte NUL? */ + je L2 /* yes => return */ + + incl %eax /* increment pointer */ + + /* 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. */ +L11: xorl %edx, %edx /* We need %edx == 0 for later */ + +L1: + 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 L3 /* previous addl caused overflow? */ + + xorl %ecx, %edx /* (word+magic)^word */ + subl $magic, %ecx /* undo previous addl to restore word */ + + andl $~magic, %edx /* any of the carry flags set? */ + + jne L3 /* 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 L3 /* previous addl caused overflow? */ + + xorl %ecx, %edx /* (word+magic)^word */ + subl $magic, %ecx /* undo previous addl to restore word */ + + andl $~magic, %edx /* any of the carry flags set? */ + + jne L3 /* 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 L3 /* previous addl caused overflow? */ + + xorl %ecx, %edx /* (word+magic)^word */ + subl $magic, %ecx /* undo previous addl to restore word */ + + andl $~magic, %edx /* any of the carry flags set? */ + + jne L3 /* 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 /* wcomplete negation of ord */ + jnc L3 /* previous addl caused overflow? */ + + xorl %ecx, %edx /* (word+magic)^word */ + subl $magic, %ecx /* undo previous addl to restore word */ + + andl $~magic, %edx /* any of the carry flags set? */ + + je L1 /* no => start loop again */ + + +L3: subl $4, %eax /* correct too early pointer increment */ + testb %cl, %cl /* lowest byte NUL? */ + + jz L2 /* yes => return */ + + inc %eax /* increment pointer */ + testb %ch, %ch /* second byte NUL? */ + + jz L2 /* yes => return */ + + shrl $16, %ecx /* make upper bytes accessible */ + incl %eax /* increment pointer */ + + cmpb $0, %cl /* is third byte NUL? */ + jz L2 /* yes => return */ + + incl %eax /* increment pointer */ + +L2: subl 4(%esp), %eax /* now compute the length as difference + between start and terminating NUL + character */ + + ret |