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/* strlen -- Compute length of NUL terminated string.
Highly optimized version for ix86, x>=5.
Copyright (C) 1995-2021 Free Software Foundation, Inc.
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
<https://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)
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