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30891f35fa
We stopped adding "Contributed by" or similar lines in sources in 2012 in favour of git logs and keeping the Contributors section of the glibc manual up to date. Removing these lines makes the license header a bit more consistent across files and also removes the possibility of error in attribution when license blocks or files are copied across since the contributed-by lines don't actually reflect reality in those cases. Move all "Contributed by" and similar lines (Written by, Test by, etc.) into a new file CONTRIBUTED-BY to retain record of these contributions. These contributors are also mentioned in manual/contrib.texi, so we just maintain this additional record as a courtesy to the earlier developers. The following scripts were used to filter a list of files to edit in place and to clean up the CONTRIBUTED-BY file respectively. These were not added to the glibc sources because they're not expected to be of any use in future given that this is a one time task: https://gist.github.com/siddhesh/b5ecac94eabfd72ed2916d6d8157e7dc https://gist.github.com/siddhesh/15ea1f5e435ace9774f485030695ee02 Reviewed-by: Carlos O'Donell <carlos@redhat.com>
182 lines
5.4 KiB
ArmAsm
182 lines
5.4 KiB
ArmAsm
/* strlen -- Compute length of NUL terminated string.
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Highly optimized version for ix86, x>=5.
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Copyright (C) 1995-2021 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<https://www.gnu.org/licenses/>. */
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#include <sysdep.h>
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#include "asm-syntax.h"
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/* This version is especially optimized for the i586 (and following?)
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processors. This is mainly done by using the two pipelines. The
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version optimized for i486 is weak in this aspect because to get
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as much parallelism we have to execute some *more* instructions.
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The code below is structured to reflect the pairing of the instructions
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as *I think* it is. I have no processor data book to verify this.
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If you find something you think is incorrect let me know. */
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/* The magic value which is used throughout in the whole code. */
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#define magic 0xfefefeff
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#define PARMS 4 /* no space for saved regs */
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#define STR PARMS
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.text
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ENTRY (strlen)
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movl STR(%esp), %eax
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movl $3, %edx /* load mask (= 3) */
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andl %eax, %edx /* separate last two bits of address */
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jz L(1) /* aligned => start loop */
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jp L(0) /* exactly two bits set */
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cmpb %dh, (%eax) /* is byte NUL? */
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je L(2) /* yes => return */
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incl %eax /* increment pointer */
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cmpb %dh, (%eax) /* is byte NUL? */
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je L(2) /* yes => return */
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incl %eax /* increment pointer */
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xorl $2, %edx
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jz L(1)
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L(0): cmpb %dh, (%eax) /* is byte NUL? */
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je L(2) /* yes => return */
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incl %eax /* increment pointer */
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xorl %edx, %edx /* We need %edx == 0 for later */
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/* We exit the loop if adding MAGIC_BITS to LONGWORD fails to
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change any of the hole bits of LONGWORD.
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1) Is this safe? Will it catch all the zero bytes?
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Suppose there is a byte with all zeros. Any carry bits
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propagating from its left will fall into the hole at its
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least significant bit and stop. Since there will be no
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carry from its most significant bit, the LSB of the
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byte to the left will be unchanged, and the zero will be
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detected.
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2) Is this worthwhile? Will it ignore everything except
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zero bytes? Suppose every byte of LONGWORD has a bit set
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somewhere. There will be a carry into bit 8. If bit 8
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is set, this will carry into bit 16. If bit 8 is clear,
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one of bits 9-15 must be set, so there will be a carry
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into bit 16. Similarly, there will be a carry into bit
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24. If one of bits 24-31 is set, there will be a carry
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into bit 32 (=carry flag), so all of the hole bits will
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be changed.
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Note: %edx == 0 in any case here. */
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L(1):
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movl (%eax), %ecx /* get word (= 4 bytes) in question */
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addl $4, %eax /* adjust pointer for *next* word */
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subl %ecx, %edx /* first step to negate word */
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addl $magic, %ecx /* add magic word */
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decl %edx /* complete negation of word */
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jnc L(3) /* previous addl caused overflow? */
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xorl %ecx, %edx /* (word+magic)^word */
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andl $~magic, %edx /* any of the carry flags set? */
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jne L(3) /* yes => determine byte */
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movl (%eax), %ecx /* get word (= 4 bytes) in question */
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addl $4, %eax /* adjust pointer for *next* word */
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subl %ecx, %edx /* first step to negate word */
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addl $magic, %ecx /* add magic word */
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decl %edx /* complete negation of word */
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jnc L(3) /* previous addl caused overflow? */
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xorl %ecx, %edx /* (word+magic)^word */
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andl $~magic, %edx /* any of the carry flags set? */
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jne L(3) /* yes => determine byte */
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movl (%eax), %ecx /* get word (= 4 bytes) in question */
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addl $4, %eax /* adjust pointer for *next* word */
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subl %ecx, %edx /* first step to negate word */
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addl $magic, %ecx /* add magic word */
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decl %edx /* complete negation of word */
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jnc L(3) /* previous addl caused overflow? */
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xorl %ecx, %edx /* (word+magic)^word */
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andl $~magic, %edx /* any of the carry flags set? */
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jne L(3) /* yes => determine byte */
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movl (%eax), %ecx /* get word (= 4 bytes) in question */
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addl $4, %eax /* adjust pointer for *next* word */
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subl %ecx, %edx /* first step to negate word */
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addl $magic, %ecx /* add magic word */
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decl %edx /* complete negation of word */
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jnc L(3) /* previous addl caused overflow? */
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xorl %ecx, %edx /* (word+magic)^word */
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andl $~magic, %edx /* any of the carry flags set? */
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je L(1) /* no => start loop again */
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L(3): subl $4, %eax /* correct too early pointer increment */
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subl $magic, %ecx
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cmpb $0, %cl /* lowest byte NUL? */
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jz L(2) /* yes => return */
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inc %eax /* increment pointer */
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testb %ch, %ch /* second byte NUL? */
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jz L(2) /* yes => return */
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shrl $16, %ecx /* make upper bytes accessible */
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incl %eax /* increment pointer */
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cmpb $0, %cl /* is third byte NUL? */
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jz L(2) /* yes => return */
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incl %eax /* increment pointer */
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L(2): subl STR(%esp), %eax /* now compute the length as difference
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between start and terminating NUL
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character */
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ret
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END (strlen)
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libc_hidden_builtin_def (strlen)
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