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189 lines
5.7 KiB
ArmAsm
189 lines
5.7 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) 1995,1996,1997,2000,2002,2003 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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Contributed by Ulrich Drepper, <drepper@gnu.ai.mit.edu>.
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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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<http://www.gnu.org/licenses/>. */
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#include <sysdep.h>
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#include "asm-syntax.h"
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#include "bp-sym.h"
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#include "bp-asm.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 LINKAGE /* no space for saved regs */
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#define STR PARMS
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.text
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ENTRY (BP_SYM (strlen))
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ENTER
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movl STR(%esp), %eax
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CHECK_BOUNDS_LOW (%eax, STR(%esp))
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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): CHECK_BOUNDS_HIGH (%eax, STR(%esp), jb)
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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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LEAVE
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ret
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END (BP_SYM (strlen))
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libc_hidden_builtin_def (strlen)
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