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348 lines
8.4 KiB
ArmAsm
348 lines
8.4 KiB
ArmAsm
/* Find character CH in a NUL terminated string.
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Highly optimized version for ix85, x>=5.
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Copyright (C) 1995-2024 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+16 /* space for 4 saved regs */
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#define RTN PARMS
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#define STR RTN
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#define CHR STR+4
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.text
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ENTRY (strchr)
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pushl %edi /* Save callee-safe registers. */
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cfi_adjust_cfa_offset (-4)
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pushl %esi
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cfi_adjust_cfa_offset (-4)
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pushl %ebx
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cfi_adjust_cfa_offset (-4)
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pushl %ebp
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cfi_adjust_cfa_offset (-4)
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movl STR(%esp), %eax
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movl CHR(%esp), %edx
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movl %eax, %edi /* duplicate string pointer for later */
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cfi_rel_offset (edi, 12)
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xorl %ecx, %ecx /* clear %ecx */
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/* At the moment %edx contains C. What we need for the
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algorithm is C in all bytes of the dword. Avoid
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operations on 16 bit words because these require an
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prefix byte (and one more cycle). */
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movb %dl, %dh /* now it is 0|0|c|c */
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movb %dl, %cl /* we construct the lower half in %ecx */
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shll $16, %edx /* now %edx is c|c|0|0 */
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movb %cl, %ch /* now %ecx is 0|0|c|c */
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orl %ecx, %edx /* and finally c|c|c|c */
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andl $3, %edi /* mask alignment bits */
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jz L(11) /* alignment is 0 => start loop */
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movb %dl, %cl /* 0 is needed below */
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jp L(0) /* exactly two bits set */
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xorb (%eax), %cl /* is byte the one we are looking for? */
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jz L(out) /* yes => return pointer */
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xorb %dl, %cl /* load single byte and test for NUL */
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je L(3) /* yes => return NULL */
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movb 1(%eax), %cl /* load single byte */
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incl %eax
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cmpb %cl, %dl /* is byte == C? */
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je L(out) /* aligned => return pointer */
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cmpb $0, %cl /* is byte NUL? */
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je L(3) /* yes => return NULL */
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incl %eax
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decl %edi
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jne L(11)
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L(0): movb (%eax), %cl /* load single byte */
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cmpb %cl, %dl /* is byte == C? */
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je L(out) /* aligned => return pointer */
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cmpb $0, %cl /* is byte NUL? */
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je L(3) /* yes => return NULL */
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incl %eax /* increment pointer */
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cfi_rel_offset (esi, 8)
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cfi_rel_offset (ebx, 4)
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cfi_rel_offset (ebp, 0)
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/* The following code is the preparation for the loop. The
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four instruction up to `L1' will not be executed in the loop
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because the same code is found at the end of the loop, but
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there it is executed in parallel with other instructions. */
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L(11): movl (%eax), %ecx
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movl $magic, %ebp
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movl $magic, %edi
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addl %ecx, %ebp
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/* The main loop: it looks complex and indeed it is. I would
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love to say `it was hard to write, so it should he hard to
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read' but I will give some more hints. To fully understand
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this code you should first take a look at the i486 version.
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The basic algorithm is the same, but here the code organized
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in a way which permits to use both pipelines all the time.
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I tried to make it a bit more understandable by indenting
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the code according to stage in the algorithm. It goes as
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follows:
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check for 0 in 1st word
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check for C in 1st word
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check for 0 in 2nd word
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check for C in 2nd word
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check for 0 in 3rd word
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check for C in 3rd word
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check for 0 in 4th word
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check for C in 4th word
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Please note that doing the test for NUL before the test for
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C allows us to overlap the test for 0 in the next word with
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the test for C. */
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L(1): xorl %ecx, %ebp /* (word^magic) */
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addl %ecx, %edi /* add magic word */
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leal 4(%eax), %eax /* increment pointer */
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jnc L(4) /* previous addl caused overflow? */
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movl %ecx, %ebx /* duplicate original word */
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orl $magic, %ebp /* (word^magic)|magic */
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addl $1, %ebp /* (word^magic)|magic == 0xffffffff? */
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jne L(4) /* yes => we found word with NUL */
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movl $magic, %esi /* load magic value */
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xorl %edx, %ebx /* clear words which are C */
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movl (%eax), %ecx
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addl %ebx, %esi /* (word+magic) */
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movl $magic, %edi
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jnc L(5) /* previous addl caused overflow? */
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movl %edi, %ebp
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xorl %ebx, %esi /* (word+magic)^word */
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addl %ecx, %ebp
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orl $magic, %esi /* ((word+magic)^word)|magic */
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addl $1, %esi /* ((word+magic)^word)|magic==0xf..f?*/
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jne L(5) /* yes => we found word with C */
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xorl %ecx, %ebp
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addl %ecx, %edi
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leal 4(%eax), %eax
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jnc L(4)
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movl %ecx, %ebx
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orl $magic, %ebp
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addl $1, %ebp
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jne L(4)
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movl $magic, %esi
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xorl %edx, %ebx
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movl (%eax), %ecx
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addl %ebx, %esi
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movl $magic, %edi
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jnc L(5)
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movl %edi, %ebp
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xorl %ebx, %esi
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addl %ecx, %ebp
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orl $magic, %esi
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addl $1, %esi
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jne L(5)
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xorl %ecx, %ebp
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addl %ecx, %edi
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leal 4(%eax), %eax
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jnc L(4)
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movl %ecx, %ebx
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orl $magic, %ebp
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addl $1, %ebp
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jne L(4)
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movl $magic, %esi
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xorl %edx, %ebx
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movl (%eax), %ecx
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addl %ebx, %esi
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movl $magic, %edi
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jnc L(5)
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movl %edi, %ebp
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xorl %ebx, %esi
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addl %ecx, %ebp
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orl $magic, %esi
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addl $1, %esi
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jne L(5)
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xorl %ecx, %ebp
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addl %ecx, %edi
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leal 4(%eax), %eax
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jnc L(4)
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movl %ecx, %ebx
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orl $magic, %ebp
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addl $1, %ebp
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jne L(4)
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movl $magic, %esi
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xorl %edx, %ebx
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movl (%eax), %ecx
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addl %ebx, %esi
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movl $magic, %edi
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jnc L(5)
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movl %edi, %ebp
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xorl %ebx, %esi
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addl %ecx, %ebp
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orl $magic, %esi
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addl $1, %esi
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je L(1)
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/* We know there is no NUL byte but a C byte in the word.
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%ebx contains NUL in this particular byte. */
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L(5): subl $4, %eax /* adjust pointer */
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testb %bl, %bl /* first byte == C? */
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jz L(out) /* yes => return pointer */
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incl %eax /* increment pointer */
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testb %bh, %bh /* second byte == C? */
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jz L(out) /* yes => return pointer */
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shrl $16, %ebx /* make upper bytes accessible */
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incl %eax /* increment pointer */
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cmp $0, %bl /* third byte == C */
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je L(out) /* yes => return pointer */
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incl %eax /* increment pointer */
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L(out): popl %ebp /* restore saved registers */
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cfi_adjust_cfa_offset (-4)
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cfi_restore (ebp)
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popl %ebx
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cfi_adjust_cfa_offset (-4)
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cfi_restore (ebx)
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popl %esi
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cfi_adjust_cfa_offset (-4)
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cfi_restore (esi)
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popl %edi
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cfi_adjust_cfa_offset (-4)
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cfi_restore (edi)
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ret
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cfi_adjust_cfa_offset (16)
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cfi_rel_offset (edi, 12)
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cfi_rel_offset (esi, 8)
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cfi_rel_offset (ebx, 4)
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cfi_rel_offset (ebp, 0)
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/* We know there is a NUL byte in the word. But we have to test
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whether there is an C byte before it in the word. */
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L(4): subl $4, %eax /* adjust pointer */
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cmpb %dl, %cl /* first byte == C? */
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je L(out) /* yes => return pointer */
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cmpb $0, %cl /* first byte == NUL? */
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je L(3) /* yes => return NULL */
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incl %eax /* increment pointer */
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cmpb %dl, %ch /* second byte == C? */
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je L(out) /* yes => return pointer */
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cmpb $0, %ch /* second byte == NUL? */
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je L(3) /* yes => return NULL */
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shrl $16, %ecx /* make upper bytes accessible */
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incl %eax /* increment pointer */
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cmpb %dl, %cl /* third byte == C? */
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je L(out) /* yes => return pointer */
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cmpb $0, %cl /* third byte == NUL? */
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je L(3) /* yes => return NULL */
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incl %eax /* increment pointer */
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/* The test four the fourth byte is necessary! */
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cmpb %dl, %ch /* fourth byte == C? */
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je L(out) /* yes => return pointer */
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L(3): xorl %eax, %eax
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jmp L(out)
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END (strchr)
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#undef index
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weak_alias (strchr, index)
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libc_hidden_builtin_def (strchr)
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