/* Find character CH in a NUL terminated string. Highly optimized version for ix85, x>=5. Copyright (C) 1995, 1996, 1997 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> #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 /* INPUT PARAMETERS: str (sp + 4) ch (sp + 8) */ .text ENTRY (strchr) pushl %edi /* Save callee-safe registers. */ pushl %esi pushl %ebx pushl %ebp movl 20(%esp), %eax /* get string pointer */ movl 24(%esp), %edx /* get character we are looking for */ movl %eax, %edi /* duplicate string pointer for later */ xorl %ecx, %ecx /* clear %ecx */ /* At the moment %edx contains C. What we need for the algorithm is C in all bytes of the dword. Avoid operations on 16 bit words because these require an prefix byte (and one more cycle). */ movb %dl, %dh /* now it is 0|0|c|c */ movb %dl, %cl /* we construct the lower half in %ecx */ shll $16, %edx /* now %edx is c|c|0|0 */ movb %cl, %ch /* now %ecx is 0|0|c|c */ orl %ecx, %edx /* and finally c|c|c|c */ andl $3, %edi /* mask alignment bits */ jz L(11) /* alignment is 0 => start loop */ movb %dl, %cl /* 0 is needed below */ jp L(0) /* exactly two bits set */ xorb (%eax), %cl /* is byte the one we are looking for? */ jz L(2) /* yes => return pointer */ xorb %dl, %cl /* load single byte and test for NUL */ je L(3) /* yes => return NULL */ movb 1(%eax), %cl /* load single byte */ incl %eax cmpb %cl, %dl /* is byte == C? */ je L(2) /* aligned => return pointer */ cmpb $0, %cl /* is byte NUL? */ je L(3) /* yes => return NULL */ incl %eax decl %edi jne L(11) L(0): movb (%eax), %cl /* load single byte */ cmpb %cl, %dl /* is byte == C? */ je L(2) /* aligned => return pointer */ cmpb $0, %cl /* is byte NUL? */ je L(3) /* yes => return NULL */ incl %eax /* increment pointer */ /* The following code is the preparation for the loop. The four instruction up to `L1' will not be executed in the loop because the same code is found at the end of the loop, but there it is executed in parallel with other instructions. */ L(11): movl (%eax), %ecx movl $magic, %ebp movl $magic, %edi addl %ecx, %ebp /* The main loop: it looks complex and indeed it is. I would love to say `it was hard to write, so it should he hard to read' but I will give some more hints. To fully understand this code you should first take a look at the i486 version. The basic algorithm is the same, but here the code organized in a way which permits to use both pipelines all the time. I tried to make it a bit more understandable by indenting the code according to stage in the algorithm. It goes as follows: check for 0 in 1st word check for C in 1st word check for 0 in 2nd word check for C in 2nd word check for 0 in 3rd word check for C in 3rd word check for 0 in 4th word check for C in 4th word Please note that doing the test for NUL before the test for C allows us to overlap the test for 0 in the next word with the test for C. */ L(1): xorl %ecx, %ebp /* (word^magic) */ addl %ecx, %edi /* add magic word */ leal 4(%eax), %eax /* increment pointer */ jnc L(4) /* previous addl caused overflow? */ movl %ecx, %ebx /* duplicate original word */ orl $magic, %ebp /* (word^magic)|magic */ addl $1, %ebp /* (word^magic)|magic == 0xffffffff? */ jne L(4) /* yes => we found word with NUL */ movl $magic, %esi /* load magic value */ xorl %edx, %ebx /* clear words which are C */ movl (%eax), %ecx addl %ebx, %esi /* (word+magic) */ movl $magic, %edi jnc L(5) /* previous addl caused overflow? */ movl %edi, %ebp xorl %ebx, %esi /* (word+magic)^word */ addl %ecx, %ebp orl $magic, %esi /* ((word+magic)^word)|magic */ addl $1, %esi /* ((word+magic)^word)|magic==0xf..f?*/ jne L(5) /* yes => we found word with C */ xorl %ecx, %ebp addl %ecx, %edi leal 4(%eax), %eax jnc L(4) movl %ecx, %ebx orl $magic, %ebp addl $1, %ebp jne L(4) movl $magic, %esi xorl %edx, %ebx movl (%eax), %ecx addl %ebx, %esi movl $magic, %edi jnc L(5) movl %edi, %ebp xorl %ebx, %esi addl %ecx, %ebp orl $magic, %esi addl $1, %esi jne L(5) xorl %ecx, %ebp addl %ecx, %edi leal 4(%eax), %eax jnc L(4) movl %ecx, %ebx orl $magic, %ebp addl $1, %ebp jne L(4) movl $magic, %esi xorl %edx, %ebx movl (%eax), %ecx addl %ebx, %esi movl $magic, %edi jnc L(5) movl %edi, %ebp xorl %ebx, %esi addl %ecx, %ebp orl $magic, %esi addl $1, %esi jne L(5) xorl %ecx, %ebp addl %ecx, %edi leal 4(%eax), %eax jnc L(4) movl %ecx, %ebx orl $magic, %ebp addl $1, %ebp jne L(4) movl $magic, %esi xorl %edx, %ebx movl (%eax), %ecx addl %ebx, %esi movl $magic, %edi jnc L(5) movl %edi, %ebp xorl %ebx, %esi addl %ecx, %ebp orl $magic, %esi addl $1, %esi je L(1) /* We know there is no NUL byte but a C byte in the word. %ebx contains NUL in this particular byte. */ L(5): subl $4, %eax /* adjust pointer */ testb %bl, %bl /* first byte == C? */ jz L(2) /* yes => return pointer */ incl %eax /* increment pointer */ testb %bh, %bh /* second byte == C? */ jz L(2) /* yes => return pointer */ shrl $16, %ebx /* make upper bytes accessible */ incl %eax /* increment pointer */ cmp $0, %bl /* third byte == C */ je L(2) /* yes => return pointer */ incl %eax /* increment pointer */ L(2): popl %ebp /* restore saved registers */ popl %ebx popl %esi popl %edi ret /* We know there is a NUL byte in the word. But we have to test whether there is an C byte before it in the word. */ L(4): subl $4, %eax /* adjust pointer */ cmpb %dl, %cl /* first byte == C? */ je L(2) /* yes => return pointer */ cmpb $0, %cl /* first byte == NUL? */ je L(3) /* yes => return NULL */ incl %eax /* increment pointer */ cmpb %dl, %ch /* second byte == C? */ je L(2) /* yes => return pointer */ cmpb $0, %ch /* second byte == NUL? */ je L(3) /* yes => return NULL */ shrl $16, %ecx /* make upper bytes accessible */ incl %eax /* increment pointer */ cmpb %dl, %cl /* third byte == C? */ je L(2) /* yes => return pointer */ cmpb $0, %cl /* third byte == NUL? */ je L(3) /* yes => return NULL */ incl %eax /* increment pointer */ /* The test four the fourth byte is necessary! */ cmpb %dl, %ch /* fourth byte == C? */ je L(2) /* yes => return pointer */ L(3): xorl %eax, %eax /* set return value = NULL */ popl %ebp /* restore saved registers */ popl %ebx popl %esi popl %edi ret END (strchr) #undef index weak_alias (strchr, index)