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Diffstat (limited to 'sysdeps/i386/rawmemchr.S')
| -rw-r--r-- | sysdeps/i386/rawmemchr.S | 218 |
1 files changed, 218 insertions, 0 deletions
diff --git a/sysdeps/i386/rawmemchr.S b/sysdeps/i386/rawmemchr.S new file mode 100644 index 0000000000..83626e0473 --- /dev/null +++ b/sysdeps/i386/rawmemchr.S @@ -0,0 +1,218 @@ +/* rawmemchr (str, ch) -- Return pointer to first occurrence of CH in STR. + For Intel 80x86, x>=3. + Copyright (C) 1994, 95, 96, 97, 98, 99 Free Software Foundation, Inc. + This file is part of the GNU C Library. + Contributed by Ulrich Drepper <drepper@gnu.ai.mit.edu> + Optimised a little by Alan Modra <Alan@SPRI.Levels.UniSA.Edu.Au> + + This version is developed using the same algorithm as the fast C + version which carries the following introduction: + + Based on strlen implementation by Torbjorn Granlund (tege@sics.se), + with help from Dan Sahlin (dan@sics.se) and + commentary by Jim Blandy (jimb@ai.mit.edu); + adaptation to memchr suggested by Dick Karpinski (dick@cca.ucsf.edu), + and implemented by Roland McGrath (roland@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" + +/* + INPUT PARAMETERS: + str (sp + 4) + c (sp + 8) +*/ + + .text +ENTRY (__rawmemchr) + /* Save callee-safe register used in this function. */ + pushl %edi + + /* Load parameters into registers. */ + movl 8(%esp), %eax /* str: pointer to memory block. */ + movl 12(%esp), %edx /* c: byte we are looking for. */ + + /* 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 */ + movl %edx, %ecx + shll $16, %edx /* Now c|c|0|0 */ + movw %cx, %dx /* And finally c|c|c|c */ + + /* Better performance can be achieved if the word (32 + bit) memory access is aligned on a four-byte-boundary. + So process first bytes one by one until boundary is + reached. Don't use a loop for better performance. */ + + testb $3, %al /* correctly aligned ? */ + je L(1) /* yes => begin loop */ + cmpb %dl, (%eax) /* compare byte */ + je L(9) /* target found => return */ + incl %eax /* increment source pointer */ + + testb $3, %al /* correctly aligned ? */ + je L(1) /* yes => begin loop */ + cmpb %dl, (%eax) /* compare byte */ + je L(9) /* target found => return */ + incl %eax /* increment source pointer */ + + testb $3, %al /* correctly aligned ? */ + je L(1) /* yes => begin loop */ + cmpb %dl, (%eax) /* compare byte */ + je L(9) /* target found => return */ + incl %eax /* increment source pointer */ + + /* 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. + + 3) But wait! Aren't we looking for C, not zero? + Good point. So what we do is XOR LONGWORD with a longword, + each of whose bytes is C. This turns each byte that is C + into a zero. */ + + + /* Each round the main loop processes 16 bytes. */ + ALIGN (4) + +L(1): movl (%eax), %ecx /* get word (= 4 bytes) in question */ + movl $0xfefefeff, %edi /* magic value */ + xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c + are now 0 */ + addl %ecx, %edi /* add the magic value to the word. We get + carry bits reported for each byte which + is *not* 0 */ + + /* According to the algorithm we had to reverse the effect of the + XOR first and then test the overflow bits. But because the + following XOR would destroy the carry flag and it would (in a + representation with more than 32 bits) not alter then last + overflow, we can now test this condition. If no carry is signaled + no overflow must have occurred in the last byte => it was 0. */ + jnc L(8) + + /* We are only interested in carry bits that change due to the + previous add, so remove original bits */ + xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */ + + /* Now test for the other three overflow bits. */ + orl $0xfefefeff, %edi /* set all non-carry bits */ + incl %edi /* add 1: if one carry bit was *not* set + the addition will not result in 0. */ + + /* If at least one byte of the word is C we don't get 0 in %edi. */ + jnz L(8) /* found it => return pointer */ + + /* This process is unfolded four times for better performance. + we don't increment the source pointer each time. Instead we + use offsets and increment by 16 in each run of the loop. But + before probing for the matching byte we need some extra code + (following LL(13) below). Even the len can be compared with + constants instead of decrementing each time. */ + + movl 4(%eax), %ecx /* get word (= 4 bytes) in question */ + movl $0xfefefeff, %edi /* magic value */ + xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c + are now 0 */ + addl %ecx, %edi /* add the magic value to the word. We get + carry bits reported for each byte which + is *not* 0 */ + jnc L(7) /* highest byte is C => return pointer */ + xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */ + orl $0xfefefeff, %edi /* set all non-carry bits */ + incl %edi /* add 1: if one carry bit was *not* set + the addition will not result in 0. */ + jnz L(7) /* found it => return pointer */ + + movl 8(%eax), %ecx /* get word (= 4 bytes) in question */ + movl $0xfefefeff, %edi /* magic value */ + xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c + are now 0 */ + addl %ecx, %edi /* add the magic value to the word. We get + carry bits reported for each byte which + is *not* 0 */ + jnc L(6) /* highest byte is C => return pointer */ + xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */ + orl $0xfefefeff, %edi /* set all non-carry bits */ + incl %edi /* add 1: if one carry bit was *not* set + the addition will not result in 0. */ + jnz L(6) /* found it => return pointer */ + + movl 12(%eax), %ecx /* get word (= 4 bytes) in question */ + movl $0xfefefeff, %edi /* magic value */ + xorl %edx, %ecx /* XOR with word c|c|c|c => bytes of str == c + are now 0 */ + addl %ecx, %edi /* add the magic value to the word. We get + carry bits reported for each byte which + is *not* 0 */ + jnc L(5) /* highest byte is C => return pointer */ + xorl %ecx, %edi /* ((word^charmask)+magic)^(word^charmask) */ + orl $0xfefefeff, %edi /* set all non-carry bits */ + incl %edi /* add 1: if one carry bit was *not* set + the addition will not result in 0. */ + jnz L(5) /* found it => return pointer */ + + /* Adjust both counters for a full round, i.e. 16 bytes. */ + addl $16, %eax + jmp L(1) + /* add missing source pointer increments */ +L(5): addl $4, %eax +L(6): addl $4, %eax +L(7): addl $4, %eax + + /* Test for the matching byte in the word. %ecx contains a NUL + char in the byte which originally was the byte we are looking + at. */ +L(8): testb %cl, %cl /* test first byte in dword */ + jz L(9) /* if zero => return pointer */ + incl %eax /* increment source pointer */ + + testb %ch, %ch /* test second byte in dword */ + jz L(9) /* if zero => return pointer */ + incl %eax /* increment source pointer */ + + testl $0xff0000, %ecx /* test third byte in dword */ + jz L(9) /* if zero => return pointer */ + incl %eax /* increment source pointer */ + + /* No further test needed we we know it is one of the four bytes. */ + +L(9): popl %edi /* pop saved register */ + + ret +END (__rawmemchr) +weak_alias (__rawmemchr, rawmemchr) |