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+/* 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)