/* strchrnul (str, chr) -- Return pointer to first occurrence of CHR in STR

   or the final NUL byte.

   For Intel 80x86, x>=3.

   Copyright (C) 1994-2018 Free Software Foundation, Inc.

   This file is part of the GNU C Library.

   Contributed by Ulrich Drepper <drepper@gnu.org>

   Some optimisations by Alan Modra <Alan@SPRI.Levels.UniSA.Edu.Au>

   The GNU C Library is free software; you can redistribute it and/or

   modify it under the terms of the GNU Lesser General Public

   License as published by the Free Software Foundation; either

   version 2.1 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

   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public

   License along with the GNU C Library; if not, see

   <http://www.gnu.org/licenses/>.  */

#include <sysdep.h>

#include "asm-syntax.h"

#define PARMS	4+4	/* space for 1 saved reg */

#define RTN	PARMS

#define STR	RTN

#define CHR	STR+4

	.text

ENTRY (__strchrnul)

	pushl %edi		/* Save callee-safe registers used here.  */

	cfi_adjust_cfa_offset (4)

	cfi_rel_offset (edi, 0)

	movl STR(%esp), %eax

	movl CHR(%esp), %edx

	/* At the moment %edx contains CHR.  What we need for the

	   algorithm is CHR 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 it is c|c|0|0 */

	movw %cx, %dx		/* and finally c|c|c|c */

	/* Before we start with the main loop we process single bytes

	   until the source pointer is aligned.  This has two reasons:

	   1. aligned 32-bit memory access is faster

	   and (more important)

	   2. we process in the main loop 32 bit in one step although

	      we don't know the end of the string.  But accessing at

	      4-byte alignment guarantees that we never access illegal

	      memory if this would not also be done by the trivial

	      implementation (this is because all processor inherent

	      boundaries are multiples of 4.  */

	testb $3, %al		/* correctly aligned ? */

	jz L(11)		/* yes => begin loop */

	movb (%eax), %cl	/* load byte in question (we need it twice) */

	cmpb %cl, %dl		/* compare byte */

	je L(6)			/* target found => return */

	testb %cl, %cl		/* is NUL? */

	jz L(6)			/* yes => return NULL */

	incl %eax		/* increment pointer */

	testb $3, %al		/* correctly aligned ? */

	jz L(11)		/* yes => begin loop */

	movb (%eax), %cl	/* load byte in question (we need it twice) */

	cmpb %cl, %dl		/* compare byte */

	je L(6)			/* target found => return */

	testb %cl, %cl		/* is NUL? */

	jz L(6)			/* yes => return NULL */

	incl %eax		/* increment pointer */

	testb $3, %al		/* correctly aligned ? */

	jz L(11)		/* yes => begin loop */

	movb (%eax), %cl	/* load byte in question (we need it twice) */

	cmpb %cl, %dl		/* compare byte */

	je L(6)			/* target found => return */

	testb %cl, %cl		/* is NUL? */

	jz L(6)			/* yes => return NULL */

	incl %eax		/* increment pointer */

	/* No we have reached alignment.  */

	jmp L(11)		/* begin loop */

      /* 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 CHR, not zero?

	 Good point.  So what we do is XOR LONGWORD with a longword,

	 each of whose bytes is CHR.  This turns each byte that is CHR

	 into a zero.  */

	/* Each round the main loop processes 16 bytes.  */

	ALIGN(4)

L(1):	addl $16, %eax		/* adjust pointer for whole round */

L(11):	movl (%eax), %ecx	/* get word (= 4 bytes) in question */

	xorl %edx, %ecx		/* XOR with word c|c|c|c => bytes of str == c

				   are now 0 */

	movl $0xfefefeff, %edi	/* magic value */

	addl %ecx, %edi		/* add the magic value to the word.  We get

				   carry bits reported for each byte which

				   is *not* CHR */

	/* 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(7)

	/* 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 CHR we don't get 0 in %edi.  */

	jnz L(7)		/* found it => return pointer */

	/* Now we made sure the dword does not contain the character we are

	   looking for.  But because we deal with strings we have to check

	   for the end of string before testing the next dword.  */

	xorl %edx, %ecx		/* restore original dword without reload */

	movl $0xfefefeff, %edi	/* magic value */

	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 NUL => return NULL */

	xorl %ecx, %edi		/* (word+magic)^word */

	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 NUL => return NULL */

	movl 4(%eax), %ecx	/* get word (= 4 bytes) in question */

	xorl %edx, %ecx		/* XOR with word c|c|c|c => bytes of str == c

				   are now 0 */

	movl $0xfefefeff, %edi	/* magic value */

	addl %ecx, %edi		/* add the magic value to the word.  We get

				   carry bits reported for each byte which

				   is *not* CHR */

	jnc L(71)		/* highest byte is CHR => 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(71)		/* found it => return pointer */

	xorl %edx, %ecx		/* restore original dword without reload */

	movl $0xfefefeff, %edi	/* magic value */

	addl %ecx, %edi		/* add the magic value to the word.  We get

				   carry bits reported for each byte which

				   is *not* 0 */

	jnc L(71)		/* highest byte is NUL => return NULL */

	xorl %ecx, %edi		/* (word+magic)^word */

	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(71)		/* found NUL => return NULL */

	movl 8(%eax), %ecx	/* get word (= 4 bytes) in question */

	xorl %edx, %ecx		/* XOR with word c|c|c|c => bytes of str == c

				   are now 0 */

	movl $0xfefefeff, %edi	/* magic value */

	addl %ecx, %edi		/* add the magic value to the word.  We get

				   carry bits reported for each byte which

				   is *not* CHR */

	jnc L(72)		/* highest byte is CHR => 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(72)		/* found it => return pointer */

	xorl %edx, %ecx		/* restore original dword without reload */

	movl $0xfefefeff, %edi	/* magic value */

	addl %ecx, %edi		/* add the magic value to the word.  We get

				   carry bits reported for each byte which

				   is *not* 0 */

	jnc L(72)		/* highest byte is NUL => return NULL */

	xorl %ecx, %edi		/* (word+magic)^word */

	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(72)		/* found NUL => return NULL */

	movl 12(%eax), %ecx	/* get word (= 4 bytes) in question */

	xorl %edx, %ecx		/* XOR with word c|c|c|c => bytes of str == c

				   are now 0 */

	movl $0xfefefeff, %edi	/* magic value */

	addl %ecx, %edi		/* add the magic value to the word.  We get

				   carry bits reported for each byte which

				   is *not* CHR */

	jnc L(73)		/* highest byte is CHR => 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(73)		/* found it => return pointer */

	xorl %edx, %ecx		/* restore original dword without reload */

	movl $0xfefefeff, %edi	/* magic value */

	addl %ecx, %edi		/* add the magic value to the word.  We get

				   carry bits reported for each byte which

				   is *not* 0 */

	jnc L(73)		/* highest byte is NUL => return NULL */

	xorl %ecx, %edi		/* (word+magic)^word */

	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.  */

	jz L(1)			/* no NUL found => restart loop */

L(73):	addl $4, %eax		/* adjust pointer */

L(72):	addl $4, %eax

L(71):	addl $4, %eax

	/* We now scan for the byte in which the character was matched.

	   But we have to take care of the case that a NUL char is

	   found before this in the dword.  */

L(7):	testb %cl, %cl		/* is first byte CHR? */

	jz L(6)			/* yes => return pointer */

	cmpb %dl, %cl		/* is first byte NUL? */

	je L(6)			/* yes => return NULL */

	incl %eax		/* it's not in the first byte */

	testb %ch, %ch		/* is second byte CHR? */

	jz L(6)			/* yes => return pointer */

	cmpb %dl, %ch		/* is second byte NUL? */

	je L(6)			/* yes => return NULL? */

	incl %eax		/* it's not in the second byte */

	shrl $16, %ecx		/* make upper byte accessible */

	testb %cl, %cl		/* is third byte CHR? */

	jz L(6)			/* yes => return pointer */

	cmpb %dl, %cl		/* is third byte NUL? */

	je L(6)			/* yes => return NULL */

	/* It must be in the fourth byte and it cannot be NUL.  */

	incl %eax

L(6):	popl %edi		/* restore saved register content */

	cfi_adjust_cfa_offset (-4)

	cfi_restore (edi)

	ret

END (__strchrnul)

weak_alias (__strchrnul, strchrnul)