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/* SPDX-License-Identifier: GPL-2.0 */
/*
* strlen() for PPC32
*
* Copyright (C) 2018 Christophe Leroy CS Systemes d'Information.
*
* Inspired from glibc implementation
*/
#include <asm/ppc_asm.h>
#include <asm/export.h>
#include <asm/cache.h>
.text
/*
* Algorithm:
*
* 1) Given a word 'x', we can test to see if it contains any 0 bytes
* by subtracting 0x01010101, and seeing if any of the high bits of each
* byte changed from 0 to 1. This works because the least significant
* 0 byte must have had no incoming carry (otherwise it's not the least
* significant), so it is 0x00 - 0x01 == 0xff. For all other
* byte values, either they have the high bit set initially, or when
* 1 is subtracted you get a value in the range 0x00-0x7f, none of which
* have their high bit set. The expression here is
* (x - 0x01010101) & ~x & 0x80808080), which gives 0x00000000 when
* there were no 0x00 bytes in the word. You get 0x80 in bytes that
* match, but possibly false 0x80 matches in the next more significant
* byte to a true match due to carries. For little-endian this is
* of no consequence since the least significant match is the one
* we're interested in, but big-endian needs method 2 to find which
* byte matches.
* 2) Given a word 'x', we can test to see _which_ byte was zero by
* calculating ~(((x & ~0x80808080) - 0x80808080 - 1) | x | ~0x80808080).
* This produces 0x80 in each byte that was zero, and 0x00 in all
* the other bytes. The '| ~0x80808080' clears the low 7 bits in each
* byte, and the '| x' part ensures that bytes with the high bit set
* produce 0x00. The addition will carry into the high bit of each byte
* iff that byte had one of its low 7 bits set. We can then just see
* which was the most significant bit set and divide by 8 to find how
* many to add to the index.
* This is from the book 'The PowerPC Compiler Writer's Guide',
* by Steve Hoxey, Faraydon Karim, Bill Hay and Hank Warren.
*/
_GLOBAL(strlen)
andi. r0, r3, 3
lis r7, 0x0101
addi r10, r3, -4
addic r7, r7, 0x0101 /* r7 = 0x01010101 (lomagic) & clear XER[CA] */
rotlwi r6, r7, 31 /* r6 = 0x80808080 (himagic) */
bne- 3f
.balign IFETCH_ALIGN_BYTES
1: lwzu r9, 4(r10)
2: subf r8, r7, r9
and. r8, r8, r6
beq+ 1b
andc. r8, r8, r9
beq+ 1b
andc r8, r9, r6
orc r9, r9, r6
subfe r8, r6, r8
nor r8, r8, r9
cntlzw r8, r8
subf r3, r3, r10
srwi r8, r8, 3
add r3, r3, r8
blr
/* Missaligned string: make sure bytes before string are seen not 0 */
3: xor r10, r10, r0
orc r8, r8, r8
lwzu r9, 4(r10)
slwi r0, r0, 3
srw r8, r8, r0
orc r9, r9, r8
b 2b
EXPORT_SYMBOL(strlen)
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