/* 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)