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Diffstat (limited to 'arch/powerpc/lib/strlen_32.S')
| -rw-r--r-- | arch/powerpc/lib/strlen_32.S | 78 |
1 files changed, 78 insertions, 0 deletions
diff --git a/arch/powerpc/lib/strlen_32.S b/arch/powerpc/lib/strlen_32.S new file mode 100644 index 000000000000..0a8d3f64d493 --- /dev/null +++ b/arch/powerpc/lib/strlen_32.S @@ -0,0 +1,78 @@ +/* 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) |