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