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213 lines
6.2 KiB
213 lines
6.2 KiB
/* SPDX-License-Identifier: GPL-2.0-only */ |
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/* |
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* Copyright (c) 2013-2021, Arm Limited. |
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* |
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* Adapted from the original at: |
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* https://github.com/ARM-software/optimized-routines/blob/98e4d6a5c13c8e54/string/aarch64/strlen.S |
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*/ |
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#include <linux/linkage.h> |
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#include <asm/assembler.h> |
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#include <asm/mte-def.h> |
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/* Assumptions: |
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* |
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* ARMv8-a, AArch64, unaligned accesses, min page size 4k. |
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*/ |
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#define L(label) .L ## label |
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/* Arguments and results. */ |
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#define srcin x0 |
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#define len x0 |
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/* Locals and temporaries. */ |
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#define src x1 |
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#define data1 x2 |
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#define data2 x3 |
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#define has_nul1 x4 |
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#define has_nul2 x5 |
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#define tmp1 x4 |
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#define tmp2 x5 |
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#define tmp3 x6 |
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#define tmp4 x7 |
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#define zeroones x8 |
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/* NUL detection works on the principle that (X - 1) & (~X) & 0x80 |
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(=> (X - 1) & ~(X | 0x7f)) is non-zero iff a byte is zero, and |
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can be done in parallel across the entire word. A faster check |
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(X - 1) & 0x80 is zero for non-NUL ASCII characters, but gives |
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false hits for characters 129..255. */ |
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#define REP8_01 0x0101010101010101 |
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#define REP8_7f 0x7f7f7f7f7f7f7f7f |
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#define REP8_80 0x8080808080808080 |
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/* |
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* When KASAN_HW_TAGS is in use, memory is checked at MTE_GRANULE_SIZE |
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* (16-byte) granularity, and we must ensure that no access straddles this |
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* alignment boundary. |
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*/ |
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#ifdef CONFIG_KASAN_HW_TAGS |
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#define MIN_PAGE_SIZE MTE_GRANULE_SIZE |
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#else |
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#define MIN_PAGE_SIZE 4096 |
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#endif |
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/* Since strings are short on average, we check the first 16 bytes |
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of the string for a NUL character. In order to do an unaligned ldp |
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safely we have to do a page cross check first. If there is a NUL |
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byte we calculate the length from the 2 8-byte words using |
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conditional select to reduce branch mispredictions (it is unlikely |
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strlen will be repeatedly called on strings with the same length). |
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If the string is longer than 16 bytes, we align src so don't need |
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further page cross checks, and process 32 bytes per iteration |
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using the fast NUL check. If we encounter non-ASCII characters, |
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fallback to a second loop using the full NUL check. |
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If the page cross check fails, we read 16 bytes from an aligned |
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address, remove any characters before the string, and continue |
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in the main loop using aligned loads. Since strings crossing a |
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page in the first 16 bytes are rare (probability of |
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16/MIN_PAGE_SIZE ~= 0.4%), this case does not need to be optimized. |
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AArch64 systems have a minimum page size of 4k. We don't bother |
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checking for larger page sizes - the cost of setting up the correct |
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page size is just not worth the extra gain from a small reduction in |
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the cases taking the slow path. Note that we only care about |
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whether the first fetch, which may be misaligned, crosses a page |
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boundary. */ |
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SYM_FUNC_START_WEAK_PI(strlen) |
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and tmp1, srcin, MIN_PAGE_SIZE - 1 |
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mov zeroones, REP8_01 |
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cmp tmp1, MIN_PAGE_SIZE - 16 |
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b.gt L(page_cross) |
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ldp data1, data2, [srcin] |
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#ifdef __AARCH64EB__ |
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/* For big-endian, carry propagation (if the final byte in the |
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string is 0x01) means we cannot use has_nul1/2 directly. |
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Since we expect strings to be small and early-exit, |
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byte-swap the data now so has_null1/2 will be correct. */ |
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rev data1, data1 |
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rev data2, data2 |
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#endif |
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sub tmp1, data1, zeroones |
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orr tmp2, data1, REP8_7f |
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sub tmp3, data2, zeroones |
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orr tmp4, data2, REP8_7f |
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bics has_nul1, tmp1, tmp2 |
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bic has_nul2, tmp3, tmp4 |
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ccmp has_nul2, 0, 0, eq |
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beq L(main_loop_entry) |
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/* Enter with C = has_nul1 == 0. */ |
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csel has_nul1, has_nul1, has_nul2, cc |
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mov len, 8 |
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rev has_nul1, has_nul1 |
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clz tmp1, has_nul1 |
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csel len, xzr, len, cc |
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add len, len, tmp1, lsr 3 |
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ret |
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/* The inner loop processes 32 bytes per iteration and uses the fast |
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NUL check. If we encounter non-ASCII characters, use a second |
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loop with the accurate NUL check. */ |
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.p2align 4 |
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L(main_loop_entry): |
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bic src, srcin, 15 |
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sub src, src, 16 |
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L(main_loop): |
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ldp data1, data2, [src, 32]! |
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L(page_cross_entry): |
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sub tmp1, data1, zeroones |
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sub tmp3, data2, zeroones |
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orr tmp2, tmp1, tmp3 |
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tst tmp2, zeroones, lsl 7 |
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bne 1f |
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ldp data1, data2, [src, 16] |
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sub tmp1, data1, zeroones |
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sub tmp3, data2, zeroones |
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orr tmp2, tmp1, tmp3 |
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tst tmp2, zeroones, lsl 7 |
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beq L(main_loop) |
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add src, src, 16 |
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1: |
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/* The fast check failed, so do the slower, accurate NUL check. */ |
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orr tmp2, data1, REP8_7f |
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orr tmp4, data2, REP8_7f |
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bics has_nul1, tmp1, tmp2 |
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bic has_nul2, tmp3, tmp4 |
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ccmp has_nul2, 0, 0, eq |
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beq L(nonascii_loop) |
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/* Enter with C = has_nul1 == 0. */ |
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L(tail): |
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#ifdef __AARCH64EB__ |
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/* For big-endian, carry propagation (if the final byte in the |
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string is 0x01) means we cannot use has_nul1/2 directly. The |
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easiest way to get the correct byte is to byte-swap the data |
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and calculate the syndrome a second time. */ |
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csel data1, data1, data2, cc |
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rev data1, data1 |
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sub tmp1, data1, zeroones |
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orr tmp2, data1, REP8_7f |
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bic has_nul1, tmp1, tmp2 |
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#else |
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csel has_nul1, has_nul1, has_nul2, cc |
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#endif |
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sub len, src, srcin |
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rev has_nul1, has_nul1 |
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add tmp2, len, 8 |
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clz tmp1, has_nul1 |
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csel len, len, tmp2, cc |
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add len, len, tmp1, lsr 3 |
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ret |
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L(nonascii_loop): |
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ldp data1, data2, [src, 16]! |
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sub tmp1, data1, zeroones |
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orr tmp2, data1, REP8_7f |
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sub tmp3, data2, zeroones |
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orr tmp4, data2, REP8_7f |
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bics has_nul1, tmp1, tmp2 |
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bic has_nul2, tmp3, tmp4 |
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ccmp has_nul2, 0, 0, eq |
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bne L(tail) |
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ldp data1, data2, [src, 16]! |
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sub tmp1, data1, zeroones |
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orr tmp2, data1, REP8_7f |
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sub tmp3, data2, zeroones |
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orr tmp4, data2, REP8_7f |
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bics has_nul1, tmp1, tmp2 |
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bic has_nul2, tmp3, tmp4 |
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ccmp has_nul2, 0, 0, eq |
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beq L(nonascii_loop) |
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b L(tail) |
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/* Load 16 bytes from [srcin & ~15] and force the bytes that precede |
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srcin to 0x7f, so we ignore any NUL bytes before the string. |
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Then continue in the aligned loop. */ |
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L(page_cross): |
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bic src, srcin, 15 |
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ldp data1, data2, [src] |
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lsl tmp1, srcin, 3 |
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mov tmp4, -1 |
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#ifdef __AARCH64EB__ |
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/* Big-endian. Early bytes are at MSB. */ |
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lsr tmp1, tmp4, tmp1 /* Shift (tmp1 & 63). */ |
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#else |
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/* Little-endian. Early bytes are at LSB. */ |
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lsl tmp1, tmp4, tmp1 /* Shift (tmp1 & 63). */ |
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#endif |
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orr tmp1, tmp1, REP8_80 |
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orn data1, data1, tmp1 |
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orn tmp2, data2, tmp1 |
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tst srcin, 8 |
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csel data1, data1, tmp4, eq |
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csel data2, data2, tmp2, eq |
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b L(page_cross_entry) |
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SYM_FUNC_END_PI(strlen) |
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EXPORT_SYMBOL_NOKASAN(strlen)
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