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225 lines
6.7 KiB
ArmAsm
225 lines
6.7 KiB
ArmAsm
/* Copyright (C) 2012-2020 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library. If not, see
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<https://www.gnu.org/licenses/>. */
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#include <sysdep.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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#ifndef STRLEN
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# define STRLEN __strlen
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#endif
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/* To test the page crossing code path more thoroughly, compile with
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-DTEST_PAGE_CROSS - this will force all calls through the slower
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entry path. This option is not intended for production use. */
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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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#ifdef TEST_PAGE_CROSS
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# define MIN_PAGE_SIZE 16
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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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ENTRY_ALIGN (STRLEN, 6)
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DELOUSE (0)
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DELOUSE (1)
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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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END (STRLEN)
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weak_alias (STRLEN, strlen)
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libc_hidden_builtin_def (strlen)
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