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162 lines
4.4 KiB
ArmAsm
162 lines
4.4 KiB
ArmAsm
/* memchr - find a character in a memory zone
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Copyright (C) 2015-2020 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
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* Neon Available.
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*/
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#ifndef MEMCHR
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# define MEMCHR __memchr
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#endif
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/* Arguments and results. */
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#define srcin x0
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#define chrin w1
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#define cntin x2
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#define result x0
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#define src x3
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#define tmp x4
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#define wtmp2 w5
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#define synd x6
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#define soff x9
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#define cntrem x10
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#define vrepchr v0
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#define vdata1 v1
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#define vdata2 v2
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#define vhas_chr1 v3
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#define vhas_chr2 v4
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#define vrepmask v5
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#define vend v6
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/*
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* Core algorithm:
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*
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* For each 32-byte chunk we calculate a 64-bit syndrome value, with two bits
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* per byte. For each tuple, bit 0 is set if the relevant byte matched the
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* requested character and bit 1 is not used (faster than using a 32bit
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* syndrome). Since the bits in the syndrome reflect exactly the order in which
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* things occur in the original string, counting trailing zeros allows to
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* identify exactly which byte has matched.
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*/
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ENTRY (MEMCHR)
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/* Do not dereference srcin if no bytes to compare. */
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cbz cntin, L(zero_length)
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/*
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* Magic constant 0x40100401 allows us to identify which lane matches
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* the requested byte.
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*/
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mov wtmp2, #0x0401
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movk wtmp2, #0x4010, lsl #16
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dup vrepchr.16b, chrin
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/* Work with aligned 32-byte chunks */
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bic src, srcin, #31
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dup vrepmask.4s, wtmp2
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ands soff, srcin, #31
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and cntrem, cntin, #31
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b.eq L(loop)
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/*
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* Input string is not 32-byte aligned. We calculate the syndrome
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* value for the aligned 32 bytes block containing the first bytes
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* and mask the irrelevant part.
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*/
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ld1 {vdata1.16b, vdata2.16b}, [src], #32
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sub tmp, soff, #32
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adds cntin, cntin, tmp
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cmeq vhas_chr1.16b, vdata1.16b, vrepchr.16b
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cmeq vhas_chr2.16b, vdata2.16b, vrepchr.16b
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and vhas_chr1.16b, vhas_chr1.16b, vrepmask.16b
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and vhas_chr2.16b, vhas_chr2.16b, vrepmask.16b
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addp vend.16b, vhas_chr1.16b, vhas_chr2.16b /* 256->128 */
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addp vend.16b, vend.16b, vend.16b /* 128->64 */
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mov synd, vend.2d[0]
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/* Clear the soff*2 lower bits */
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lsl tmp, soff, #1
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lsr synd, synd, tmp
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lsl synd, synd, tmp
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/* The first block can also be the last */
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b.ls L(masklast)
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/* Have we found something already? */
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cbnz synd, L(tail)
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L(loop):
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ld1 {vdata1.16b, vdata2.16b}, [src], #32
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subs cntin, cntin, #32
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cmeq vhas_chr1.16b, vdata1.16b, vrepchr.16b
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cmeq vhas_chr2.16b, vdata2.16b, vrepchr.16b
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/* If we're out of data we finish regardless of the result */
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b.ls L(end)
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/* Use a fast check for the termination condition */
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orr vend.16b, vhas_chr1.16b, vhas_chr2.16b
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addp vend.2d, vend.2d, vend.2d
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mov synd, vend.2d[0]
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/* We're not out of data, loop if we haven't found the character */
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cbz synd, L(loop)
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L(end):
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/* Termination condition found, let's calculate the syndrome value */
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and vhas_chr1.16b, vhas_chr1.16b, vrepmask.16b
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and vhas_chr2.16b, vhas_chr2.16b, vrepmask.16b
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addp vend.16b, vhas_chr1.16b, vhas_chr2.16b /* 256->128 */
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addp vend.16b, vend.16b, vend.16b /* 128->64 */
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mov synd, vend.2d[0]
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/* Only do the clear for the last possible block */
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b.hi L(tail)
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L(masklast):
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/* Clear the (32 - ((cntrem + soff) % 32)) * 2 upper bits */
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add tmp, cntrem, soff
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and tmp, tmp, #31
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sub tmp, tmp, #32
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neg tmp, tmp, lsl #1
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lsl synd, synd, tmp
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lsr synd, synd, tmp
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L(tail):
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/* Count the trailing zeros using bit reversing */
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rbit synd, synd
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/* Compensate the last post-increment */
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sub src, src, #32
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/* Check that we have found a character */
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cmp synd, #0
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/* And count the leading zeros */
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clz synd, synd
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/* Compute the potential result */
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add result, src, synd, lsr #1
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/* Select result or NULL */
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csel result, xzr, result, eq
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ret
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L(zero_length):
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mov result, #0
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ret
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END (MEMCHR)
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weak_alias (MEMCHR, memchr)
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libc_hidden_builtin_def (memchr)
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