strlen.S (6353B)
1/* SPDX-License-Identifier: GPL-2.0-only */ 2/* 3 * Copyright (c) 2013-2021, Arm Limited. 4 * 5 * Adapted from the original at: 6 * https://github.com/ARM-software/optimized-routines/blob/98e4d6a5c13c8e54/string/aarch64/strlen.S 7 */ 8 9#include <linux/linkage.h> 10#include <asm/assembler.h> 11#include <asm/mte-def.h> 12 13/* Assumptions: 14 * 15 * ARMv8-a, AArch64, unaligned accesses, min page size 4k. 16 */ 17 18#define L(label) .L ## label 19 20/* Arguments and results. */ 21#define srcin x0 22#define len x0 23 24/* Locals and temporaries. */ 25#define src x1 26#define data1 x2 27#define data2 x3 28#define has_nul1 x4 29#define has_nul2 x5 30#define tmp1 x4 31#define tmp2 x5 32#define tmp3 x6 33#define tmp4 x7 34#define zeroones x8 35 36 /* NUL detection works on the principle that (X - 1) & (~X) & 0x80 37 (=> (X - 1) & ~(X | 0x7f)) is non-zero iff a byte is zero, and 38 can be done in parallel across the entire word. A faster check 39 (X - 1) & 0x80 is zero for non-NUL ASCII characters, but gives 40 false hits for characters 129..255. */ 41 42#define REP8_01 0x0101010101010101 43#define REP8_7f 0x7f7f7f7f7f7f7f7f 44#define REP8_80 0x8080808080808080 45 46/* 47 * When KASAN_HW_TAGS is in use, memory is checked at MTE_GRANULE_SIZE 48 * (16-byte) granularity, and we must ensure that no access straddles this 49 * alignment boundary. 50 */ 51#ifdef CONFIG_KASAN_HW_TAGS 52#define MIN_PAGE_SIZE MTE_GRANULE_SIZE 53#else 54#define MIN_PAGE_SIZE 4096 55#endif 56 57 /* Since strings are short on average, we check the first 16 bytes 58 of the string for a NUL character. In order to do an unaligned ldp 59 safely we have to do a page cross check first. If there is a NUL 60 byte we calculate the length from the 2 8-byte words using 61 conditional select to reduce branch mispredictions (it is unlikely 62 strlen will be repeatedly called on strings with the same length). 63 64 If the string is longer than 16 bytes, we align src so don't need 65 further page cross checks, and process 32 bytes per iteration 66 using the fast NUL check. If we encounter non-ASCII characters, 67 fallback to a second loop using the full NUL check. 68 69 If the page cross check fails, we read 16 bytes from an aligned 70 address, remove any characters before the string, and continue 71 in the main loop using aligned loads. Since strings crossing a 72 page in the first 16 bytes are rare (probability of 73 16/MIN_PAGE_SIZE ~= 0.4%), this case does not need to be optimized. 74 75 AArch64 systems have a minimum page size of 4k. We don't bother 76 checking for larger page sizes - the cost of setting up the correct 77 page size is just not worth the extra gain from a small reduction in 78 the cases taking the slow path. Note that we only care about 79 whether the first fetch, which may be misaligned, crosses a page 80 boundary. */ 81 82SYM_FUNC_START(__pi_strlen) 83 and tmp1, srcin, MIN_PAGE_SIZE - 1 84 mov zeroones, REP8_01 85 cmp tmp1, MIN_PAGE_SIZE - 16 86 b.gt L(page_cross) 87 ldp data1, data2, [srcin] 88#ifdef __AARCH64EB__ 89 /* For big-endian, carry propagation (if the final byte in the 90 string is 0x01) means we cannot use has_nul1/2 directly. 91 Since we expect strings to be small and early-exit, 92 byte-swap the data now so has_null1/2 will be correct. */ 93 rev data1, data1 94 rev data2, data2 95#endif 96 sub tmp1, data1, zeroones 97 orr tmp2, data1, REP8_7f 98 sub tmp3, data2, zeroones 99 orr tmp4, data2, REP8_7f 100 bics has_nul1, tmp1, tmp2 101 bic has_nul2, tmp3, tmp4 102 ccmp has_nul2, 0, 0, eq 103 beq L(main_loop_entry) 104 105 /* Enter with C = has_nul1 == 0. */ 106 csel has_nul1, has_nul1, has_nul2, cc 107 mov len, 8 108 rev has_nul1, has_nul1 109 clz tmp1, has_nul1 110 csel len, xzr, len, cc 111 add len, len, tmp1, lsr 3 112 ret 113 114 /* The inner loop processes 32 bytes per iteration and uses the fast 115 NUL check. If we encounter non-ASCII characters, use a second 116 loop with the accurate NUL check. */ 117 .p2align 4 118L(main_loop_entry): 119 bic src, srcin, 15 120 sub src, src, 16 121L(main_loop): 122 ldp data1, data2, [src, 32]! 123L(page_cross_entry): 124 sub tmp1, data1, zeroones 125 sub tmp3, data2, zeroones 126 orr tmp2, tmp1, tmp3 127 tst tmp2, zeroones, lsl 7 128 bne 1f 129 ldp data1, data2, [src, 16] 130 sub tmp1, data1, zeroones 131 sub tmp3, data2, zeroones 132 orr tmp2, tmp1, tmp3 133 tst tmp2, zeroones, lsl 7 134 beq L(main_loop) 135 add src, src, 16 1361: 137 /* The fast check failed, so do the slower, accurate NUL check. */ 138 orr tmp2, data1, REP8_7f 139 orr tmp4, data2, REP8_7f 140 bics has_nul1, tmp1, tmp2 141 bic has_nul2, tmp3, tmp4 142 ccmp has_nul2, 0, 0, eq 143 beq L(nonascii_loop) 144 145 /* Enter with C = has_nul1 == 0. */ 146L(tail): 147#ifdef __AARCH64EB__ 148 /* For big-endian, carry propagation (if the final byte in the 149 string is 0x01) means we cannot use has_nul1/2 directly. The 150 easiest way to get the correct byte is to byte-swap the data 151 and calculate the syndrome a second time. */ 152 csel data1, data1, data2, cc 153 rev data1, data1 154 sub tmp1, data1, zeroones 155 orr tmp2, data1, REP8_7f 156 bic has_nul1, tmp1, tmp2 157#else 158 csel has_nul1, has_nul1, has_nul2, cc 159#endif 160 sub len, src, srcin 161 rev has_nul1, has_nul1 162 add tmp2, len, 8 163 clz tmp1, has_nul1 164 csel len, len, tmp2, cc 165 add len, len, tmp1, lsr 3 166 ret 167 168L(nonascii_loop): 169 ldp data1, data2, [src, 16]! 170 sub tmp1, data1, zeroones 171 orr tmp2, data1, REP8_7f 172 sub tmp3, data2, zeroones 173 orr tmp4, data2, REP8_7f 174 bics has_nul1, tmp1, tmp2 175 bic has_nul2, tmp3, tmp4 176 ccmp has_nul2, 0, 0, eq 177 bne L(tail) 178 ldp data1, data2, [src, 16]! 179 sub tmp1, data1, zeroones 180 orr tmp2, data1, REP8_7f 181 sub tmp3, data2, zeroones 182 orr tmp4, data2, REP8_7f 183 bics has_nul1, tmp1, tmp2 184 bic has_nul2, tmp3, tmp4 185 ccmp has_nul2, 0, 0, eq 186 beq L(nonascii_loop) 187 b L(tail) 188 189 /* Load 16 bytes from [srcin & ~15] and force the bytes that precede 190 srcin to 0x7f, so we ignore any NUL bytes before the string. 191 Then continue in the aligned loop. */ 192L(page_cross): 193 bic src, srcin, 15 194 ldp data1, data2, [src] 195 lsl tmp1, srcin, 3 196 mov tmp4, -1 197#ifdef __AARCH64EB__ 198 /* Big-endian. Early bytes are at MSB. */ 199 lsr tmp1, tmp4, tmp1 /* Shift (tmp1 & 63). */ 200#else 201 /* Little-endian. Early bytes are at LSB. */ 202 lsl tmp1, tmp4, tmp1 /* Shift (tmp1 & 63). */ 203#endif 204 orr tmp1, tmp1, REP8_80 205 orn data1, data1, tmp1 206 orn tmp2, data2, tmp1 207 tst srcin, 8 208 csel data1, data1, tmp4, eq 209 csel data2, data2, tmp2, eq 210 b L(page_cross_entry) 211SYM_FUNC_END(__pi_strlen) 212SYM_FUNC_ALIAS_WEAK(strlen, __pi_strlen) 213EXPORT_SYMBOL_NOKASAN(strlen)