diff options
Diffstat (limited to 'sysdeps/aarch64/multiarch/strlen_asimd.S')
| -rw-r--r-- | sysdeps/aarch64/multiarch/strlen_asimd.S | 168 |
1 files changed, 168 insertions, 0 deletions
diff --git a/sysdeps/aarch64/multiarch/strlen_asimd.S b/sysdeps/aarch64/multiarch/strlen_asimd.S new file mode 100644 index 0000000000..1d1c6abb82 --- /dev/null +++ b/sysdeps/aarch64/multiarch/strlen_asimd.S @@ -0,0 +1,168 @@ +/* Strlen implementation that uses ASIMD instructions for load and NULL checks. + Copyright (C) 2018-2019 Free Software Foundation, Inc. + + This file is part of the GNU C Library. + + The GNU C Library is free software; you can redistribute it and/or + modify it under the terms of the GNU Lesser General Public + License as published by the Free Software Foundation; either + version 2.1 of the License, or (at your option) any later version. + + The GNU C Library is distributed in the hope that it will be useful, + but WITHOUT ANY WARRANTY; without even the implied warranty of + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU + Lesser General Public License for more details. + + You should have received a copy of the GNU Lesser General Public + License along with the GNU C Library. If not, see + <https://www.gnu.org/licenses/>. */ + +#include <sysdep.h> + +/* Assumptions: + + ARMv8-a, AArch64, ASIMD, unaligned accesses, min page size 4k. */ + +/* To test the page crossing code path more thoroughly, compile with + -DTEST_PAGE_CROSS - this will force all calls through the slower + entry path. This option is not intended for production use. */ + +/* Arguments and results. */ +#define srcin x0 +#define len x0 + +/* Locals and temporaries. */ +#define src x1 +#define data1 x2 +#define data2 x3 +#define has_nul1 x4 +#define has_nul2 x5 +#define tmp1 x4 +#define tmp2 x5 +#define tmp3 x6 +#define tmp4 x7 +#define zeroones x8 +#define dataq q2 +#define datav v2 +#define datab2 b3 +#define dataq2 q3 +#define datav2 v3 + +#ifdef TEST_PAGE_CROSS +# define MIN_PAGE_SIZE 16 +#else +# define MIN_PAGE_SIZE 4096 +#endif + + /* Since strings are short on average, we check the first 16 bytes + of the string for a NUL character. In order to do an unaligned load + safely we have to do a page cross check first. If there is a NUL + byte we calculate the length from the 2 8-byte words using + conditional select to reduce branch mispredictions (it is unlikely + strlen_asimd will be repeatedly called on strings with the same + length). + + If the string is longer than 16 bytes, we align src so don't need + further page cross checks, and process 16 bytes per iteration. + + If the page cross check fails, we read 16 bytes from an aligned + address, remove any characters before the string, and continue + in the main loop using aligned loads. Since strings crossing a + page in the first 16 bytes are rare (probability of + 16/MIN_PAGE_SIZE ~= 0.4%), this case does not need to be optimized. + + AArch64 systems have a minimum page size of 4k. We don't bother + checking for larger page sizes - the cost of setting up the correct + page size is just not worth the extra gain from a small reduction in + the cases taking the slow path. Note that we only care about + whether the first fetch, which may be misaligned, crosses a page + boundary. */ + +ENTRY_ALIGN (__strlen_asimd, 6) + DELOUSE (0) + DELOUSE (1) + and tmp1, srcin, MIN_PAGE_SIZE - 1 + cmp tmp1, MIN_PAGE_SIZE - 16 + b.gt L(page_cross) + ldr dataq, [srcin] +#ifdef __AARCH64EB__ + rev64 datav.16b, datav.16b +#endif + + /* Get the minimum value and keep going if it is not zero. */ + uminv datab2, datav.16b + mov tmp1, datav2.d[0] + cbnz tmp1, L(main_loop_entry) + + cmeq datav.16b, datav.16b, #0 + mov data1, datav.d[0] + mov data2, datav.d[1] + cmp data1, 0 + csel data1, data1, data2, ne + mov len, 8 + rev data1, data1 + clz tmp1, data1 + csel len, xzr, len, ne + add len, len, tmp1, lsr 3 + ret + +L(main_loop_entry): + bic src, srcin, 15 + +L(main_loop): + ldr dataq, [src, 16]! +L(page_cross_entry): + /* Get the minimum value and keep going if it is not zero. */ + uminv datab2, datav.16b + mov tmp1, datav2.d[0] + cbnz tmp1, L(main_loop) + +L(tail): +#ifdef __AARCH64EB__ + rev64 datav.16b, datav.16b +#endif + /* Set te NULL byte as 0xff and the rest as 0x00, move the data into a + pair of scalars and then compute the length from the earliest NULL + byte. */ + cmeq datav.16b, datav.16b, #0 + mov data1, datav.d[0] + mov data2, datav.d[1] + cmp data1, 0 + csel data1, data1, data2, ne + sub len, src, srcin + rev data1, data1 + add tmp2, len, 8 + clz tmp1, data1 + csel len, len, tmp2, ne + add len, len, tmp1, lsr 3 + ret + + /* Load 16 bytes from [srcin & ~15] and force the bytes that precede + srcin to 0xff, so we ignore any NUL bytes before the string. + Then continue in the aligned loop. */ +L(page_cross): + mov tmp3, 63 + bic src, srcin, 15 + and tmp1, srcin, 7 + ands tmp2, srcin, 8 + ldr dataq, [src] + lsl tmp1, tmp1, 3 + csel tmp2, tmp2, tmp1, eq + csel tmp1, tmp1, tmp3, eq + mov tmp4, -1 +#ifdef __AARCH64EB__ + /* Big-endian. Early bytes are at MSB. */ + lsr tmp1, tmp4, tmp1 + lsr tmp2, tmp4, tmp2 +#else + /* Little-endian. Early bytes are at LSB. */ + lsl tmp1, tmp4, tmp1 + lsl tmp2, tmp4, tmp2 +#endif + mov datav2.d[0], tmp1 + mov datav2.d[1], tmp2 + orn datav.16b, datav.16b, datav2.16b + b L(page_cross_entry) +END (__strlen_asimd) +weak_alias (__strlen_asimd, strlen_asimd) +libc_hidden_builtin_def (strlen_asimd) |