| /* |
| * strlen - calculate the length of a string |
| * |
| * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. |
| * See https://llvm.org/LICENSE.txt for license information. |
| * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception |
| */ |
| |
| /* Assumptions: |
| * |
| * ARMv8-a, AArch64, unaligned accesses, min page size 4k. |
| */ |
| |
| #include "../asmdefs.h" |
| |
| /* 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 |
| |
| /* NUL detection works on the principle that (X - 1) & (~X) & 0x80 |
| (=> (X - 1) & ~(X | 0x7f)) is non-zero iff a byte is zero, and |
| can be done in parallel across the entire word. A faster check |
| (X - 1) & 0x80 is zero for non-NUL ASCII characters, but gives |
| false hits for characters 129..255. */ |
| |
| #define REP8_01 0x0101010101010101 |
| #define REP8_7f 0x7f7f7f7f7f7f7f7f |
| #define REP8_80 0x8080808080808080 |
| |
| #ifdef TEST_PAGE_CROSS |
| # define MIN_PAGE_SIZE 15 |
| #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 ldp |
| 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_aarch64 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 32 bytes per iteration |
| using the fast NUL check. If we encounter non-ASCII characters, |
| fallback to a second loop using the full NUL check. |
| |
| 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 (__strlen_aarch64) |
| and tmp1, srcin, MIN_PAGE_SIZE - 1 |
| mov zeroones, REP8_01 |
| cmp tmp1, MIN_PAGE_SIZE - 16 |
| b.gt L(page_cross) |
| ldp data1, data2, [srcin] |
| #ifdef __AARCH64EB__ |
| /* For big-endian, carry propagation (if the final byte in the |
| string is 0x01) means we cannot use has_nul1/2 directly. |
| Since we expect strings to be small and early-exit, |
| byte-swap the data now so has_null1/2 will be correct. */ |
| rev data1, data1 |
| rev data2, data2 |
| #endif |
| sub tmp1, data1, zeroones |
| orr tmp2, data1, REP8_7f |
| sub tmp3, data2, zeroones |
| orr tmp4, data2, REP8_7f |
| bics has_nul1, tmp1, tmp2 |
| bic has_nul2, tmp3, tmp4 |
| ccmp has_nul2, 0, 0, eq |
| beq L(main_loop_entry) |
| |
| /* Enter with C = has_nul1 == 0. */ |
| csel has_nul1, has_nul1, has_nul2, cc |
| mov len, 8 |
| rev has_nul1, has_nul1 |
| clz tmp1, has_nul1 |
| csel len, xzr, len, cc |
| add len, len, tmp1, lsr 3 |
| ret |
| |
| /* The inner loop processes 32 bytes per iteration and uses the fast |
| NUL check. If we encounter non-ASCII characters, use a second |
| loop with the accurate NUL check. */ |
| .p2align 4 |
| L(main_loop_entry): |
| bic src, srcin, 15 |
| sub src, src, 16 |
| L(main_loop): |
| ldp data1, data2, [src, 32]! |
| L(page_cross_entry): |
| sub tmp1, data1, zeroones |
| sub tmp3, data2, zeroones |
| orr tmp2, tmp1, tmp3 |
| tst tmp2, zeroones, lsl 7 |
| bne 1f |
| ldp data1, data2, [src, 16] |
| sub tmp1, data1, zeroones |
| sub tmp3, data2, zeroones |
| orr tmp2, tmp1, tmp3 |
| tst tmp2, zeroones, lsl 7 |
| beq L(main_loop) |
| add src, src, 16 |
| 1: |
| /* The fast check failed, so do the slower, accurate NUL check. */ |
| orr tmp2, data1, REP8_7f |
| orr tmp4, data2, REP8_7f |
| bics has_nul1, tmp1, tmp2 |
| bic has_nul2, tmp3, tmp4 |
| ccmp has_nul2, 0, 0, eq |
| beq L(nonascii_loop) |
| |
| /* Enter with C = has_nul1 == 0. */ |
| L(tail): |
| #ifdef __AARCH64EB__ |
| /* For big-endian, carry propagation (if the final byte in the |
| string is 0x01) means we cannot use has_nul1/2 directly. The |
| easiest way to get the correct byte is to byte-swap the data |
| and calculate the syndrome a second time. */ |
| csel data1, data1, data2, cc |
| rev data1, data1 |
| sub tmp1, data1, zeroones |
| orr tmp2, data1, REP8_7f |
| bic has_nul1, tmp1, tmp2 |
| #else |
| csel has_nul1, has_nul1, has_nul2, cc |
| #endif |
| sub len, src, srcin |
| rev has_nul1, has_nul1 |
| add tmp2, len, 8 |
| clz tmp1, has_nul1 |
| csel len, len, tmp2, cc |
| add len, len, tmp1, lsr 3 |
| ret |
| |
| L(nonascii_loop): |
| ldp data1, data2, [src, 16]! |
| sub tmp1, data1, zeroones |
| orr tmp2, data1, REP8_7f |
| sub tmp3, data2, zeroones |
| orr tmp4, data2, REP8_7f |
| bics has_nul1, tmp1, tmp2 |
| bic has_nul2, tmp3, tmp4 |
| ccmp has_nul2, 0, 0, eq |
| bne L(tail) |
| ldp data1, data2, [src, 16]! |
| sub tmp1, data1, zeroones |
| orr tmp2, data1, REP8_7f |
| sub tmp3, data2, zeroones |
| orr tmp4, data2, REP8_7f |
| bics has_nul1, tmp1, tmp2 |
| bic has_nul2, tmp3, tmp4 |
| ccmp has_nul2, 0, 0, eq |
| beq L(nonascii_loop) |
| b L(tail) |
| |
| /* Load 16 bytes from [srcin & ~15] and force the bytes that precede |
| srcin to 0x7f, so we ignore any NUL bytes before the string. |
| Then continue in the aligned loop. */ |
| L(page_cross): |
| bic src, srcin, 15 |
| ldp data1, data2, [src] |
| lsl tmp1, srcin, 3 |
| mov tmp4, -1 |
| #ifdef __AARCH64EB__ |
| /* Big-endian. Early bytes are at MSB. */ |
| lsr tmp1, tmp4, tmp1 /* Shift (tmp1 & 63). */ |
| #else |
| /* Little-endian. Early bytes are at LSB. */ |
| lsl tmp1, tmp4, tmp1 /* Shift (tmp1 & 63). */ |
| #endif |
| orr tmp1, tmp1, REP8_80 |
| orn data1, data1, tmp1 |
| orn tmp2, data2, tmp1 |
| tst srcin, 8 |
| csel data1, data1, tmp4, eq |
| csel data2, data2, tmp2, eq |
| b L(page_cross_entry) |
| |
| END (__strlen_aarch64) |