diff options
Diffstat (limited to 'string/aarch64/strlen.S')
| -rw-r--r-- | string/aarch64/strlen.S | 274 |
1 files changed, 134 insertions, 140 deletions
diff --git a/string/aarch64/strlen.S b/string/aarch64/strlen.S index 2293f73..a1b164a 100644 --- a/string/aarch64/strlen.S +++ b/string/aarch64/strlen.S @@ -1,84 +1,88 @@ /* - * strlen - calculate the length of a string + * strlen - calculate the length of a string. * - * Copyright (c) 2013, Arm Limited. + * Copyright (c) 2020, Arm Limited. * SPDX-License-Identifier: MIT */ /* Assumptions: * - * ARMv8-a, AArch64, unaligned accesses, min page size 4k. + * ARMv8-a, AArch64, Advanced SIMD, unaligned accesses. + * Not MTE compatible. */ #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 srcin x0 +#define len x0 + +#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 maskv v0 +#define maskd d0 +#define dataq1 q1 +#define dataq2 q2 +#define datav1 v1 +#define datav2 v2 +#define tmp x2 +#define tmpw w2 +#define synd x3 +#define shift x4 + +/* For the first 32 bytes, NUL detection works on the principle that + (X - 1) & (~X) & 0x80 (=> (X - 1) & ~(X | 0x7f)) is non-zero if a + byte is zero, and can be done in parallel across the entire word. */ #define REP8_01 0x0101010101010101 #define REP8_7f 0x7f7f7f7f7f7f7f7f -#define REP8_80 0x8080808080808080 + +/* 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. */ #ifdef TEST_PAGE_CROSS -# define MIN_PAGE_SIZE 15 +# define MIN_PAGE_SIZE 32 #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. */ +/* Core algorithm: + + Since strings are short on average, we check the first 32 bytes of the + string for a NUL character without aligning the string. In order to use + unaligned loads safely we must 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 will be repeatedly called on strings with the same length). + + If the string is longer than 32 bytes, align src so we don't need further + page cross checks, and process 32 bytes per iteration using a fast SIMD + loop. + + If the page cross check fails, we read 32 bytes from an aligned address, + and ignore any characters before the string. If it contains a NUL + character, return the length, if not, continue in the main loop. */ ENTRY (__strlen_aarch64) + PTR_ARG (0) and tmp1, srcin, MIN_PAGE_SIZE - 1 - mov zeroones, REP8_01 - cmp tmp1, MIN_PAGE_SIZE - 16 - b.gt L(page_cross) + cmp tmp1, MIN_PAGE_SIZE - 32 + b.hi L(page_cross) + + /* Look for a NUL byte in the first 16 bytes. */ ldp data1, data2, [srcin] + mov zeroones, REP8_01 + #ifdef __AARCH64EB__ /* For big-endian, carry propagation (if the final byte in the string is 0x01) means we cannot use has_nul1/2 directly. @@ -94,113 +98,103 @@ ENTRY (__strlen_aarch64) bics has_nul1, tmp1, tmp2 bic has_nul2, tmp3, tmp4 ccmp has_nul2, 0, 0, eq - beq L(main_loop_entry) + b.eq L(bytes16_31) - /* Enter with C = has_nul1 == 0. */ + /* Find the exact offset of the first NUL byte in the first 16 bytes + from the string start. 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 + clz tmp1, has_nul1 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] + .p2align 3 + /* Look for a NUL byte at offset 16..31 in the string. */ +L(bytes16_31): + ldp data1, data2, [srcin, 16] +#ifdef __AARCH64EB__ + rev data1, data1 + rev data2, data2 +#endif 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 + 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.eq L(loop_entry) - /* 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 + /* Find the exact offset of the first NUL byte at offset 16..31 from + the string start. Enter with C = has_nul1 == 0. */ csel has_nul1, has_nul1, has_nul2, cc -#endif - sub len, src, srcin + mov len, 24 rev has_nul1, has_nul1 - add tmp2, len, 8 + mov tmp3, 16 clz tmp1, has_nul1 - csel len, len, tmp2, cc + csel len, tmp3, len, 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) +L(loop_entry): + bic src, srcin, 31 - /* 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 + .p2align 5 +L(loop): + ldp dataq1, dataq2, [src, 32]! + uminp maskv.16b, datav1.16b, datav2.16b + uminp maskv.16b, maskv.16b, maskv.16b + cmeq maskv.8b, maskv.8b, 0 + fmov synd, maskd + cbz synd, L(loop) + + /* Low 32 bits of synd are non-zero if a NUL was found in datav1. */ + cmeq maskv.16b, datav1.16b, 0 + sub len, src, srcin + tst synd, 0xffffffff + b.ne 1f + cmeq maskv.16b, datav2.16b, 0 + add len, len, 16 +1: + /* Generate a bitmask and compute correct byte offset. */ #ifdef __AARCH64EB__ - /* Big-endian. Early bytes are at MSB. */ - lsr tmp1, tmp4, tmp1 /* Shift (tmp1 & 63). */ + bic maskv.8h, 0xf0 #else - /* Little-endian. Early bytes are at LSB. */ - lsl tmp1, tmp4, tmp1 /* Shift (tmp1 & 63). */ + bic maskv.8h, 0x0f, lsl 8 +#endif + umaxp maskv.16b, maskv.16b, maskv.16b + fmov synd, maskd +#ifndef __AARCH64EB__ + rbit synd, synd #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) + clz tmp, synd + add len, len, tmp, lsr 2 + ret + + .p2align 4 + +L(page_cross): + bic src, srcin, 31 + mov tmpw, 0x0c03 + movk tmpw, 0xc030, lsl 16 + ld1 {datav1.16b, datav2.16b}, [src] + dup maskv.4s, tmpw + cmeq datav1.16b, datav1.16b, 0 + cmeq datav2.16b, datav2.16b, 0 + and datav1.16b, datav1.16b, maskv.16b + and datav2.16b, datav2.16b, maskv.16b + addp maskv.16b, datav1.16b, datav2.16b + addp maskv.16b, maskv.16b, maskv.16b + fmov synd, maskd + lsl shift, srcin, 1 + lsr synd, synd, shift + cbz synd, L(loop) + + rbit synd, synd + clz len, synd + lsr len, len, 1 + ret END (__strlen_aarch64) |