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Diffstat (limited to 'string/aarch64/strnlen.S')
| -rw-r--r-- | string/aarch64/strnlen.S | 160 |
1 files changed, 160 insertions, 0 deletions
diff --git a/string/aarch64/strnlen.S b/string/aarch64/strnlen.S new file mode 100644 index 0000000..b02c846 --- /dev/null +++ b/string/aarch64/strnlen.S @@ -0,0 +1,160 @@ +/* + * strnlen - calculate the length of a string with limit. + * + * Copyright (c) 2013, Arm Limited. + * SPDX-License-Identifier: MIT + */ + +/* Assumptions: + * + * ARMv8-a, AArch64 + */ + +/* Arguments and results. */ +#define srcin x0 +#define len x0 +#define limit x1 + +/* Locals and temporaries. */ +#define src x2 +#define data1 x3 +#define data2 x4 +#define data2a x5 +#define has_nul1 x6 +#define has_nul2 x7 +#define tmp1 x8 +#define tmp2 x9 +#define tmp3 x10 +#define tmp4 x11 +#define zeroones x12 +#define pos x13 +#define limit_wd x14 + + .macro def_fn f p2align=0 + .text + .p2align \p2align + .global \f + .type \f, %function +\f: + .endm + +#define REP8_01 0x0101010101010101 +#define REP8_7f 0x7f7f7f7f7f7f7f7f +#define REP8_80 0x8080808080808080 + + .text + .p2align 6 +.Lstart: + /* Pre-pad to ensure critical loop begins an icache line. */ + .rep 7 + nop + .endr + /* Put this code here to avoid wasting more space with pre-padding. */ +.Lhit_limit: + mov len, limit + ret + +def_fn __strnlen_aarch64 + cbz limit, .Lhit_limit + mov zeroones, #REP8_01 + bic src, srcin, #15 + ands tmp1, srcin, #15 + b.ne .Lmisaligned + /* Calculate the number of full and partial words -1. */ + sub limit_wd, limit, #1 /* Limit != 0, so no underflow. */ + lsr limit_wd, limit_wd, #4 /* Convert to Qwords. */ + + /* 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. */ + /* The inner loop deals with two Dwords at a time. This has a + slightly higher start-up cost, but we should win quite quickly, + especially on cores with a high number of issue slots per + cycle, as we get much better parallelism out of the operations. */ + + /* Start of critial section -- keep to one 64Byte cache line. */ +.Lloop: + ldp data1, data2, [src], #16 +.Lrealigned: + sub tmp1, data1, zeroones + orr tmp2, data1, #REP8_7f + sub tmp3, data2, zeroones + orr tmp4, data2, #REP8_7f + bic has_nul1, tmp1, tmp2 + bic has_nul2, tmp3, tmp4 + subs limit_wd, limit_wd, #1 + orr tmp1, has_nul1, has_nul2 + ccmp tmp1, #0, #0, pl /* NZCV = 0000 */ + b.eq .Lloop + /* End of critical section -- keep to one 64Byte cache line. */ + + orr tmp1, has_nul1, has_nul2 + cbz tmp1, .Lhit_limit /* No null in final Qword. */ + + /* We know there's a null in the final Qword. The easiest thing + to do now is work out the length of the string and return + MIN (len, limit). */ + + sub len, src, srcin + cbz has_nul1, .Lnul_in_data2 +#ifdef __AARCH64EB__ + mov data2, data1 +#endif + sub len, len, #8 + mov has_nul2, has_nul1 +.Lnul_in_data2: +#ifdef __AARCH64EB__ + /* For big-endian, carry propagation (if the final byte in the + string is 0x01) means we cannot use has_nul directly. The + easiest way to get the correct byte is to byte-swap the data + and calculate the syndrome a second time. */ + rev data2, data2 + sub tmp1, data2, zeroones + orr tmp2, data2, #REP8_7f + bic has_nul2, tmp1, tmp2 +#endif + sub len, len, #8 + rev has_nul2, has_nul2 + clz pos, has_nul2 + add len, len, pos, lsr #3 /* Bits to bytes. */ + cmp len, limit + csel len, len, limit, ls /* Return the lower value. */ + ret + +.Lmisaligned: + /* Deal with a partial first word. + We're doing two things in parallel here; + 1) Calculate the number of words (but avoiding overflow if + limit is near ULONG_MAX) - to do this we need to work out + limit + tmp1 - 1 as a 65-bit value before shifting it; + 2) Load and mask the initial data words - we force the bytes + before the ones we are interested in to 0xff - this ensures + early bytes will not hit any zero detection. */ + sub limit_wd, limit, #1 + neg tmp4, tmp1 + cmp tmp1, #8 + + and tmp3, limit_wd, #15 + lsr limit_wd, limit_wd, #4 + mov tmp2, #~0 + + ldp data1, data2, [src], #16 + lsl tmp4, tmp4, #3 /* Bytes beyond alignment -> bits. */ + add tmp3, tmp3, tmp1 + +#ifdef __AARCH64EB__ + /* Big-endian. Early bytes are at MSB. */ + lsl tmp2, tmp2, tmp4 /* Shift (tmp1 & 63). */ +#else + /* Little-endian. Early bytes are at LSB. */ + lsr tmp2, tmp2, tmp4 /* Shift (tmp1 & 63). */ +#endif + add limit_wd, limit_wd, tmp3, lsr #4 + + orr data1, data1, tmp2 + orr data2a, data2, tmp2 + + csinv data1, data1, xzr, le + csel data2, data2, data2a, le + b .Lrealigned + .size __strnlen_aarch64, . - .Lstart /* Include pre-padding in size. */ |