/*
* strnlen - calculate the length of a string with limit.
*
* 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
*/
#include "../asmdefs.h"
/* 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
#define REP8_01 0x0101010101010101
#define REP8_7f 0x7f7f7f7f7f7f7f7f
#define REP8_80 0x8080808080808080
.text
.p2align 6
L(start):
/* 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. */
L(hit_limit):
mov len, limit
ret
ENTRY_ALIGN (__strnlen_aarch64, 0)
cbz limit, L(hit_limit)
mov zeroones, #REP8_01
bic src, srcin, #15
ands tmp1, srcin, #15
b.ne L(misaligned)
/* 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 critical section -- keep to one 64Byte cache line. */
L(loop):
ldp data1, data2, [src], #16
L(realigned):
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 L(loop)
/* End of critical section -- keep to one 64Byte cache line. */
orr tmp1, has_nul1, has_nul2
cbz tmp1, L(hit_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, L(nul_in_data2)
#ifdef __AARCH64EB__
mov data2, data1
#endif
sub len, len, #8
mov has_nul2, has_nul1
L(nul_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
L(misaligned):
/* 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 L(realigned)
END (__strnlen_aarch64)
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