root/arch/arm64/lib/strnlen.S

   1 /* SPDX-License-Identifier: GPL-2.0-only */
   2 /*
   3  * Copyright (C) 2013 ARM Ltd.
   4  * Copyright (C) 2013 Linaro.
   5  *
   6  * This code is based on glibc cortex strings work originally authored by Linaro
   7  * be found @
   8  *
   9  * http://bazaar.launchpad.net/~linaro-toolchain-dev/cortex-strings/trunk/
  10  * files/head:/src/aarch64/
  11  */
  12 
  13 #include <linux/linkage.h>
  14 #include <asm/assembler.h>
  15 
  16 /*
  17  * determine the length of a fixed-size string
  18  *
  19  * Parameters:
  20  *      x0 - const string pointer
  21  *      x1 - maximal string length
  22  * Returns:
  23  *      x0 - the return length of specific string
  24  */
  25 
  26 /* Arguments and results.  */
  27 srcin           .req    x0
  28 len             .req    x0
  29 limit           .req    x1
  30 
  31 /* Locals and temporaries.  */
  32 src             .req    x2
  33 data1           .req    x3
  34 data2           .req    x4
  35 data2a          .req    x5
  36 has_nul1        .req    x6
  37 has_nul2        .req    x7
  38 tmp1            .req    x8
  39 tmp2            .req    x9
  40 tmp3            .req    x10
  41 tmp4            .req    x11
  42 zeroones        .req    x12
  43 pos             .req    x13
  44 limit_wd        .req    x14
  45 
  46 #define REP8_01 0x0101010101010101
  47 #define REP8_7f 0x7f7f7f7f7f7f7f7f
  48 #define REP8_80 0x8080808080808080
  49 
  50 WEAK(strnlen)
  51         cbz     limit, .Lhit_limit
  52         mov     zeroones, #REP8_01
  53         bic     src, srcin, #15
  54         ands    tmp1, srcin, #15
  55         b.ne    .Lmisaligned
  56         /* Calculate the number of full and partial words -1.  */
  57         sub     limit_wd, limit, #1 /* Limit != 0, so no underflow.  */
  58         lsr     limit_wd, limit_wd, #4  /* Convert to Qwords.  */
  59 
  60         /*
  61         * NUL detection works on the principle that (X - 1) & (~X) & 0x80
  62         * (=> (X - 1) & ~(X | 0x7f)) is non-zero iff a byte is zero, and
  63         * can be done in parallel across the entire word.
  64         */
  65         /*
  66         * The inner loop deals with two Dwords at a time.  This has a
  67         * slightly higher start-up cost, but we should win quite quickly,
  68         * especially on cores with a high number of issue slots per
  69         * cycle, as we get much better parallelism out of the operations.
  70         */
  71 .Lloop:
  72         ldp     data1, data2, [src], #16
  73 .Lrealigned:
  74         sub     tmp1, data1, zeroones
  75         orr     tmp2, data1, #REP8_7f
  76         sub     tmp3, data2, zeroones
  77         orr     tmp4, data2, #REP8_7f
  78         bic     has_nul1, tmp1, tmp2
  79         bic     has_nul2, tmp3, tmp4
  80         subs    limit_wd, limit_wd, #1
  81         orr     tmp1, has_nul1, has_nul2
  82         ccmp    tmp1, #0, #0, pl    /* NZCV = 0000  */
  83         b.eq    .Lloop
  84 
  85         cbz     tmp1, .Lhit_limit   /* No null in final Qword.  */
  86 
  87         /*
  88         * We know there's a null in the final Qword. The easiest thing
  89         * to do now is work out the length of the string and return
  90         * MIN (len, limit).
  91         */
  92         sub     len, src, srcin
  93         cbz     has_nul1, .Lnul_in_data2
  94 CPU_BE( mov     data2, data1 )  /*perpare data to re-calculate the syndrome*/
  95 
  96         sub     len, len, #8
  97         mov     has_nul2, has_nul1
  98 .Lnul_in_data2:
  99         /*
 100         * For big-endian, carry propagation (if the final byte in the
 101         * string is 0x01) means we cannot use has_nul directly.  The
 102         * easiest way to get the correct byte is to byte-swap the data
 103         * and calculate the syndrome a second time.
 104         */
 105 CPU_BE( rev     data2, data2 )
 106 CPU_BE( sub     tmp1, data2, zeroones )
 107 CPU_BE( orr     tmp2, data2, #REP8_7f )
 108 CPU_BE( bic     has_nul2, tmp1, tmp2 )
 109 
 110         sub     len, len, #8
 111         rev     has_nul2, has_nul2
 112         clz     pos, has_nul2
 113         add     len, len, pos, lsr #3       /* Bits to bytes.  */
 114         cmp     len, limit
 115         csel    len, len, limit, ls     /* Return the lower value.  */
 116         ret
 117 
 118 .Lmisaligned:
 119         /*
 120         * Deal with a partial first word.
 121         * We're doing two things in parallel here;
 122         * 1) Calculate the number of words (but avoiding overflow if
 123         * limit is near ULONG_MAX) - to do this we need to work out
 124         * limit + tmp1 - 1 as a 65-bit value before shifting it;
 125         * 2) Load and mask the initial data words - we force the bytes
 126         * before the ones we are interested in to 0xff - this ensures
 127         * early bytes will not hit any zero detection.
 128         */
 129         ldp     data1, data2, [src], #16
 130 
 131         sub     limit_wd, limit, #1
 132         and     tmp3, limit_wd, #15
 133         lsr     limit_wd, limit_wd, #4
 134 
 135         add     tmp3, tmp3, tmp1
 136         add     limit_wd, limit_wd, tmp3, lsr #4
 137 
 138         neg     tmp4, tmp1
 139         lsl     tmp4, tmp4, #3  /* Bytes beyond alignment -> bits.  */
 140 
 141         mov     tmp2, #~0
 142         /* Big-endian.  Early bytes are at MSB.  */
 143 CPU_BE( lsl     tmp2, tmp2, tmp4 )      /* Shift (tmp1 & 63).  */
 144         /* Little-endian.  Early bytes are at LSB.  */
 145 CPU_LE( lsr     tmp2, tmp2, tmp4 )      /* Shift (tmp1 & 63).  */
 146 
 147         cmp     tmp1, #8
 148 
 149         orr     data1, data1, tmp2
 150         orr     data2a, data2, tmp2
 151 
 152         csinv   data1, data1, xzr, le
 153         csel    data2, data2, data2a, le
 154         b       .Lrealigned
 155 
 156 .Lhit_limit:
 157         mov     len, limit
 158         ret
 159 ENDPIPROC(strnlen)
 160 EXPORT_SYMBOL_NOKASAN(strnlen)