source: trunk/libs/newlib/src/newlib/libc/machine/aarch64/strnlen.S @ 444

/* strnlen - calculate the length of a string with limit.

   Copyright (c) 2013, Linaro Limited
   All rights reserved.

   Redistribution and use in source and binary forms, with or without
   modification, are permitted provided that the following conditions are met:
       * Redistributions of source code must retain the above copyright
         notice, this list of conditions and the following disclaimer.
       * Redistributions in binary form must reproduce the above copyright
         notice, this list of conditions and the following disclaimer in the
         documentation and/or other materials provided with the distribution.
       * Neither the name of the Linaro nor the
         names of its contributors may be used to endorse or promote products
         derived from this software without specific prior written permission.

   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
   A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
   HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
   SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
   LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
   DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
   THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
   (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
   OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. */

#if (defined (__OPTIMIZE_SIZE__) || defined (PREFER_SIZE_OVER_SPEED))
/* See strlen-stub.c  */
#else

/* 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
        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, . - .Lstart    /* Include pre-padding in size.  */

#endif