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

/* Copyright (c) 2012-2013, 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. */

/* Assumptions:
 *
 * ARMv8-a, AArch64
 */

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

        .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

/* Parameters and result.  */
#define src1            x0
#define src2            x1
#define result          x0

/* Internal variables.  */
#define data1           x2
#define data1w          w2
#define data2           x3
#define data2w          w3
#define has_nul         x4
#define diff            x5
#define syndrome        x6
#define tmp1            x7
#define tmp2            x8
#define tmp3            x9
#define zeroones        x10
#define pos             x11

        /* Start of performance-critical section  -- one 64B cache line.  */
def_fn strcmp p2align=6
        eor     tmp1, src1, src2
        mov     zeroones, #REP8_01
        tst     tmp1, #7
        b.ne    .Lmisaligned8
        ands    tmp1, src1, #7
        b.ne    .Lmutual_align
        /* 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.  */
.Lloop_aligned:
        ldr     data1, [src1], #8
        ldr     data2, [src2], #8
.Lstart_realigned:
        sub     tmp1, data1, zeroones
        orr     tmp2, data1, #REP8_7f
        eor     diff, data1, data2      /* Non-zero if differences found.  */
        bic     has_nul, tmp1, tmp2     /* Non-zero if NUL terminator.  */
        orr     syndrome, diff, has_nul
        cbz     syndrome, .Lloop_aligned
        /* End of performance-critical section  -- one 64B cache line.  */

#ifndef __AARCH64EB__
        rev     syndrome, syndrome
        rev     data1, data1
        /* The MS-non-zero bit of the syndrome marks either the first bit
           that is different, or the top bit of the first zero byte.
           Shifting left now will bring the critical information into the
           top bits.  */
        clz     pos, syndrome
        rev     data2, data2
        lsl     data1, data1, pos
        lsl     data2, data2, pos
        /* But we need to zero-extend (char is unsigned) the value and then
           perform a signed 32-bit subtraction.  */
        lsr     data1, data1, #56
        sub     result, data1, data2, lsr #56
        ret
#else
        /* For big-endian we cannot use the trick with the syndrome value
           as carry-propagation can corrupt the upper bits if the trailing
           bytes in the string contain 0x01.  */
        /* However, if there is no NUL byte in the dword, we can generate
           the result directly.  We can't just subtract the bytes as the
           MSB might be significant.  */
        cbnz    has_nul, 1f
        cmp     data1, data2
        cset    result, ne
        cneg    result, result, lo
        ret
1:
        /* Re-compute the NUL-byte detection, using a byte-reversed value.  */
        rev     tmp3, data1
        sub     tmp1, tmp3, zeroones
        orr     tmp2, tmp3, #REP8_7f
        bic     has_nul, tmp1, tmp2
        rev     has_nul, has_nul
        orr     syndrome, diff, has_nul
        clz     pos, syndrome
        /* The MS-non-zero bit of the syndrome marks either the first bit
           that is different, or the top bit of the first zero byte.
           Shifting left now will bring the critical information into the
           top bits.  */
        lsl     data1, data1, pos
        lsl     data2, data2, pos
        /* But we need to zero-extend (char is unsigned) the value and then
           perform a signed 32-bit subtraction.  */
        lsr     data1, data1, #56
        sub     result, data1, data2, lsr #56
        ret
#endif

.Lmutual_align:
        /* Sources are mutually aligned, but are not currently at an
           alignment boundary.  Round down the addresses and then mask off
           the bytes that preceed the start point.  */
        bic     src1, src1, #7
        bic     src2, src2, #7
        lsl     tmp1, tmp1, #3          /* Bytes beyond alignment -> bits.  */
        ldr     data1, [src1], #8
        neg     tmp1, tmp1              /* Bits to alignment -64.  */
        ldr     data2, [src2], #8
        mov     tmp2, #~0
#ifdef __AARCH64EB__
        /* Big-endian.  Early bytes are at MSB.  */
        lsl     tmp2, tmp2, tmp1        /* Shift (tmp1 & 63).  */
#else
        /* Little-endian.  Early bytes are at LSB.  */
        lsr     tmp2, tmp2, tmp1        /* Shift (tmp1 & 63).  */
#endif
        orr     data1, data1, tmp2
        orr     data2, data2, tmp2
        b       .Lstart_realigned

.Lmisaligned8:
        /* We can do better than this.  */
        ldrb    data1w, [src1], #1
        ldrb    data2w, [src2], #1
        cmp     data1w, #1
        ccmp    data1w, data2w, #0, cs  /* NZCV = 0b0000.  */
        b.eq    .Lmisaligned8
        sub     result, data1, data2
        ret
        .size   strcmp, .-strcmp

#endif