libc/string/memmove.c

/*
 * Copyright (C) 2008 The Android Open Source Project
 * 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.
 *
 * 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 OWNER 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.
 */
#include <string.h>

void *memmove(void *dst, const void *src, size_t n)
{
  const char *p = src;
  char *q = dst;
  /* We can use the optimized memcpy if the destination is below the
   * source (i.e. q < p), or if it is completely over it (i.e. q >= p+n).
   */
  if (__builtin_expect((q < p) || ((size_t)(q - p) >= n), 1)) {
    return memcpy(dst, src, n);
  } else {
#define PRELOAD_DISTANCE 64
      /* a semi-optimized memmove(). we're preloading the src and dst buffers
       * as we go */
    size_t c0, c1, i;
    p += n;
    q += n;
    /* note: we preload the destination as well, because the 1-byte at a time
     * copy below doesn't take advantage of the write-buffer, we need
     * to use the cache instead as a poor man's write-combiner */
    __builtin_prefetch(p-1);
    __builtin_prefetch(q-1);
    if (PRELOAD_DISTANCE > 32) {
        __builtin_prefetch(p-(32+1));
        __builtin_prefetch(q-(32+1));
    }
    /* do the prefetech as soon as possible, prevent the compiler to
     * reorder the instructions above the prefetch */
    asm volatile("":::"memory");
    c0 = n & 0x1F; /* cache-line is 32 bytes */
    c1 = n >> 5;
    while ( c1-- ) {
        /* ARMv6 can have up to 3 memory access outstanding */
      __builtin_prefetch(p - (PRELOAD_DISTANCE+1));
      __builtin_prefetch(q - (PRELOAD_DISTANCE+1));
      asm volatile("":::"memory");
      for (i=0 ; i<32 ; i++) {
        *--q = *--p;
      }
    }
    while ( c0-- ) {
      *--q = *--p;
    }
  }

  return dst;
}