duff.c
author Markus Bröker <mbroeker@largo.dyndns.tv>
Mon, 07 Jun 2010 23:20:47 +0200
changeset 132 54a04f5f141d
parent 63 5a82f89d607e
permissions -rw-r--r--
Adjusted to show side effects and their handling When dealing with static buffers, successive calls point to the same memory location and overwrite formers ones... committer: Markus Bröker <mbroeker@largo.homelinux.org>
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/**
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 * Duff's Device
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 * fast copy algorithm
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 * performs 8 times faster on long strings.
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 */
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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char *duffcopy (char *to, char *from, size_t len)
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{
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    size_t n, pos = 0;
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    n = (len + 7) / 8;
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    switch (len % 8) {
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    case 0:
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        do {
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            to[pos] = from[pos];
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            pos++;
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    case 7:
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            to[pos] = from[pos];
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            pos++;
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    case 6:
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            to[pos] = from[pos];
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            pos++;
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    case 5:
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            to[pos] = from[pos];
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            pos++;
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    case 4:
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            to[pos] = from[pos];
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            pos++;
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    case 3:
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            to[pos] = from[pos];
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            pos++;
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    case 2:
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            to[pos] = from[pos];
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            pos++;
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    case 1:
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            to[pos] = from[pos];
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            pos++;
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        } while (--n > 0);
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    }
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    return to;
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}
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int main (int argc, char **argv)
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{
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    char *to, *from;
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    size_t len;
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    if (argc != 2) {
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        printf ("Usage: %s <string>\n", argv[0]);
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        return EXIT_FAILURE;
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    }
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    from = argv[1];
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    len = strlen (from) + 1;
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    if ((to = malloc (len)) == NULL) {
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        perror ("MALLOC");
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        return EXIT_FAILURE;
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    }
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    printf ("DUFF: %s\n", duffcopy (to, from, len));
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    if (to != NULL) {
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        free (to);
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    }
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    return EXIT_SUCCESS;
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}