c - 什么是复制未对齐位数组的高效算法？

Question

过去我不得不这样做很多次，但我对结果从未感到满意。

谁能建议一种将连续位数组从源复制到目标的快速方法，其中源和目标可能未在方便的处理器边界上对齐(右移)？

如果源和目标都没有对齐，问题可以很快变成只有其中一个没有对齐(在第一个副本之后)。

作为起点，我的代码最终不可避免地看起来像下面这样(未经测试，忽略副作用，这只是一个即兴的例子):

const char mask[8] = { 1, 3, 7, 15, 31, 63, 127, 255 };
/* Assume:
 * - destination is already zeroed,
 * - offsets are right shifts
 * - bits to copy is big (> 32 say)
 */
int bitarray_copy(char * src, int src_bit_offset, int src_bit_len,
                  char * dst, int dst_bit_offset) {
    if (src_bit_offset == dst_bit_offset) { /* Not very interesting */ 
    } else {
        int bit_diff_offset = src_bit_offset - dst_bit_offset; /* assume positive */
        int loop_count;
        char c;
        char mask_val = mask[bit_diff_offset];

        /* Get started, line up the destination. */
        c  = (*src++ << bit_diff_offset) | ((*src >> (8 - bit_diff_offset)) & mask_val);
        c &= mask[8-dst_bit_offset];

        *dst++ |= c;

        src_bit_len -= 8 - dst_bit_offset;
        loop_count = src_bit_len >> 3;

        while (--loop_count >= 0) 
            * dst ++ = (*src++ << bit_diff_offset) | ((*src >> (8 - bit_diff_offset)) & mask_val);

        /* Trailing tail copy etc ... */
        if (src_bit_len % 8) /* ... */
    }
}

(其实这个比我之前做的好多了，看起来还不错)

Answer 1

这就是我最终所做的。 (编辑 于 2014 年 8 月 21 日更改为单个位复制错误。)

#include <limits.h>
#include <string.h>
#include <stddef.h>

#define PREPARE_FIRST_COPY()                                      \
    do {                                                          \
    if (src_len >= (CHAR_BIT - dst_offset_modulo)) {              \
        *dst     &= reverse_mask[dst_offset_modulo];              \
        src_len -= CHAR_BIT - dst_offset_modulo;                  \
    } else {                                                      \
        *dst     &= reverse_mask[dst_offset_modulo]               \
              | reverse_mask_xor[dst_offset_modulo + src_len];    \
         c       &= reverse_mask[dst_offset_modulo + src_len];    \
        src_len = 0;                                              \
    } } while (0)


static void
bitarray_copy(const unsigned char *src_org, int src_offset, int src_len,
                    unsigned char *dst_org, int dst_offset)
{
    static const unsigned char mask[] =
        { 0x00, 0x01, 0x03, 0x07, 0x0f, 0x1f, 0x3f, 0x7f, 0xff };
    static const unsigned char reverse_mask[] =
        { 0x00, 0x80, 0xc0, 0xe0, 0xf0, 0xf8, 0xfc, 0xfe, 0xff };
    static const unsigned char reverse_mask_xor[] =
        { 0xff, 0x7f, 0x3f, 0x1f, 0x0f, 0x07, 0x03, 0x01, 0x00 };

    if (src_len) {
        const unsigned char *src;
              unsigned char *dst;
        int                  src_offset_modulo,
                             dst_offset_modulo;

        src = src_org + (src_offset / CHAR_BIT);
        dst = dst_org + (dst_offset / CHAR_BIT);

        src_offset_modulo = src_offset % CHAR_BIT;
        dst_offset_modulo = dst_offset % CHAR_BIT;

        if (src_offset_modulo == dst_offset_modulo) {
            int              byte_len;
            int              src_len_modulo;
            if (src_offset_modulo) {
                unsigned char   c;

                c = reverse_mask_xor[dst_offset_modulo]     & *src++;

                PREPARE_FIRST_COPY();
                *dst++ |= c;
            }

            byte_len = src_len / CHAR_BIT;
            src_len_modulo = src_len % CHAR_BIT;

            if (byte_len) {
                memcpy(dst, src, byte_len);
                src += byte_len;
                dst += byte_len;
            }
            if (src_len_modulo) {
                *dst     &= reverse_mask_xor[src_len_modulo];
                *dst |= reverse_mask[src_len_modulo]     & *src;
            }
        } else {
            int             bit_diff_ls,
                            bit_diff_rs;
            int             byte_len;
            int             src_len_modulo;
            unsigned char   c;
            /*
             * Begin: Line things up on destination. 
             */
            if (src_offset_modulo > dst_offset_modulo) {
                bit_diff_ls = src_offset_modulo - dst_offset_modulo;
                bit_diff_rs = CHAR_BIT - bit_diff_ls;

                c = *src++ << bit_diff_ls;
                c |= *src >> bit_diff_rs;
                c     &= reverse_mask_xor[dst_offset_modulo];
            } else {
                bit_diff_rs = dst_offset_modulo - src_offset_modulo;
                bit_diff_ls = CHAR_BIT - bit_diff_rs;

                c = *src >> bit_diff_rs     &
                    reverse_mask_xor[dst_offset_modulo];
            }
            PREPARE_FIRST_COPY();
            *dst++ |= c;

            /*
             * Middle: copy with only shifting the source. 
             */
            byte_len = src_len / CHAR_BIT;

            while (--byte_len >= 0) {
                c = *src++ << bit_diff_ls;
                c |= *src >> bit_diff_rs;
                *dst++ = c;
            }

            /*
             * End: copy the remaing bits; 
             */
            src_len_modulo = src_len % CHAR_BIT;
            if (src_len_modulo) {
                c = *src++ << bit_diff_ls;
                c |= *src >> bit_diff_rs;
                c     &= reverse_mask[src_len_modulo];

                *dst     &= reverse_mask_xor[src_len_modulo];
                *dst |= c;
            }
        }
    }
}