U32 reverseBuffer( register U64 *p, const U32 start, const U32 size ) {
    register U32 i, swaps = 0;
    const U32 half = size >> 1;

    for( i = start; i < half; ++i ) {
        swapElems( p, i, size -1 -i );
        ++swaps;
    }
    return swaps;
}

U32 xchgBufferV( U64 *p, U32 size, U32 offset ) {
    register U32 swaps = 0;
    swaps += reverseBuffer( p, 0, size );
    swaps += reverseBuffer( p, 0, size - offset );
    swaps += reverseBuffer( p + (size - offset), 0, offset );
    return swaps;
}