I think the method I proposed above would look something like this, if rendered in a manner similar to the snippet in the OP -- but please just think of this as pseudo-code (and take it with a good bit of salt), since I'm not as fluent or comfortable as I once was with the more C-like syntax you used up there, and I don't have the means to test this.

void swapBufferChunks( U8 *buf, U32 start1, U32 start2, U32 bufsize ) 
+{
    register U64 *p1 = (U64*)&buf[ start1 ];
    register U64 *p2 = (U64*)&buf[ start2 ];
    register U64 temp;

    shift_size = p2 - p1;
    end_buf = p1 + bufsize;   # assuming bufsize is byte count
    item_count = bufsize / 8; # assuming elements are 8-bytes each
    iter_count = 0;

    bgn_ofs = p1;
    temp = *p1;

    while ( iter_count < item_count ) {
        if ( p2 = bgn_ofs ) {
            *p1 = temp;
            iter_count++;
            last if ( iter_count == item_count ); # break out of while
+ loop as soon as this is true
            p1++;
            p2++;
            bgn_ofs = p1;
            temp = *p1;
        }
        *p1 = *p2;
        iter_count++;
        p1 = p2;
        p2 += shift_size;
        p2 -= bufsize if ( p2 >= end_buf );
    }
}
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(updated include a line inside the while loop to increment iter_count - rather a stupid thing to forget.)

(Updating again to mention that my snippet is bound to have a problem when it hits the last iteration; I haven't tried to fix that in the snippet (left as an exercise for the reader) -- hint: must increment iter_count every time a value is assigned to *p1, and must exit the while loop as soon as iter_count reaches the value of item_count.

(final update: last line used to say ... if ( p2 > end_buf ) which was bound to be wrong. Hell, while I'm at. might as well fix the last-iteration problem too. Still, this sort of thing needs really careful testing, which I haven't done.)