And we have a (provisional) winner! (Needs more testing.)

Your (my C version of) tail recursive algorithm beats (my C version of) roboticus' implementation of his/graff's/hdb's/bitingduck's iterative algorithm on a 3GB buffer with a 2GB offset.

The output shows 10 elements at the beginning; either side of the offset; and at the end; before and after the swap:

C:\test\C>bufswap 402653184 268435456
size:402653184 offset;268435456
[   0    1    2    3    4    5    6    7    8    9 ...268435446 268435
+447 268435448 268435449 268435450 268435451 268435452 268435453 26843
+5454 268435455 268435456 268435457 268435458 268435459 268435460 2684
+35461 268435462 268435463 268435464 268435465 ...402653174 402653175 
+402653176 402653177 402653178 402653179 402653180 402653181 402653182
+ 402653183 ]
[268435456 268435457 268435458 268435459 268435460 268435461 268435462
+ 268435463 268435464 268435465 ...134217718 134217719 134217720 13421
+7721 134217722 134217723 134217724 134217725 134217726 134217727 1342
+17728 134217729 134217730 134217731 134217732 134217733 134217734 134
+217735 134217736 134217737 ...268435446 268435447 268435448 268435449
+ 268435450 268435451 268435452 268435453 268435454 268435455 ]
iterative took 3.217107510

[   0    1    2    3    4    5    6    7    8    9 ...268435446 268435
+447 268435448 268435449 268435450 268435451 268435452 268435453 26843
+5454 268435455 268435456 268435457 268435458 268435459 268435460 2684
+35461 268435462 268435463 268435464 268435465 ...402653174 402653175 
+402653176 402653177 402653178 402653179 402653180 402653181 402653182
+ 402653183 ]
[268435456 268435457 268435458 268435459 268435460 268435461 268435462
+ 268435463 268435464 268435465 ...134217718 134217719 134217720 13421
+7721 134217722 134217723 134217724 134217725 134217726 134217727 1342
+17728 134217729 134217730 134217731 134217732 134217733 134217734 134
+217735 134217736 134217737 ...268435446 268435447 268435448 268435449
+ 268435450 268435451 268435452 268435453 268435454 268435455 ]
recursive took 2.445034532
[download]

The C code (needs some sanity checks):

/* Drawn from roboticus' (1118303) & hdb's(1118256) iterative implemen
+tations (Algorithm also described by graff & bitingduck) */
void xlateBuffersI( register U64 *p, const U32 size, const U32 offset 
+) {
    U32 register cnt = size, dst = 0, cmp = 0;
    U64 register tmp = p[ dst ];

    assert( offset < size );
    assert( ( (U64)p & 7ull ) == 0 );

    while( cnt-- ) {
        U32 register src = ( dst + offset ) % size;
        if( src == cmp ) {
            p[ dst ] = tmp;
            dst = ++cmp;
            tmp = p[ dst ];
        }
        else {
            p[ dst ] = p[ src ];
            dst = src;
        }
    }
    return;
}

__forceinline void swapElems( register U64 *p, const U32 x, const U32 
+y ) {
    register U64 temp = p[ x ];
    p[ x ] = p[ y ];
    p[ y ] = temp;
}
/* Adapted from anonymonk's post 1118262 */
void xlateBuffersR( U64 *p, U32 size, U32 x, U32 y ) {
    U32 i, savedY = y;

    if( y == size ) return;

    for( i = x; i < savedY; ++i ) {
        if( y == size ) y = savedY;
        swapElems( p, x++, y++ );
    }
    xlateBuffersR( p, size, x, y );
}
[download]