[can't edit previous post]

In fact replacing the nopw that is used for alignment by a prefetch instruction gives me something slightly even faster:

  00000000004004e0 <prefetchit>:
  4004e0:       31 c0                   xor    %eax,%eax
  4004e2:       0f 18 0d 87 04 20 00    prefetcht0 0x200487(%rip)        # 600970 <counter>
  4004e9:       48 8b 15 80 04 20 00    mov    0x200480(%rip),%rdx        # 600970 <counter>
  4004f0:       48 01 c2                add    %rax,%rdx
  4004f3:       48 83 c0 01             add    $0x1,%rax
  4004f7:       48 3d 00 84 d7 17       cmp    $0x17d78400,%rax
  4004fd:       48 89 15 6c 04 20 00    mov    %rdx,0x20046c(%rip)        # 600970 <counter>
  400504:       75 dc                   jne    4004e2 <prefetchit+0x2>
  400506:       f3 c3                   repz retq 
  400508:       0f 1f 84 00 00 00 00    nopl   0x0(%rax,%rax,1)
  40050f:       00 

     2,445,505,116 cycles                    #    0.000 GHz                      ( +-  0.52% )
     2,800,857,971 instructions              #    1.15  insns per cycle          ( +-  0.00% )

       0.643010038 seconds time elapsed