Array append performance

[Walter Bright]

The bugzilla report on this suggests replacing []=[] with memcpy. But 
this benchmark shows this isn't a consistent win:

import std.c.string;

long timer()
{
     asm
     {   naked                   ;
         rdtsc                   ;
         ret                     ;
     }
}

void test1(byte[] a, byte[] b)
{
     a[] = b[];
}

void test2(byte[] a, byte[] b)
{
     memcpy(a.ptr, b.ptr, a.length);
}

void main()
{
     for (int i = 4; i < 100_000_000; i *= 2)
     {
         auto a = new byte[i];
         auto b = new byte[i];

         auto start = timer();
         test1(a, b);
         auto end = timer();
         auto r1 = end - start;

         start = timer();
         test2(a, b);
         end = timer();
         auto r2 = end - start;

         printf("i: %8d,\t[]=[]: %8lld,\tmemcpy: %8lld\n", i, r1, r2);
     }
}

Running this program produces:

i:        4,    []=[]:      144,        memcpy:      568
i:        8,    []=[]:      144,        memcpy:      300
i:       16,    []=[]:      172,        memcpy:      324
i:       32,    []=[]:      204,        memcpy:      344
i:       64,    []=[]:      288,        memcpy:      276
i:      128,    []=[]:      288,        memcpy:      272
i:      256,    []=[]:      352,        memcpy:      364
i:      512,    []=[]:      372,        memcpy:      424
i:     1024,    []=[]:      552,        memcpy:      564
i:     2048,    []=[]:      684,        memcpy:     1384
i:     4096,    []=[]:     1344,        memcpy:     1772
i:     8192,    []=[]:     2900,        memcpy:     3216
i:    16384,    []=[]:     5292,        memcpy:     5472
i:    32768,    []=[]:    11496,        memcpy:    10388
i:    65536,    []=[]:    29484,        memcpy:    27480
i:   131072,    []=[]:   110464,        memcpy:    67784
i:   262144,    []=[]:   655580,        memcpy:   562400
i:   524288,    []=[]:  1204124,        memcpy:  1107256
i:  1048576,    []=[]:  2364588,        memcpy:  2272552
i:  2097152,    []=[]:  4516440,        memcpy:  4417764
i:  4194304,    []=[]:  8996992,        memcpy:  8817176
i:  8388608,    []=[]: 20223908,        memcpy: 17717748
i: 16777216,    []=[]: 35774952,        memcpy: 36094652
i: 33554432,    []=[]: 71008068,        memcpy: 71246896
i: 67108864,    []=[]: 142982284,       memcpy: 145473300

There's not much of a consistent conclusion to be drawn from that.