Not only is this an obvious optimization:

benchmark                           old MB/s     new MB/s     speedup
BenchmarkMemmove1-4                 35.35        29.65        0.84x
BenchmarkMemmove2-4                 63.78        52.53        0.82x
BenchmarkMemmove3-4                 89.72        73.96        0.82x
BenchmarkMemmove4-4                 109.94       95.73        0.87x
BenchmarkMemmove5-4                 127.60       112.80       0.88x
BenchmarkMemmove6-4                 143.59       126.67       0.88x
BenchmarkMemmove7-4                 157.90       138.92       0.88x
BenchmarkMemmove8-4                 167.18       231.81       1.39x
BenchmarkMemmove9-4                 175.23       252.07       1.44x
BenchmarkMemmove10-4                165.68       261.10       1.58x
BenchmarkMemmove11-4                174.43       263.31       1.51x
BenchmarkMemmove12-4                180.76       267.56       1.48x
BenchmarkMemmove13-4                189.06       284.93       1.51x
BenchmarkMemmove14-4                186.31       284.72       1.53x
BenchmarkMemmove15-4                195.75       281.62       1.44x
BenchmarkMemmove16-4                202.96       439.23       2.16x
BenchmarkMemmove32-4                264.77       775.77       2.93x
BenchmarkMemmove64-4                306.81       1209.64      3.94x
BenchmarkMemmove128-4               357.03       1515.41      4.24x
BenchmarkMemmove256-4               380.77       2066.01      5.43x
BenchmarkMemmove512-4               385.05       2556.45      6.64x
BenchmarkMemmove1024-4              381.23       2804.10      7.36x
BenchmarkMemmove2048-4              379.06       2814.83      7.43x
BenchmarkMemmove4096-4              387.43       3064.96      7.91x
BenchmarkMemmoveUnaligned1-4        28.91        25.40        0.88x
BenchmarkMemmoveUnaligned2-4        56.13        47.56        0.85x
BenchmarkMemmoveUnaligned3-4        74.32        69.31        0.93x
BenchmarkMemmoveUnaligned4-4        97.02        83.58        0.86x
BenchmarkMemmoveUnaligned5-4        110.17       103.62       0.94x
BenchmarkMemmoveUnaligned6-4        124.95       113.26       0.91x
BenchmarkMemmoveUnaligned7-4        142.37       130.82       0.92x
BenchmarkMemmoveUnaligned8-4        151.20       205.64       1.36x
BenchmarkMemmoveUnaligned9-4        166.97       215.42       1.29x
BenchmarkMemmoveUnaligned10-4       148.49       221.22       1.49x
BenchmarkMemmoveUnaligned11-4       159.47       239.57       1.50x
BenchmarkMemmoveUnaligned12-4       163.52       247.32       1.51x
BenchmarkMemmoveUnaligned13-4       167.55       256.54       1.53x
BenchmarkMemmoveUnaligned14-4       175.12       251.03       1.43x
BenchmarkMemmoveUnaligned15-4       192.10       267.13       1.39x
BenchmarkMemmoveUnaligned16-4       190.76       378.87       1.99x
BenchmarkMemmoveUnaligned32-4       259.02       562.98       2.17x
BenchmarkMemmoveUnaligned64-4       317.72       842.44       2.65x
BenchmarkMemmoveUnaligned128-4      355.43       1274.49      3.59x
BenchmarkMemmoveUnaligned256-4      378.17       1815.74      4.80x
BenchmarkMemmoveUnaligned512-4      362.15       2180.81      6.02x
BenchmarkMemmoveUnaligned1024-4     376.07       2453.58      6.52x
BenchmarkMemmoveUnaligned2048-4     381.66       2568.32      6.73x
BenchmarkMemmoveUnaligned4096-4     398.51       2669.36      6.70x
BenchmarkMemclr5-4                  113.83       107.93       0.95x
BenchmarkMemclr16-4                 223.84       389.63       1.74x
BenchmarkMemclr64-4                 421.99       1209.58      2.87x
BenchmarkMemclr256-4                525.94       2411.58      4.59x
BenchmarkMemclr4096-4               581.66       4372.20      7.52x
BenchmarkMemclr65536-4              565.84       4747.48      8.39x
BenchmarkGoMemclr5-4                194.63       160.31       0.82x
BenchmarkGoMemclr16-4               295.30       630.07       2.13x
BenchmarkGoMemclr64-4               480.24       1884.03      3.92x
BenchmarkGoMemclr256-4              540.23       2926.49      5.42x

but it turns out that it's necessary to avoid the GC seeing partially written
pointers.

It's of course possible to be more sophisticated (using ldp/stp to move 16
bytes at a time in the core loop and unrolling the tail copying loops being
the obvious ideas) but I wanted something simple and (reasonably) obviously
correct.

Fixes #12552

Change-Id: Iaeaf8a812cd06f4747ba2f792de1ded738890735
Reviewed-on: https://go-review.googlesource.com/14813Reviewed-by: Austin Clements <austin@google.com>