>I tend to the opposite view: those saying "do not do X" are in fact obligated to explain why X should be avoided. But perhaps this is just a difference in worldview.

For me it depends on the context. Here aligned access makes more sense so unaligned should be defended.

I hacked together a test, feel free to point out mistakes.

c part: http://pastebin.com/zMha8Fre

asm part(SSE and SSE2 for nt): http://pastebin.com/mxEFC8Cw

results:

aligned: 0 sec, 69049070 nsec

unaligned on aligned data: 0 sec, 69210069 nsec

unaligned on one byte unaligned data: 0 sec, 70278354 nsec

unaligned on three bytes unaligned data: 0 sec, 70315162 nsec

aligned nontemporal: 0 sec, 42549571 nsec

naive: 0 sec, 67741031 nsec

Repeating the test only shows non-temporal to be of benefit. The difference of, on average, 1-2% is not much, that i yield. But it is measurable.

But that is not all! Changing the copy size to something that fits in the cache (1MB) showed completely different results.

aligned: 0 sec, 160536 nsec

unaligned on aligned data: 0 sec, 179999 nsec

unaligned on one byte unaligned data: 0 sec, 375108 nsec

aligned nontemporal: 0 sec, 374811 nsec // usually a bit slower then one byte unaligned

And, out of interest, i made all the copy-s skip every second 16 bytes, (relative) results are the same as the original test except non-temporal being over 3x slower then anything else.

And this is on a amd fx8320 that has the misalignsse flag. On my former cpu (can't remember if it was the celeron or the amd 3800+) the results were very much in favor of aligned access.

So yea, align things. It's not hard to just add " __attribute__ ((aligned (16))) " (for gcc, idk anything else).

PS It may seem like the naive way is good, but memcpy is a bit more complicated then that.