Wilco (Wilco.Dijkstra@ntlworld.com) on 1/12/11 wrote:
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>Code with lots of misaligned accesses typically doesn't run fast - not even on
>x86 as there is always a penalty. There certainly are a few key examples where they
>improve performance, but apart from those, unaligned accesses don't help much, even there wasn't any penalty.

About a year ago I redesigned a hashtable class and
spent a while trying out different code sequences and
data layouts on Nehalem-based machines. The conclusion
of that effort was the penalty for misaligned accesses
within a cacheline appeared to be very small; and that
for what I was doing (essentially storing an array of
48bit values) it would be fastest to pack the values
tightly and do read accesses with non-aligned 64-bit
reads (followed by masking as necessary).

In the light of that experience - which was somewhat
surprising to me - I would suggest that "code with lots
of misaligned accesses typically doesn't run fast"
may be incorrect for Nehalem and later.

Though if the use of misaligned accesses is due to
careless programming rather than conscious design, there's
a likelihood that such programs will have other
performance issues due to other manifestations of the
carelessness ...