Re: [RFC] Documentation about unaligned memory access

Andi Kleen
2007-11-23T02:07:48Z

Robert Hancock <hancockr@shaw.ca> writes:
>
> Also, x86 doesn't prohibit unaligned accesses,

That depends, e.g. for SSE2 they can be forbidden.

> but I believe they have
> a significant performance cost and are best avoided where possible.

On Opteron the typical cost of a misaligned access is a single cycle
and some possible penalty to load-store forwarding.

On Intel it is a bit worse, but not all that much. Unless you do
a lot of accesses of it in a loop it's not really worth something
caring about too much.

-Andi