[Rd] suggestion how to use memcpy in duplicate.c

[Seth Falcon]

On 4/21/10 10:45 AM, Simon Urbanek wrote:
> Won't that miss the last incomplete chunk? (and please don't use
> DATAPTR on INTSXP even though the effect is currently the same)
>
> In general it seems that the it depends on nt whether this is
> efficient or not since calls to short memcpy are expensive (very
> small nt that is).
>
> I ran some empirical tests to compare memcpy vs for() (x86_64, OS X)
> and the results were encouraging - depending on the size of the
> copied block the difference could be quite big: tiny block (ca. n =
> 32 or less) - for() is faster small block (n ~ 1k) - memcpy is ca. 8x
> faster as the size increases the gap closes (presumably due to RAM
> bandwidth limitations) so for n = 512M it is ~30%.
>

> Of course this is contingent on the implementation of memcpy,
> compiler, architecture etc. And will only matter if copying is what
> you do most of the time ...

Copying of vectors is something that I would expect to happen fairly 
often in many applications of R.

Is for() faster on small blocks by enough that one would want to branch 
based on size?

+ seth

-- 
Seth Falcon | @sfalcon | http://userprimary.net/