memcpy speed

[Sebastien Barre]

At 13:06 26/03/00 -0500, David Gobbi a écrit:

>Well, I decided to run a benchmark of memcpy() versus
>loops of the form
>
>j = count;
>while (--j >= 0)
>{
>   *cp1++ = *cp2++;
>}
>
>and loops of the form
>
>for (j = 0; j < count; j++)
>{
>   *cp1++ = *cp2++;
>}
>
>
>Depending on the architecture and the data type, you need to copy
>at least 32 bytes or memcpy is much slower than copying
>the data in a loop.  There is often a factor of >5 improvement
>in looping over using memcpy!

I've also observed that "feature" sometimes. It depends on the 
architecture, but it might be worth having a front-end for memcpy that will 
use either memcpy or that kind of loop (choice made at compilation time 
depening on a #DEFINE for exemple).

>Also, with gcc, the 'j = count; while (--j >= 0)' form of looping is
>around 15% to 30% faster than 'for (j = 0; j < count; j++)' form
>for copying less than 16 bytes.

Oh yes, that's common optimization trick :) (actually I thought it was true 
for more than 16 bytes also, I guess compilers get better and better in 
that game).

BTW, I'm quite sure that there is a lot of places in VTK where optimization 
might be done by replacing loop indices like mat[i][j] = ... by a pointer 
notation like *ptr = ... (not **ptr, but really *ptr).

--
Sebastien BARRE
IRCOM-SIC, UMR-CNRS 6615 - Université de Poitiers
Bât. SP2MI, Bvd 3 - Téléport 2, BP 179 F-86960 Futuroscope Cedex
Tel. : +33 (0)5 49 49 65 95 / 65 83, Fax : +33 (0)5 49 49 65 70
http://www-sic.univ-poitiers.fr/barre/ ou  http://www.hds.utc.fr/~barre/