[Rd] optim-Bug (PR#6720)

[Prof Brian Ripley]

I think S_alloc is little used (at least in R), but I have altered it to
use memset in R-devel. As I understand it all decent compilers recognize
memset calls and try to inline them with optimized code (probably even
optimizing the bzero case).

On 21 Apr 2004, Douglas Bates wrote:

> Prof Brian Ripley <ripley at stats.ox.ac.uk> writes:
> ...
> > Zeroing the workspace is not a requirement of the original L-BFGS-B code 
> > that I can see.  Given that it was originally in Fortran, and Fortran 
> > often does zero it seems a likely symptom, but it does mean that a 
> > variable is being used uninitialized somewhere in the code (converted to 
> > C).  It would be better to leave vect alone and to zero the workspace with 
> > a memset call in lbfgsb.  (Incidentally, I don't know why S_alloc does not 
> > use memset -- we do require standard C and use in several other places.)
> 
> I had planned to suggest that we use memset more widely and perhaps
> add a macro Memset, defined like the current Memcpy, to R_ext/RS.h.  I
> felt that memset was likely to be more efficient for zeroing large
> areas of memory than running a loop would be.  Recently Peter Dalgaard
> and Andy Liaw and I discussed this with regard to a test to see if
> vectors using more than 4 GB of memory could be allocated on an
> Opteron.  Andy installed my suggested modification to do_makevector to
> use memset for zeroing freshly allocated vectors and found that the
> running time for a test on an Opteron/Linux system actually increased.
> It appears that, at least on that system, memset runs a loop at the
> level of characters.

(You mean char* bytes?  S_alloc was a byte-level loop, but looking in the
headers suggest that gcc3 has optimized inlined code for memset(s, 0, n).)

> I still think it would be a good idea to use memset but we would want
> to do some timing comparisons to make sure we don't slow things down.

That's very machine- and compiler- dependent though.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595