[Rd] request: allow inline functions in R

[Prof Brian Ripley]

On Fri, 14 May 2004, Li Long wrote:

> Hi, R core developers,
> 
> I work in the Swiss Institute of Bioinformatics.  We have two clusters of
> Intel Itanium2 clusters for bioinformaticians to crank their data.  One
> piece of software they use heavily is R and BioConductors.  I ported the R
> codes and R packages to this platform already, and am working on
> optimizing their performance.  I'm using Intel C/C++ compiler on this
> platform running Linux.  One of my findings is that turning some functions
> in R to "inline" functions boost performance significantly.
> 
> While R follows strict C89 standard right now, there're quite some good
> reasons to relax the rule somewhat.  From my experience in software
> development in industry, I understand very well both the portability issue
> and backward compatability issue, I also see the hidden cost of holding
> back for too long and not fully achieving the potential of new technology,

Could you then please quantify that hidden cost?

> I recommend that we allow "inline" functions in R's C codes.

In what sense do `we' not allow it?  And who is `we'?

The problem is that very few compilers fully support C99, and others have
different ways to indicate inlining.  So a configure test is needed. I am
sure that if you provide one together with patches to parts of the code
where you find inlining beneficial, the real `we' would consider it
carefully.  Especially if the `hidden cost' is noticeable.

....

> In R, there are quite some simple functions that are called extremely
> often, such as "R_IsNaNorNA", "R_finite", etc.  They are used in heavy
> loops quite a lot.  They disrupt the pipelining, and negatively affect the
> performance of the software.  For instance, on IA64, system call of
> "isnan" cost 4 cycles, while a wrapper like "R_IsNaNorNA" could cost
> several times more.

However, one of the motivations of eliminating support for non-IEEE-754
platforms in R 2.0.0 is to enable some of this baggage to be eliminated.  
But the wrapper is there for a good reason: to get the right answer.

Since I gather you have suitably modified code, it would be helpful to 
your case to provide data 

 - on real problems
 - on a mainstream platform.

of the actual performance impact of not inlining.

-- 
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