[Haskell-cafe] Re: blas bindings, why are they so much slower the C?

[patperry at stanford.edu]

Thanks for the patch, Don.  I'm on vacation now, but I'll apply it  
when I get back.

As for why "getDot" involves pattern matching, it is because a vector  
is either a pointer to raw storage, or a conjugated vector.  I wrote a  
blog post about this for those of you who are interested (  
http://quantile95.com/2008/06/12/blas-data-types/ ).

The current data type is this:

data DVector t n e =
       DV { fptr   :: !(ForeignPtr e)
          , offset :: !Int
          , len    :: !Int
          , stride :: !Int
          }
     | C !(DVector t n e)

Now, it isn't strictly necessary to use an algebraic data type-- I  
could have just used a boolean flag, but the code is a little cleaner  
this way.  I haven't done any tests to see what the difference in  
performance is.  I would expect that a pattern match between two  
alternatives would be just as fast as checking a boolean flag.

The good news is that this is a constant-time overhead.  For larger  
vectors, getDot should approach the performance of a raw cblas_ddot  
call.



Patrick