Neil Mitchell wrote:
Hi Adrian
The GHC users guide says overloading "is death to performance if left to linger in an inner loop" and one thing I noticed while playing about with the AVL lib was that using a HOF and passing the (overloaded) compare function as an explicit argument at the start seemed to give noticable a performance boost (compared with dictionary passing presumably).
I'm not sure why that should be, but has anyone else noticed this?
A HOF is one box, the Ord dictionary is an 8-box tuple containing HOF boxes. When you invoke compare out of Ord, you are taking something out of the tuple, whereas when you use the HOF its directly there.
Well I can understand why overloading might be slow compared to a direct call (presumably this is what you get from specialisation). But I wouldn't have thought this additional indirection cost of method lookup was very significant compared with the HOF approach (at least not after everything was in cache). IOW I would have expected HOFs to just about as deathly to performance as (unspecialised) overloading, but it seems this isn't the case.
This is also the reason you get a performance decrease moving from a 1-element class to a 2-element class.
Why is that? Does ghc pass just the single method rather than a one entry dictionary in such cases? Regards -- Adrian Hey