tar@sevak.isi.edu (Thomas A. Russ) writes:

> "jurgen_defurne" <jurgen.defurne@pandora.be> writes:
>
> > And I know about CLOS and generic functions, but I have tested CLOS,
> > and while it might certainly have applications in more complex
> > settings, I want to extract the last drop of performance for my
> > program. A test program (in SBCL) written to test the access time to a
> > structure or an object, showed the access time to a structure to be two
> > times faster than CLOS.
>
> To add to your CLOS knowledge, there is nothing that stops you from
> using generic functions that dispatch on types introduced by DEFSTRUCT.
> So you can still use the generic function dispatch to decide which
> function to call at run-time while enjoying the faster access times of
> structs versus classes. This lets you use Pascal's solution to the
> problem of run-time dispatch.
>
> But the next question is whether you really need the last drop of
> performance for your program. How much of the TOTAL runtime of the
> program is spent performing slot accesses? Can you reduce that by using
> local variables instead? How much of an improvement does subsituting
> structs for classes make in the final runtime of the program?

These are all good questions, but since the poster is using a specific
implementation (SBCL) some other questions raise themselves; are you
losing anything by using structs versus defclass? Although the spec
doesn't have defined semantics for structure redefinition, SBCL does.
You still only get single inheritance, and no before/after/around
methods on accessors, but that may be good enough for a given module.
Are you storing the types of values in your slots which you want to
declare? The implementation treats structure slot definitions as
assertions, and is able to use that information when doing type
inference elsewhere. A couple of :type (unsigned-byte 7) (vector
single-float) declarations in defstructs can be really helpful when
compiling at (speed 3), while keeping your code readable overall.
This last one is the big win for structures in SBCL, IMNSHO.