Re: [Caml-list] Just how slow are classes?

[Gerd Stolpmann <info@gerd-stolpmann.de>]

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Am Freitag, den 18.12.2015, 00:19 -0800 schrieb Jordan W:
> I am aware of some of the reasons why OCaml classes and objects are
> slower than records. The lack of nominal subtyping makes determining
> memory layout at compile time much more difficult.
> 
> 
> While I am also curious why "nice cases" of objects/classes couldn't
> be optimized (perhaps some link time check), when most of the objects
> adhere to a nice hierarchy, I would like to defer that discussion
> until another time.

Oh, this is quickly explained. There is no hierarchy at runtime you can
exploit. Remember that object X can be a subtype of object Y even if
there is no inheritance relationship. This is very different from other
OO languages.

>  For now, my primary question is: Just *how* slow are objects/classes?
> Has there been any relevant benchmarking efforts that could shed some
> light on this?

It is essentially an additional hash table lookup.

> For example, how much more expensive in terms of memory are objects
> (compared to records). How much slower are property accesses and
> method invocations than record accesses and module function
> invocations?

It is hard to give a number because function invocations can be cheap
but also quite expensive when there are lots of arguments and many
register spills. It also depends on whether you always call the same
function or whether it changes every time (at the same point in the
program, i.e. per generated indirect jump).

Note that you can factor out the additional lookup when there are method
arguments, e.g. if you have

method foo : s -> t

you can do this:

let f = obj#foo

and then later, e.g. in a loop

f arg

which is then only the function call, w/o method lookup. This doesn't
work, though, when there are no method arguments.

Gerd

-- 
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Gerd Stolpmann, Darmstadt, Germany    gerd@gerd-stolpmann.de
My OCaml site:          http://www.camlcity.org
Contact details:        http://www.camlcity.org/contact.html
Company homepage:       http://www.gerd-stolpmann.de
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