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[Caml-list] Optimizing false polymorphic local functions
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Date: -- (:)
From: Xavier Leroy <xavier.leroy@i...>
Subject: Re: [Caml-list] Optimizing false polymorphic local functions
> The other day, I ran into a significant speedup improvement.
> [...]
> Now consider the slightly different version where "loop" is forced into
> a monomorphic function:
> [...]
> On my computer in native code, the speedup is really significant: more
> than 6 times faster (OK this example was built on purpose...). The
> reason is that in the first case, the operator <= is replaced by a call
> to the internal polymorphic compare_val function, whereas is the second
> case a direct comparison between integers is performed.
> 
> I suspect there are other cases in which the compiler can produce a
> better code when it knows more precisely the types involved. 

Yes: besides comparisons, array and bigarray accesses can be compiled
more efficiently if the exact types of the data are known statically.

> So my question is: would it be possible to help him in this way by
> enforcing the type checker to infer a monomorphic type in such
> situations ? By "such situations", I mean: local polymorphic
> functions that are used in exactly one monomorphic setting
> afterwards. Of course, this is not desirable for global functions,
> since it may break the calculus; but for local functions, it should
> be of no harm since we know all the places where they are used, and
> it would not change the type of the wrapper, thus being transparent
> for the user...
> Any comment ?

The following paper formalizes exactly this idea, and gives a type
inference algorithm that avoids unecessary polymorphism like you suggest:

  http://citeseer.nj.nec.com/bjorner94minimal.html

- Xavier Leroy
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