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Instruction selection in the OCaml compiler: Modules or classes?
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Date: -- (:)
From: Xavier Leroy <Xavier.Leroy@i...>
Subject: Re: [Caml-list] Instruction selection in the OCaml compiler: Modules or classes?
> I'm reading through the icfp99 slides on classes vs modules in OCaml.
> The part that I'm interested in starts on p27 where instruction
> selection in the OCaml compiler is discussed.
>
> I'm not well-versed in the OCaml compiler code yet so I thought I  would
> ask the list: does the compiler use a module or class solution?

It uses classes, inheritance and overriding (of generic code by
processor-dependent code) for a few passes: instruction selection,
reloading of spilled registers, and instruction scheduling.  The other
passes are either processor-independent or can be parameterized in a
simpler way (e.g. register allocation, which is parameterized by the
number of hardware registers in each register class).

> The slides seem to favor the class-based solution but then I hear  that
> classes in OCaml are slow and people like Markus Mottl just  plain
> despise them :-). What does everyone else opine?

Method dispatch is slightly slower than calls to unknown functions,
but the compiler passes that use objects are not speed-critical anyway
(most of the compilation time is spent elsewhere).

I don't despise objects and classes: it's just that the kind of code
that I usually write does not need them often.  But there are some
cases where they work better than other forms of parameterization
available in OCaml.  Ensuring that the native-code compiler is not
polluted all over the place by Intel x86-specific hacks is one of them.

- Xavier Leroy