Re: [Caml-list] Comments welcomed to progr. style / speed of my first ocaml program "Error correcting LDPC decoder"

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

Andries Hekstra said:
> Attached please find the first program I wrote. It has about the same
> speed as its C++ counterpart (slightly faster). I welcome comments w.r.t.
> programming style, esp. when it affects the speed of the program.

As you are doing a lot of array manipulation, this tip may increase
performance significantly: because "float array" is stored with a
different layout than all other arrays, the compiler generates a dynamic
check for every access when the type of the array elements cannot be
statically determined. For example, your function

let swapElement i j a = let x = a.(i) in a.(i) <- a.(j); a.(j) <- x

is polymorphic in the element type. If you define instead

let swapFloatElement i j (a : float array) =
  let x = a.(i) in a.(i) <- a.(j); a.(j) <- x

better code is generated because the mentioned check can be omitted by the
compiler. This applies only if the function is not inlined; swapElement
could be small enough (but qsort, for instance, is definitely not).

Unfortunately, one cannot define swapNonFloatElement because there is no
way to constrain a type that it is not float. So the type system cannot
represent this duality float/non-float, and one must fall back to as many
definitions as element types are actually used.

Gerd
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Gerd Stolpmann * Viktoriastr. 45 * 64293 Darmstadt * Germany
gerd@gerd-stolpmann.de          http://www.gerd-stolpmann.de
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